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What is amperage
Amperage is the current of electrons being push through a conductor by volts
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Use only 600 volt wire.
Lamp cord, extension cords are not rated 600 volt.
Use copper wire only. Aluminum wire is fire risk and should be avoided or installed by professional.

30 amp breaker use 10 gauge /
120-240 volt 30 amp outlet can be installed on 30 amp breaker only/ use 10 gauge wire ... cannot be connected to 15-20-40 amp breaker.

Orange/ #10 gauge wire, with ground ... 30 amp capacity. Safe maximum: 30 x 80% = 24 amps.
Buy:
10-2 gauge/ 30 amp
10-3/ 30 amp
Southwire electric tools
Yellow 12 gauge 20 amp
120 volt 20 amp outlet can be installed on 20 amp breaker, but not 15 amp breaker/ use 12 ga wire.
... cannot be connected to 30-40 amp breaker. 1

Yellow/ #12 gauge wire, with ground ... 20 amp capacity. Safe maximum 16 amps.
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12-2 gauge/ 20 amp
12-3/ 20 amp

NMB is house wiring
UF is underground
Rolls of stranded wire
HOOK UP Wires
White 14 gauge 15 amp
120 volt 15 amp outlet, AFCI, GFCI, timer, switch etc can be installed on 15 or 20 amp breaker. Never connect 15 gauge wire to 20-30-40 amp breaker.

White/ #14 gauge wire,  with ground ... 15 amp capacity. Safe maximum 12 amps.
Buy:
14-2 gauge/ 15 amp
14-3/ 15 amp

NMB is house wiring
UF is underground
50-60 amp breaker use 6 gauge /
240 volt 50 outlet can be installed on 50 amp breaker only
6-2 wire
Buy:
6-2 wire
Southwire electric tools
NMB is house wiring
UF is underground
40-50 amp breaker use 8 gauge /
240 volt 40 amp outlet can be installed on 40 or 50 amp breaker only
6-2 wire
Buy:
8-2 wire
Southwire electric tools
NMB is house wiring
UF is underground
     
Copper ground wire.
Every device, load, metal enclosure etc must be grounded. Ground wire must be continuous throughout installation, never switched on-off, never used as a Neutral wire.
Generally ... use same size as other wire in circuit
Buy:
12 gauge copper ground wire
Ground wire
Green ground wire
Ground pigtails
Ground rods/ ground clamps at Amazon
armored cable
Non metallic flexible cables must carry ground wire, but do not have hazard of short circuit causing injury from shock.

Armored steel cable can be used as a grounded connection, and will protect wires from damage. Metal can be energized from an insulation failure.

All conduit ...metal, plastic ... flexible and rigid ... must be attached to structure, and attached to enclosures, boxes.
Movement, damage and deterioration are major cause of electrical failure.
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Non-metallic flexible conduit
Power whip
Armored cable
Southwire armored cable cutter
Pull boxes
Southwire armored cable cutter
Electrical tools must be insulated.
Always best to disconnect power, but insulation failure, lack of proper grounding, grounded neutral, lack of GFCI, out-of-code wiring, generator operating without transfer switch, and other problems still pose a risk to anyone working on electric power ... even when breaker is off.

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Electrician tools kits
KLEIN TOOLS
Tools kits
IRWIN tools
KNIPEX

Telephone cable knife
Low Voltage Mounting Bracket for Telephone
stranded wire
600 volt 12 ga Stranded wire ot THHN wire is good for conduit with multiple wires ... but stranded cannot be installed under screw terminals on outlets, switches, timers etc without risk ... of heat causing splayed strands ....that come loose ... and begin to arc. Connect stranded to short piece of solid copper wire, and attach solid to screw terminal. Do not solder residential or commercial wiring.

Project wire, extension cords, thermostat, bell and automotive wire are not rated for residential or commercial wiring. Only wire maked 600 volt can be used for permanent household wiring.
Buy:
Southwire 600 volt stranded wire
Rolls of stranded wire
HOOK UP Wires

Protect wiring from damage
Use nipper instead of pliers or screwdriver for removing staples (and nails). Do not damage cable or wires inside cable.

-Code says: Cable SHALL BE secured without damage to the outer covering. NEC sec. 336-15
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End cutting nippers at Amazon


Electrically insulated tools
When removing insulation from wire, do NOT score or put cuts on surface of copper wire. Doing so increases resistance and heat on wire and creates possible weak point.
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KLEIN TOOLS
Wire strippers at Amazon
Electric Wire Stripping Machine
Linesman pliers
Utility knife at Amazon
multimeter
Multimeters
Voltage is tested across two separate wires. Ohms or resistance is tested across both ends of same wire. Amperage is tested along one or two points on same wire.
Buy:
Analog multimeter
Multimeters at Amazon
Klein multimeter
Electric testers at Amazon
Clampmeter for testing amp flow on line

GFCI circuit breaker
A GFCI circuit breaker will protect all boxes on circuit.

Circuit must be grounded. Ground wire must be continuous and connected to each piece of equipment.

White wire on circuit breaker must be connected to Neutral busbar inside the breaker panel.

