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How many amps of electricity does home use
30 amp breaker use 10 gauge / 

10-2 gauge/ 30 amp
10-3/ 30 amp
Southwire electric tools
Yellow 12 gauge 20 amp

Yellow/ #12 gauge wire for 20 amp
with ground
12-2 gauge/ 20 amp
12-3/ 20 amp

Rolls of stranded wire

NMB is house wiring
UF is underground
White 14 gauge 15 amp

White/ #14 gauge wire for 15 amp
with ground
14-2 gauge/ 15 amp
14-3/ 15 amp

NMB is house wiring
UF is underground
50-60 amp breaker use 6 gauge / 
6-2 wire
6-2 wire
Southwire electric tools
NMB is house wiring
UF is underground
40-50 amp breaker use 8 gauge / 
6-2 wire
8-2 wire
Southwire electric tools
NMB is house wiring
UF is underground

Copper ground wire
Use same size as other wire in circuit

Spools of ground wire
Main breaker has amp rating
Typical home might have 150-250 amp service. A service is the installation from power company transformer to meter box, and then to the household breaker box.
Residential service in US is typically 120-240 volt single phase that requires 2 wires to complete each circuit: 2 Hot wires for 240 volt, and Hot-Neutral for 120 volt.
Service amperage can be seen by looking at amp rating of main circuit breaker.
The main breaker rating is the maximum amperage available at the house.
How many amps does typical house use?
Most situations, the amperage consumed by a home is less than the main breaker rating. If not, the main breaker will start to warm and then trip. The main will also trip if breaker has gone bad.
No estimate of average amperage is possible for all users. Some homes consume massive amounts of power to run swimming pool, air conditioner etc. While other homes run a few light bulbs, or consume nothing. Some homes have all electric appliances, while others use gas heater, dryer, and stove.
Converting to LED bulbs has changed the consumption of amperage, however light bulbs are normally not the biggest usage at a home. Air conditioners, motors, and heaters consume more power in a few minutes than a light bulb running all night.
The summer season is the time of highest demand because air conditioning requires electricity, yet many people have no air conditioning.
How to figure power usage for your home.
Usage varies per day, but most accurate measure is the electric meter.
The power company installs a meter that records Kwh or kilo watt hours ... which is 1000 watt hours.
Wattage is a measure of electric power.
Figure cost of running light bulbs
If you run 100 watt bulb for 10 hours => that's 100 watts of power x 10 hours = 1000 watt hour = 1 Kwh costing 10 cents. Run 4 bulbs every night for 1 year costs $12.
If you run 7 watt LED bulb for 10 hours > that's 7 x 10 hours = 70 watt hours = .07 Kwh costing .007 cents. Run 4 bulbs every night for 1 year costs 81 cents.
Of course all costs are energy, so up-front price of bulbs is consideration.
Figure the amp consumption for entire house?
Let's say, your house uses 1350 Kwh of electricity per month. Volts x amps = watts. Divide 1,350,000 watt hours by 240 volts = 5625 amp hours
Except house also uses 120 volts. Divided 1,350,000 watt hours by 120 votls = 10,833 amp hours.
The two numbers are not the same, so this method is not accurate, and 'amp hours' is not a useful way to measure power because it's a factor. Just like volts is a factor. Both volts and amps are needed to calculate total watt (power) consumption. Volts x Amps = Watts (power)

For best accuracy at amp usage for entire home, add up all usage at your home using the rating plate for each item.
For eacmple, 100 watt incandescent bulb runs on 120 volt. This information is printed on end of each bulb.
100 watts divided by 120 volt = .83 amps
Electric water heater has a label that shows 240 volt 4500 watts.
4500 watt divided by 240 volts = 18.75 amps.
The two calculations show that electric water heaters, and 240 volt appliances in general, consume much more electricity than light bulb, because they do more work.
Examples: Basic formula: Volts x Amps = Watts  
Volts x amps = watts ... this is same formula as E (volts) x I (amps) = P (power or watts)
Each electrical device has a label that gives at least two of the three factors needed for the formula.

Example: Microwave oven label shows 120 volts 1500 watts
Apply basic formula to 120 volt and 1500 watts: 1500 watts divided by 120 volts = 12.5 amps.
This means a microwave oven will consume 12.5 amps when it is running.
If microwave is NOT running, it consumes a small fraction of electricity to operate the digital display.

Example: 24Look at main breaker raiting0 volt 4500 watt water heater.
Using basic formula: 4500 watts divided by 240 volt = 18.75 amps.