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GFCI circuit breakers
Push down
single-pole breaker
1" wide Push down breaker
Plug-in breaker
Buy single-pole:
Choose from 15 to 60 amp
Eaton CL series
Siemens 15-60 amp

Single-pole tandem breaker
Tandem breakers
Push down
double-pole breaker
2" wide Push down breaker
Plug-in breakers
Buy double-pole:
Choose from 15 to 125 amp
Choose from 15 amp to 200 amp
Eaton CL series circuit breaker
Siemens circuit breaker
Double pole GFCI
What is amperage?
Amperage is the current (flow) of electrons being pushed through the matrix (atomic structure) of a conductor (wire) by voltage.
To produce household and commercial electricity, a magnet is passed over a conductor ... or more specifically over a coil of wire ... which causes electrons to come loose from orbit ... and begin jumping from atom to atom in a current of electrons that are pushed down the wire.
This process is called magnetic induction ... and is the method invented to produce alternating current (AC) that is found on the electric grid.
The stronger the magnetic field, the higher the voltage. More voltage means more force pushing the electrons ... which is the basis for long distance transmission and distribution of usable AC electricity.
Resource:
Electricity from power plant to end user

Other forces produce amperage, such as lighting, where warm and cold air masses collide in a moist atmosphere. The air masses moving against each other cause friction  like rubbing socks across a carpet) that can dislodge the lightweight and negatively-charge electrons from orbit around the heavier, positively-charged protons. This creates conditions that can result in lighting.
How it works. Air is not a good conductor of electricity, because it offers high resistance against the movement of electrons. 
Contrast air with copper wire and other metals that are good conductors and offer less resistance to the movement of electrons.

The high resistance of air causes the dislodged electrons inside storm clouds to gather into pockets, where the potential negative charge can build up depending on number of electrons.
Likewise, other areas of the clouds accumulate pockets of the positively charged protons that were stripped of their electrons.

If the potential charge between the negative electrons and positive protons becomes great enough, the natural attraction of opposite charges can overcome the resistance of air ... resulting in a high voltage lighting bolt that pushes electrons and protons together where the electrons re-enter orbit around the protons and equalize the charge.
Lightning occurs only when the positive and negative charges are so strong, that the voltage can overcome the resistance of air.

There is also a natural attraction between the pockets of charged particles inside clouds and the earth below. Again, if the different charges are strong enough, a bolt of lighting will strike the earth.
The amount of voltage from a lighting bolt is so massive, that it can travel long distances, and still have enough force to push the current of amperage (electrons) completely through the trunk of a tree and then bounce and flow across the ground, following whatever path offers the least resistance. The bolt lasts until the the unequal charges are equalized, or until the differential charge is less than the resistance of the air.

Resistance and heat
The conductor (for example a copper or aluminum wire) always offers resistance to the flow of electrons ... resistance is like friction ... that produces heat when electrons are pushed through the wire by the voltage.
This explains the bright flash of high heat that accompanies a lighting bolt that is pushing electrons through the resistance of air.
Resistance also explains why wires get hot, melt and pop when too many electrons (amperage) are being pushed through an undersized conductor by the voltage.

Heat is the challenge for controlling man-made electricity
Lightning and other forms of natural occurring electricity are generally unpredictable and uncontrollable ... while man-made electricity can be controlled using very specific techniques and materials. This makes the transmission, distribution and use of electricity possible.
For example, circuit breakers (overcurrent protection) are required throughout the grid, and at end user locations.
Circuit breakers are rated by amperage. They are designed to limit the amount of amperage that can flow into a wire. Breakers protect conductors (wires) from too much amperage ... because too much amperage will encounter increased resistance that will overheat the wire. Overheated wires lead to less predictable events such as insulation failure, damaged equipment, short circuit, melted wires, fire, explosion etc.

By code, all electricity must be routed through a circuit breaker or fuse that is designed to trip when heat on wire exceeds breaker rating.
Breakers and fuses etc are called 'overcurrent' protection. Current is amperage, so 'overcurrent' is too much amperage.
Each breaker or fuse is rated by a maximum number of amps that can pass before the overcurrent protection trips and cuts off the flow of electrons.
Overcurrent protection devices are rated by amps, but they actually respond to heat on the wire.
If heat on the wire exceeds rating of circuit breaker (or fuse) then the breaker trips.
This also means that ambient temperature can affect actual tripping point for a breaker, with cool temperatures allowing more amperage to flow than hot temperatures.
Each wire has a rating for maximum number of amps that can flow before the wire overheats.
If heat on the wire is allowed to exceed safe limits of the wire, the wire can melt and cause fire. That's why it's important to match wire size and breaker rating.
Resource:
Wire and breaker sizes
Amp rating of power lines

Measuring Volts and Amps
Except for brief surges throughout the day and night, voltage on the electric grid remains a steady potential between out-of-phase hot wires or hot wires and a neutral. Voltage is measured across two different conductors.
On the grid, even when no lights or motors or loads are turned on, the voltage potential remains the same. For example you can insert a tester into two prongs of an electric plug, and the tester will light up and show full voltage, even though nothing is plugged into outlet.
Buy: Electrical tester
In a storm cloud, the voltage would be measured as a potential between the pockets of negatively and positively charged particles. Even if a bolt of lighting never occurs, the potential voltage between the pockets of different charges is still there ... until a bolt of lighting occurs or the storm dissipates and the charges slowly equalize without lighting.

Amperage on the grid rises and falls as loads are turned on-off. If more air conditioners are running, then amperage on the line is higher.
Amperage flowing on the wire varies depending on usage, and is measured as the flow of electrons along one conductor.
A clampmeter is commonly used to measure amps.
Buy: Clampmeter
In a lighting bolt event, the amperage would be measured as the number of electrons flowing along the bolt of lighting. If there is no lightning bolt, then there is no amperage. The voltage potential between positive and negative charges is still present until the storm dissipates, but if no electrons are moving, then there is no amperage.

On the grid, the wires are energized with voltage to deliver the flow of electrons (amperage) ... voltage is held steady ... but amperage varies depending on end-user demand. If consumption of power (P or wattage or VA or volts x amps) goes up because more air conditioners are operating during the day, the voltage does not increase, only the flow of electrons (amperage) goes up.

The more amperage flowing on the wire, the higher the heat. Resistance causes heat on the wire, and amperage and resistance are correlates ... when amperage goes up, the resistance and corresponding heat on the wire goes up
Definitions
"(V or E) Volt: The unit of measurement used to quantify electrical pressure or the force that causes electrons to flow.'' Voltage is the electrical potential difference between two points.

''(I or A or current) Amp: the unit of measurement used to quantify the rate of electrical current.'' ''Electrical current is movement of the electrons.''

(R) Ohm: The unit of measurement used to quantify the opposition or “resistance” to the flow of electricity.
Factors that affect resistance include wire size, material used for the wire, length, wind and heat.

(P or W) Watt or Power (volts x amps = watts or power): The unit of measurement used to quantify the amount of electrical energy or power being used."

Basic formulas
(V) Volts x (I) Amps = (P) Watts
(I²) amps² x (R) resistance = (P) Watts
Ohms = Volts divided by Amperes
Any material can be a conductor if enough voltage is present
For example a lightning strike carries extremely high voltage ... and produces so much force that the flow of electrons can blow a tree apart. While a lesser voltage cannot do the same because wood offers fairly high resistance against the movement of electrons.

Some materials are better conductors than others.
Wood and air are poor conductors. A lightning bolt must carry immense potential to overcome both the resistance of air and the resistance of a tree.

The power company uses the insulating value of air space to help insulate the bare high voltage lines that transmit power into communities.
The higher the voltage, the father apart the high voltage wires are spaced from each other, and the higher the wires are suspended from the ground. This is to keep the high voltage from arcing to the ground, to other electric wires, conductors etc. This illustrates that the 370,000-500,000+ volt transmission lines carry much lower voltage than a lightning strike.
The power company also utilizes the insulating value of wood poles to carry 69,000 volt sub-transmission lines, and carry local 4500-7200 volt distribution lines to individual homes and businesses.

The grid is a balance of cost and functionality. Wood poles offer fairly high resistance, they are abundant, moderately priced, strong, and last 25+ years. The air surrounding electric wires is free, as long as wire can be raised to the correct height and insulated away from contact with poles, other conductors, and flammable materials .. so wood and air, and other non-conductive material like glass, porcelain, rubber etc are utilized by the grid to isolate and insulate electric power away from causing damage, fires etc ... and meet the dual objectives of function and affordability.

Conductive materials like metals offer much lower resistance than materials like rubber. Conductive materials like copper and aluminum are used as conductors on the grid and inside homes and businesses because they offer low resistance to the flow of electrons (amperage) compared with other materials. The relative abundance of these metals, and the workability of each make them ideal for electrical wiring.
Silver is likewise a very good conductor ... it is abundant and workable ... but too expensive, and too soft for large applications outside of electronics.

No matter which conductor is specified for the application .... whether it's copper, aluminum, steel, silver etc ... each material offers resistance against the flow of electrons (amperage), creating practical limits that must be met for safe electrical installations.
Resource: Safe electric wiring
what is amperage One ampere is the current which one volt can send through a resistance of one ohm

Basic formula for electricity: Volts x amps = watts
Watts are combination of volts and amps.
Watts are total consumption of power by end user
Resistance becomes the heat that results when volt push too many electrons (amperage) through a conductor,

Example: let's suppose:
Volts are the number of men who show up for work, they are the force that causes electrons to flow.
 
Amps are the flow of energy (effort) through each man.
They are the rate of electrical current.

Watts are the amount of rock that must be produced to meet demand.
(Volts) Men x (Amps) effort = (watts) total amount of work.
What is amperage Volts x Amps = Watts
Volts and amps are inversely proportional. If volts (number of men working) are reduced, then amps (effort from each man) are increased.

The inverse is also true: ... if volts are increased, then amps are decreased

Example:
If fewer men (volts) come to work, the average effort (amps) of each man must be increased to meet the total demand for rock (power or watts).

Men work harder, they get hotter. When each man works harder, he sweats more because he is putting off more heat.

Same with amps. When amps get pushed too hard, then heat on the wire increases.

How do we know amperage causes heat?
transmissionReason 1) Electric transmission
The power plant generates electricity that is transmitted long distance across high voltage lines. The wires are high voltage and low amperage to save heat loss on the wires.