How much wattage can 20 amp breaker supply?
If you have 20 amp 240 volt breaker: 20 amp x 240 volt = 4800 watts
If you have 20 amp 120 volt breaker: 20 amp x 120 volt = 2400 watt
Caution must be used for safety. To calculate the safe maximum for a breaker, you must multiply result by 80%.

Exceeding 80% on a circuit breaker is not necessarily dangerous, but it can lead to problems over time because an over clocked breaker will get warm. Whenever a wire, switch, outlet, extension cord or breaker is getting warm, it is causing energy loss, and eventually connection points can begin to deteriorate and loosen. This causes heat and sparking, leading to potential fire. For example a circuit breaker that is getting warm or hot will eventually loosen on the busbar. The resulting fire is contained inside the breaker box enclosure, but the breaker and busbar are damaged. A damaged busbar mean a slot inside the breaker panel is lost to further use.

Calculate safe maximum for circuit breaker:
If you have 20 amp 240 volt breaker: 20 amp x 240 volts= 4800 watts x .80 = 3800 watt.
       3800 watt is too small for 240 volt 4500 watt electric water heater, so you would use a 30 amp breaker.
If you have 20 amp 120 volt breaker: 20 amp x 120 volt = 2400 watt x .80 = 1920 watts
       1920 watts is fine for 120 volt 1500 watt microwave

What if you have 120 volt 20 amp breaker that supplies electricity to 1500 watt microwave and 12 amp refrigerator??
Often, appliances with motors show amperage. Inductive loads, like motors draw high amps during start-up, and then run with lower amps. Household circuit breakers are slow acting, meaning the amp draw on a breaker can exceed rating for a short time before breaker trips. The start-up for a household motor is very brief and rarely does a breaker trip, but sometimes lights may dim momentarily when large loads like HVAC turn on.
Use the basic formula to figure watts for 12 amp refrigerator. 120 volts x 12 amps =  1440 watts.
Add watts from both refrigerator and microwave: 1440 + 1500 watts = 2940 watts.
This exceeds the capacity of 20 amp breaker.
Solutions: Code today requires a dedicated circuit breaker for microwave. Dedicated circuit means that only the microwave is on that one circuit.
If you have too many watts on a circuit breaker, then add another circuit.
Short term strategy:  Unplug refrigerator when microwave is being used.

What size circuit breaker for 1 horsepower motor
Usually the wire and breaker size for motors is determined by using a chart:
See chart:
HP, breaker and wire chart

Electrical formulas

Basic household wiring
Figure volts amps watts for water heater
Not enough space in breaker box
Match wire and breaker size
Main breaker
Larger image
Ordinary Main panel for home has 120 volt and 240 volt circuits.
This is called single-phase electric power. Phasing is determined by the power company.

- Each 120 volt circuit can include up to 12 outlets, and supply power to lights inside a room.
- Generally each 120 volt circuit sends power to one area of home. This is because the wire leaves breaker box and goes to first junction box inside a room. From the junction box, the wire splits off and goes to each individual box, jumping from box to box until the final box is reached. So each box will have 1 black hot wire, 1 white neutral wire, and 1 ground wire that connect back to main breaker box.
- Some circuit breakers are dedicated to 1 specific appliance. For example the kitchen microwave is usually wired to 1 circuit breaker and no other outlets or lights are connected to that breaker. This is because microwave draws so much amperage. That is called dedicated circuit, and dedicated breaker.

- Each 240 volt circuit is usually dedicated to one appliance. For example electric water heater is connected to 1 240 volt breaker. The electric dryer is connected to another 240 volt breaker. The heat/AC is connected to another 240 volt circuit breaker, and so on.

See inside main breaker box
How to replace circuit breaker
How to install subpanel
Figure volts amps watts for electric water heater
wire size chart
Larger image

Match wire size and breaker amp rating
If your home was wired by professional electrician who pulled permits, then circuit breaker and wire size will match. If manufactured home or travel trailer has not been altered by homeowner then breaker and wire size will match.

Circuit breaker and wire size must match
Chart on right shows that 30 amp breaker matches 10 gauge copper wire.
Do not use aluminum wire. Copper wire is safe. Aluminum wire expands when electricity travels through wire. Expansion causes loose connections and fire hazard.

If breaker is oversized for wire, then breaker can let too much electricity travel through the wire. Wire can overheat and cause fire.