Each conductor on the overhead conductor has an amp rating, or maximum number of electrons that can flow along the wire before the wire simply cannot carry any more either breaks, melts, catches fire or trips substation circuit breaker. Amp rating of power lines
If amperage was high instead of volts, it would require larger diameter wire. Larger heavier wire means the transmission towers would not be spread as far apart, the costs would be much higher, and electricity couldn't travel far before losing much energy through heat loss.

Transmission wires with 300,000-500,000 volts have low amps and low heat which is good for long-distance transmission. But the switchgear and wiring for 500,000 volts is too large, expensive and dangerous for household use.
Using transformers, the power company changes volt-amp ratio to accomplish different objectives the grid. The result is household receives low volt, high amp power. It works nicely because 120-240 voltage can be safely controlled by small switches, relays, cell phone chargers etc contained within steel and plastic enclosures, while the amperage (and resulting heat from resistance of conductive materials) is controlled by circuit breakers and then distributed to outlets, switches, dryer etc using correctly sized wire to match amp rating of breaker. Resource: Power plant to end user

Super conductivity: reduce heat loss using super cooled temperatures, allows long distance transmission of power without loss. Again, it's heat on the wire that causes loss, and illustrates why long-distance transmissions use high volt low amp electricity.
Large scale The practical application  of super conductivity is probably impossible. And even in super-cool temperatures, there would be a limit to electrons based on wire diameter and material.

Reason 2) Circuit breakers
Circuit breakers are overcurrent (amperage) devices. They are rated by amps, and trip when amperage or flow of electrons exceeds breaker rating. The breaker responds to heat on the wire and trips.

Household electric service is 120-240 volts. The voltage does not vary. Voltage stays the same throughout the day, no matter how many appliances are using power. There are excepting when voltage can spike due to surge, malfunction of power company transformer, lightning strike, but generally voltage should be steady.

When a circuit is wired, all the electrical plugs and switches etc are wired in parallel. If there are 12 boxes on the circuit, each containing an outlet, the voltage at the first outlet is same as last outlet.
When the desk light oven turns on, the electrons or amperage on wire increase. When the computer turns on, more electrons begin to flow. The voltage stays the same (assuming correct size wire is used for distance from breaker box ). Resource: Color code wire

When there is overamperage, due to too many devices drawing power, the wire begins to heat up. When the heat exceeds breaker rating, the breaker trips off the circuit. If wire is undersized for the breaker amp rating, the line can get hot inside wall and cause fire. This means it is important that wire size and breaker size match. ResourceMatch wire and breaker size

When there is overamperage, due to a short circuit, the wires can melt. Wires melt because of heat caused by too many electrons (amperage) getting pushed down the wire by voltage. In ordinary circumstances, the voltage didn't change. The transformer that supplies power to house didn't suddenly increase the voltage. Instead it's the increase in amperage creating high resistance against the wire that melts wires.
Resource
Color code for breaker

Reason 3) Wire rating
By code, all wire and cable used for permanent installation in homes and businesses are rated 600 volt.
This does not include extension cords that cannot be used for permanent wiring and have strictly limited applications.
The reason:
600 volts is the maximum supply voltage that distribution transformers can supply to business, and then 600 volts is only supplied to commercial installations when that specific service is required. Homes never receive 600 volt service, and there are no household devices or appliances made for such voltage.
Most businesses receive 120-208-240-277-480 volts depending on requirements. 600 volt service is  not common.
Homes receive standard 120-240 volt service, which can range from 115 to 250 volts since voltages are never precise, but expected to be within range to ensure appliances and devices can be mass-produced and operate correctly. The voltage might vary slightly over time, but is generally stable and unchanging.

Reason 4) Mathematics of electricity
If there is a short circuit, where electrons start running wildly into the ground wire (assuming installation is correctly grounded), the wires can melt.
a) The voltage does not increase during a short circuit. The voltage didn't spike over 600 volts. The grid transformer doesn't suddenly change it's construction parameters and deliver more voltage.
b) The amp rating of the copper wire does not change. Each wire has a maximum amp rating. For example 10 gauge wire has maximum 30 amps. NEC Code says: Maximum is 80% of rating ... 80% x 30 amps = safe maximum 24 amps for 10 ga wire. Wire are like lanes on a freeway, and amps are like the cars. Wires only allow so many cars before cars slow down and get backed up. Larger wires have more lanes.

c) So what changes when a short circuit happens? The flow of amps on wire increases because the short has allowed the electrons to flow into earth, and the electrons start to back up because the roadway doesn't have enough lanes.
d) But why does the wire get hot when more electrons are on the wire? Why does a wire melt?
e) The answer is, more electrons pushing against the matrix of a conductor, causes resistance to go up. The copper wire is a fixed size, and the copper matrix will only allow so many electrons to flow on the wire before the resistance limits the flow of electrons. Electrons begin to pile up and push harder, like an impatient traffic jam. The more electrons on the wire, the more the resistance slows the electrons, resulting in more heat on wires.

f) Mathematics
First formula: Volts x Amps = P (Power or watts) illustrates the concept. Volts do not change, but more electrons (amps) are flowing down the wire, so amps are rising. This means P (power or watts) is rising.
Second formula: I² (amps) x R (resistance) = P (Power or watts). The first formula tells us that Power is going up. If P is going up, then [amps x resistance] must be going up. But the wire limits how many amps (electrons) can flow on the wire. Once the number of electrons approaches the maximum, then the number of electrons is limited. This means the resistance must go up../. which slows the electrons and backs them up like too many cars on the road. Too many electrons on the wire and the wire gets hot causing the breaker to trip. Breakers respond to heat, but breakers are slow-acting, so the amperage can melt the wire almost instantly before the breaker trips.

This tells us that amps (the flow of electrons on the wire), held back by resistance, is the cause of heat. If there was no amperage flowing, there would be no heat.

circuit breakerEach circuit breaker has Amp rating.
If amps on wire exceed the breaker rating, the breaker responds to heat on the wire, and trips.
Breakers and fuses trip because of heat on wire. Amperage is heat.

Most breakers and fuses are not fast acting, and will allow heat to exceed breaker rating for a window of time before tripping. This allows the operation of motors and other inductive loads that might draw more amps than breaker rating for a brief period during start up.
If breaker is too large for wire, it will let too much amperage enter the wire. If the wire gets too hot it can cause a fire inside the walls. This is why it is important to match wire size and breaker.
Color code for wire

GFCI and AFCI are specialized types of breaker. They respond to heat on wire, same as regular breaker, but also respond to unequal flow of electrons through the breaker, and instantly trip when anomalies signal a ground fault or arcing condition somewhere on circuit. In that sense, GFCI and AFCI are fast acting under certain conditions of fault and arc, but are still slow acting to allow inductive loads.

Breaker types in illustration:
Tandem, GFCI or AFCI, single-pole, single pole, double-pole, and 3-pole for commercial 3-phase applications.
Resistance R = watts ÷ amps squared
Resistance R of an element = rated volts squared ÷ rated watts
Resistance on a wire = resistivity of material used for wire x [length of wire ÷ cross sectional area of wire]
"One ohm is the resistance offered to the passage of one ampere when impelled by one volt"
All electrical wires and appliances consume energy. This energy is consumed.

Let's say resistance is the amount of energy each man consumes while working
When a man is working he is using energy. When you slam the pick downward, and it strikes the surface, that action consumes energy. When you lift the pick back into the air, that action consumes energy.
If work used up no energy, each man could work forever without rest
Let's say, the energy that each man consumes during work is equivalent to resistance on the electric wire.
Transformers:  Current, amperage from the grid.
The current is inversely proportional to both voltage and number of turns. E volts N turns I amps
If primary volts E1 from the distribution line on the grid are 7200 volts, and secondary volts E2 going to home are 240 volts and secondary amps I2 are 250 amps (the amp rating of main breaker in breaker box), then the turns ratio on the transformer is 30:1, and the primary lines must be able to deliver 8 amps to the primary coil on the transformer ... 250 amp divided by 30.
So a residential home will draw 5-8 amps from the grid at maximum usage. Voltage remains unchanged, 7200 on the primary and 240 volt on the secondary, no matter how many amps are pulled by the household, because all circuits are wired in parallel, not in series ... each Load (transformer, outlet, air conditioner etc) connect to the Hot lead, and not to the previous Load, so Loads do not experience voltage drop.

Since electricity is dynamic, and delivered on demand, 8 amps would be consumed off the primary only when the home was using maximum amps on all circuits. The 250 amp main breaker in household panel would be near to tripping off.
The effect is multiplied when the distribution line supplies power to 100's or 1000s of homes and businesses. If everybody on the line was consuming maximum amps during a record heat wave, then the amperage on the power line would begin to heat the circuit breaker at the substation. If the substation breaker tripped off on the line, another circuit might pick up the load, but sometimes at reduced power, resulting in brownout where the voltage drops. Grid improvements for reliability have minimized the problem for short-run, but as summer temperatures increase, the practice of reducing consumption is best choice. Rooftop solar panels can likewise assist.
Resources:
Source page 6
Overheat power line proximity devices 20034865 page 3
How to wire generator transfer switch
What causes electrocution
Override air conditioner to reduce run time

Ohm's lawOhm's law

Nobody can say exactly what electricity is.
We can describe what it does... it is the force that moves electrons across the surface of a conductor.
For example a copper wire is a conductor, and electricity can be used to move electrons down the wire.
When the electrons arrive at incandescent light bulb, voltage pushes the current of electrons (amperage) across a narrow filament inside the bulb.
The filament is a type of wire, except smaller than regular electric wire.
When electrons are pushed across the small wire, the wire becomes hot, and begins to glow.
The wire doesn't burn out because the bulb is a vacuum ... there is no oxygen or air.
The glowing wire becomes the light emitted from the light bulb.

How do we know what size wire and how much electricity is needed to cause the light bulb to give off light without also causing the larger wire to get hot?
That's where ohm's laws is used.
While we can't say exactly what electricity is, we can predict what it will do using ohm's law.
Ohm's laws are a series of formulas that help electricians and engineers design functional electrical and electronic systems. The basic formulas are shown in the ohms law wheel.

Ohms laws have been used in common practice for many decades.
Over time, reports arise about the peculiar things electricity can do... such as jump across wires and cause shorts and fires.
The practical experience over time, plus application of ohms laws has given rise to the National Electric Code.
The code is a guideline that standardizes wire and breaker sizes, and establishes best practices.
The practical application of ohm's law and the code lets electricians safely wire a house or a business.
Control the heat, and you control amps
The electric grid is based on controlling the heat caused by amps ...
Heat on the grid is controlled by:
1) keeping voltages uniform... and then making electric products that meet the uniform voltage.
2) using transformers to control the volt-amp relationship
3) using fuses and breakers that trip, and stop power, when heat exceeds rating
4) using correct wire size to meet the amp rating of fuse
5) Using the correct volt and amp combination to meet total demand for power.
1) Uniform voltages help control heat from amps
1) First step for controlling heat is done by keeping voltages uniform.
Uniform voltages help standardize the grid so peak loads on the grid can be calculated
Electric grids around the world offer specific voltages.
In the US, transmission and distribution lines carry high voltages such as 500,000 or 7200 volts.
Of course the end-user receives lower voltages. Lower voltages are necessary at end user because high voltage appliances are expensive to buy, hard to maintain, and very unsafe.
For example in the US, common end-user voltages are 120, 208, 240, 277, 440, 600 volts
Other parts of the world might offer 230 Volt or different voltages etc.
Products are made for these specific voltages. Generally, nobody offers products that are rated for 156 volts, because 156 volt electric service is not available.
Uniform voltages are first step for controlling heat from amps.
2) Transformers control volts and amps

Electric transmission lines are high voltage and low amperage
.... to reduce heat loss during transmission 2) Transformers are used control the volt-amp relationship.
Smaller power company transformers are air cooled, while bigger transformers stay cool using special oils that are inside the transformer
Transformers are engineered to raise or lower the voltage, which causes higher or lower amps in same proportion.
When power company transmits electricity long distance, they use transformer located at generation plant to increase the voltage from 30,000 to 500,000 volts, etc.
When the transformer increases voltage, it also lowers amperage, so there is less heat-loss (or power loss) during transmission.
Long distance transmission of electric power requires low heat or low amperage, and this is accomplished by raising voltage..
If amps were allowed to be high, then long-distance transmission lines would be hotter, and much of the electric power would be consumed by heating the wire. Heat loss would reduce efficiency, reliability and decrease maximum distance for transmission.
Other transmission problems from high amps: hot wires would sag requiring closer transmission towers, and thus more towers, wires would be larger diameter and made of different materials, insulators would be more robust to hold the weight and withstand the heat. Cost of transmission would be higher if amps were higher.
Lowering heat loss by lowering amps and raising voltage allows long distance transmission of electricity. This applies to both AC transmission and HVDC (high voltage DC) transmission.
Long distance transmission wires travel to local substations. Transformers located at local distribution substations reduce the 500,000 volt transmission lines into lower voltages and higher amperage for shorter distance transmission to homes and businesses and other substations.
When electric power arrives at the end user location, more transformers are used to step down the voltage and increase amperage again.
For example a transformer located at typical US home might receive 7200 volt power line from the grid. The transformer converts 7200 volts into usable 240 Volt and 200 amp service for home.
Household low voltage and high amperage is transmitted very short distance from transformer to home.
Most high voltage transmission lines are bare wire suspended high above the ground.
Wires going to end user are sheathed with insulation.

Summary: Transformers can step up voltage, or step down voltage anywhere along the grid.
Step-up and step-down transformers are used to control the heat caused by amps. The use of transformers maximizes the efficiency of electric transmission.
Transformers are a method for controlling heat from amps.
3) Fuses and circuit breakers trip when heat from amps exceeds rating
Each fuse and circuit breaker has specific amp rating
When too many amps are drawn across the wire, the wire gets hot.
Fuses and breakers respond to heat on the wire.
When heat exceeds amp rating, the fuse or breaker trips and stops flow of electric power.
All electric power that arrives at end user must have a fused breaker panel
Summary: To prevent overheated wire and resulting fire, the circuit breaker trips when heat on wire exceeds amp rating of fuse or breaker.
4) Wire size must match amp rating or wire will overheat and cause fire

Each wire in the electric grid is protected by a fuse or circuit breaker
Generally, the fuse protects full length of wire from too many amps
When heat on wire exceeds amp rating of fuse, the fuse trips before wire gets too hot
It is important that each wire match the amp rating of the fuse
If the wire is too small, or amp rating of fuse too large for wire size, then wire can overheat and cause melted wire, arcing and fire.
 
AC or alternating current The electric grid is AC power.
AC is alternating current
AC is generated by rotating a magnet inside inside a magnetic field.
Each time the north and south pole of magnet rotate past a coil, the current reverses direction and flows opposite direction.
In the US, AC power reverses directions back and forth 60 times per second. This is because the generator rotates 60 times per second
When AC reverses direction, the electrons inside AC wires move back and forth
AC and DC
AC alternating current and DC direct current are two different types of electricity.
Both types of electricity cause electrons to move along surface of wire or other conductive material. If voltage is high enough using AC or DC, almost any material can conduct electricity.
DC is direct current. 
Example: D-size lithium battery with + and - side. Or car battery produces 12 Volt DC. Or solar panel creates DC electricity and has + and - connections. Or use DC generator. Or convert AC electricity into DC using rectifier. Or convert DC into AC using inverter.

Electrons in DC power travel only one direction. They do not reverse direction like AC.
With DC power, electrons flow from positive to negative terminals. Or positive to ground.
Summary: With AC power, the electrons move back and forth 60 times per second. With DC power, the electrons move 1 direction, from positive to negative.
Use DC to illustrate Volts
We can use DC to demonstrate Volts.
If you put 3 D-size batteries end-to-end, or in series, the voltage is multiplied, but amps stay the same.
Each D battery is 1.5 volts so total voltage: 3 batteries x 1.5 volts = 4.5 volts.
DC car battery to illustrate Amps  
We can use DC to demonstrate Amps.
Most people know a car battery is about 12 volt.... but why are battery wires so large in diameter?
Why are 12 volt car battery wires so much bigger in diameter than household wires that carry 120 volts to outlet?
Why are jumper cables so large diameter, when they just carry 12 volts?
The answer is amperage. Car batteries produce high amperage.
The big wire used for 12 volt car battery show that car batteries carry short burst of high amperage needed to start the car.

Unlike household 120 volt electricity which can kill you. The 12 volt car battery is not enough voltage to kill you.
Even 48 volt DC, commonly used in DC applications, is generally not enough to kill you. So 12-48 DC power is safe, compared with 120-240 volt AC or DC.
DC car battery to illustrate heat from Amps
Here is real-world example using car battery to show high heat from amperage
We know car batteries have a large wire so they can supply short burst of high amperage to the starter.
We can demonstrate heat using car jumper cables.
Jumper cables get stiff in the cold weather. But after you use the cables to jump another car, the cables are no longer stiff. Instead, the cables roll up easy because they got warm or hot carrying the amperage needed to start car.
We can also demonstrate need for correct wire size using car jumper cables.
The starter pull high amps. If jumper cables are not big enough in diameter, then you cannot get enough amps to jump the other car. Use larger cables, and the other car is more likely to start.
This example shows the heat from amps, and shows that wire size is important for carrying amps.
AC is wired in parallel
Appliances and loads rated for AC power are always wired in parallel
All household plugs and lights are wired in parallel
If you wire AC in series instead of parallel, you change voltage as it passes through each load, and that affects the control of amps, and the performance of each load.
If two light bulbs are wired in series, then the voltage drops after the first light. The second light receives less voltage. The overall circuit is affected, and bulbs do not perform as expected.
You want all AC loads to receive equal voltage.
AC generation produces three-phase electricity
Three phase is transmitted over three separate hot wires: Hot 1, Hot 2, Hot 3

Each wire is rated for the amount of amps that it carries
To calculate wire size and transmission distance, total maximum load is calculated.
Tap into electric wire
Larger image

Higher wires shown in photo are the 3-phase distribution lines that run down the street. These lines are high voltage.
Lower wires are same voltage as distribution wires.
Lower wires are drop wires that cross the street and supply
3-phase power to Walgreen in Rosenberg Texas

Unseen in photograph are the 3 fuses that protect wires and transformers from over-amperage. Also unseen are the 3 transformers that convert high voltage into usable voltage for Walgreen. For example Walgreen might receive 240-480 Volt 3 phase that can be configured many different ways once inside the mechanical room and breaker panel.

Often when you see drop wires in this configuration, each drop wire is protected by a heat-rated fuse/ lightning arrestor. These drop wires do not have fuse because it is short distance over to Walgreen's service pole which has fuses.
AC electric grid has bare wires.
Connections can be made by tapping into a wire
Any bare wire carrying AC power can be tapped anywhere along the line, using another wire to distribute electricity many directions.
Often in other countries, a kunda line or hook is thrown over a bare wire to get free electricity. Absolutely this is not done without risk of death and the human body burned into carbon.

Illustration shows 3 hot wires located at top of pole, and three taps that send power into 3 lower wires. 
Wires located highest on the pole come from generation side, arriving from nearby substation. Lower wires supply power to end user.
Note each wire is carefully separated from other wires.
Each wire is insulated away from the wooden pole. Insulators are made of specialized and purified glass. Without insulator, the wooden pole could become a conductor to ground, and catch fire, and disrupt integrity of power source.
It is important for the grid to maintain integrity of power.
AC power is a wave form. The 3 hot wires create a specific waveform. If the wave is disrupted, then clean power is not available at end user, causing malfunctions in electronics, overheated equipment, and inefficiencies that consume more power and cost more.
This illustration does not show the system neutral which is connected lower on pole, to which the ground wire is connected. All poles have ground wire.
3-phase electricity from generator to end user
Mathematics
From Singh's book on power generation

In alternating current systems - the volts and amperes might not be 100% synchronous, meaning that the oscillating waveform for volts is same as waveform for amps, so that amps do not lag volts

When synchronous the volt amperes equals the watts on a watt meter
When not synchronous volt amperes exceed watts - reactive power
Calculations show need for high voltage, low amperage during transmission

"Power P = volts V x amps I x cosineθ where amps lag voltage along transmission line"
      AC power is dynamic, and follows a waveform that oscillates at calculable angles
      As the waveform oscillates, or rises and falls, the power also rises and falls
      Power P is the power factor, which is the average power
      If you have high power factor, then that means the average power is high
"Power-loss PL due to flow of current or amperage during transmission  = I² x R"
       
PL = I² x R means amperage squared multiplied by resistance of wire
       Power loss is the amount of power it takes to transmit electricity
"Power loss in a conductor in terms of operating voltage and transmitted power:
PL = [P² x R] ÷ [V²  x  cosθ²]"
    Power loss = [Power squared x resistance] divided by [voltage squared x cos squared]

►"Formula above shows the power loss PL in conductors will be minimized with high-voltage and high power factor."
Conductor is the wire
We know that high-voltage means amps must be reduced.
We know that amps cause heat.
Therefore heat from amps must be reduced to minimize power loss PL and increase efficiency of transmission
"R = p x L/a
Resistance = resistivity of conductor x [length of conductor ÷ cross sectional area of conductor]"
This calculation is about choosing the correct diameter of transmission wire based on resistivity of wire and length of wire.
Each type of wire or conductive material has different resistivity.
Resistance consumes electricity in the form of heat. Too much resistance and wire will get hot and transmission costs more.
Transmission wires are chosen based on combination of many factors including resistivity, strength, and cost. For example copper is commonly used for household wiring, offering low resistivity for the purpose it serves, but copper wire is not strong enough to span several thousand feet between transmission towers. Moreover, the cost of copper is too high compared with other conductive materials.
Transmission and practical cost factors

►Using basic formulas above, the electric company can determine correct wire diameter for each length of transmission, based on volts and amps, and based on choice of available wire types and sizes.
►Economic costs are also important for calculating transmission of power.
Heavier wire costs more, so keeping voltage high means smaller diameter wire, which is less expensive than using bigger diameter wire.
However, if voltages are too high, then cost of switchgear and transformers goes up.
The mathematics of electricity must be balanced with economic costs.
►Cost for the end user is also important. High voltage appliances are impractical because they are expensive and dangerous and hard to maintain. As a result, the electric company cannot deliver 500,000 volts to each home. The voltage must be stepped down at local substation transformers, and then stepped down again using transformer located at each end user to meet practical economics of end-user consumption.
AC electrical devices are engineered to be wired in parallel,,,, so that voltage stays the same... and amperage is predictable...

120 volt 60 watt light bulb: watts divided by volts = .5 amp per bulb.
Ten bulbs wired in parallel.... each bulb receives uniform 120 volts and burns uniform 60 watt each.
Total amperage on line is 10 bulbs x .5 amp per bulb = 5 amps....
So amps are multiplied... and this lets the electrician know exactly which size breaker and wire are needed to prevent fire

If you connect appliances or water heaters differently than parallel ... then the uniformity of the electric grid is compromised, and the world will blow up as your electricians predicted.
Or more likely, catch fire.
Of course electricity does strange things but formulas that predict electric power do not vary by geography.
The physics that govern electricity is generally uniform across the globe, but transmission and practical electric code vary by country.

What I know about international electric code came from guy in Costa Rica last year:
His electrician connected 18 gauge lamp cord directly to 100 amp main breaker
... ran the 18 gauge wire out of small hole in the box
... taped the wire to metal fence up to point where he spliced 18 gauge into 14 gauge wire coming from gate opener
... the splice was covered with black electric tape, and then taped directly to metal fence.
Tape appeared loose.
... additional photos showed #4 wires dropping down from transformer to feed 100 amp breaker.
... and then #4 wires leaving main breaker box and going to large house located several yards away
... main breaker box was hanging on metal fence using 2 u-bolts
...no ground wires were visible at the main breaker box ... maybe metal fence was the ground?

He wanted to know why smoke kept coming out of wall sockets in his home.
Answer: amperage overheating the wires, and wires were catching fire.

The #4 wires are too small for the 100 amp main breaker. When the #4 wire gets hot, it's not hot enough to trip 100 amp breaker, so more amperage is carried onto the wire than should be allowed. Breaker should be 70 amp so wire stays cool and will not carry too many amps.
The little 18 gauge lamp cord should never be connected to 100 amp breaker. If there was a short circuit on the line, 100 amps would turn that little wire into a light bulb filament and the wire would explode in a flash. That's a firecracker. But anybody standing nearby would be sprayed with liquid metal that would seriously burn or kill. Likewise, a person touching the metal fence could be injured or killed.
Adding 10 amp line fuse to a small wire might reduce the problem IF the wire was rated for 600 volts (lamp cord is not rated 600 volts), and If the wire and the splice were inside plastic conduit instead of taped to the metal fence, and IF the main box was grounded.

So why was smoke coming out of wall sockets? Inside the house was another breaker box, and all those breakers were too large for the wires, and each breaker served too many loads and appliances.
The 100 amp main breaker allowed too much amperage into the household breaker box, and the house was wired incorrectly and consuming too many amps for the electric service.
Wires were catching fire behind the walls, and smoke was coming out where electric outlets were located.
Did the house burn down? I don't know.
I am hoping the internet, and my website will help folks stay safe around the world.
How to match wire and breaker size
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