If you suspect wiring is not correct, call electrician. Absolute electrocution danger when removing cover off main panel. More information below.
Main breaker terminals are still Hot when main breaker is turned off.
Maximum wattage for household NM-B and DU-F wires
Wire diameter, weight, resistance
circuit breakers are warm Check if breakers are warm
Put hand on each circuit breaker. Including main breaker. No shock hazard touching circuit breakers when box cover is in place.
Always stand on totally dry surface when touching anything electrical. For example stand on dry boards.
Breakers should not be warm.
Warm circuit breaker indicates problem
Make sure cover is put over main breaker box and that box door is kept closed to protect against spark and fire

What does warm circuit breaker mean? It means breaker is Overheating
1) Breaker is going bad
2) Breaker is not big enough to handle Load, so amp draw is larger than breaker.
3) Breaker has been getting warm over time and is loose on the busbar.
4) There are loose wires causing arcs and heating
Vacuum cleaner cord gets hot Vacuum cleaner cord gets hot
Put hand on extension cords, and household wires. Feel for warmth.
If cord or wire is warm, then that indicates appliance or tool is exceeding safe-maximum. It shows the cord is too small for amp-rating of tool. Or the wire is undersized for load.
Use larger size extension cord. For example, upgrade from 14 gauge to 12 gauge or upgrade more to 10 gauge.
Cords frequently get warm, and that is not good. Warm indicates high load. Hot is dangerous. Cords and wires should never get HOT.

If cord is hot, unplug immediately.
Vacuum cleaner cords frequently get hot. This is because manufacturers are under-sizing the cord for the amp-draw of the appliance motor.
Over time, the warm cord means motor is starving for electricity. Motor runs under-speed. This causes motor to get warm. Motor begins to weaken.
Motor finally gets hot and begins to short. Cord get hot. Vacuum cleaner needs to be replaced.
Foreign manufacturer makes more money selling cheap products, while corporate owners complain about government codes and regulation. Then your house burns down and insurance company says you need to sue appliance manufacturer. Win-win for all except you.
Space heater cords frequently get warm
This means the cord is too small for amp-draw of space heater.
Space heaters should never be operated for long hours, and should always be unplugged when not in use
Space heaters come with 14 gauge wire, but safe-maximum rule says 14 gauge wire is rated for 1440 watts.
14 gauge wire is OK for space heater, but load is slightly above safe-maximum so wire can feel warm.
Always unplug appliances that are not being used
Outlet is warm Outlet is warm/ Switch is warm
Use hand to check if outlets and switches are warm.
If switches and/or outlets are warm, then call electrician.
Turn off power. Remove cover and inspect for loose wires. Replace switch or outlet. Monitor if new switch/outlet gets warm.

Aluminum wiring is absolute fire hazard
If wires are aluminum, then this is absolute fire hazard. Professional electrician must be called to solve problem.
Aluminum wire must be replaced with copper. Special outlets-switches required for aluminum wire. Special connections required for aluminum wire.

All electrical boxes, including outlets and switches must have cover plate to keep sparks and overheating from causing fire.
  Intermatic EI500   Westek TE06WHB   Intermatic EJ351 Wall timer is getting warm.

Replace timer. See bottom of this for links to timers and manuals.
Use product manual to check load rating for timer. Check watt and amp draw of Load.
For example, if timer is rated for 500 watts incandescent light bulbs, but timer is connected to 600 watt TV or 1Hp pool pump, then timer will get warm and burn out.
Dimmer getting hot
Typical 600 watt rotary dimmer
Cover plate must be installed for safety
Dimmer is getting hot

Change light bulbs to lower wattage. Replace dimmer with new dimmer.
Turn off power. Pull dimmer out and look at watt rating printed on side of dimmer.
Add up total watts of all light bubs controlled by dimmer. Compare dimmer rating with actual watt rating.
For example if dimmer is rated for 600 watts, then four 150 watt light bulbs will cause dimmer to get warm.
Change to four 100 watt light bulbs, or buy dimmer rated for 1000 watts.

Some dimmers have aluminum fins that work as heat sinks.
Aluminum dissipates heat very fast. The aluminum fins can be warm, and that is normal.
Aluminum fins should not be hot.
Main breaker stress test
Using 80% rule: Safe maximum for 125Amp main breaker is 100 Amps
Safe maximum for 60 Amp main breaker is 48 amps
Main breaker is getting warm
Main breaker is tripping
Calculate PEAK amp draw.
Add up total amp usage using example below.
Formula: Volts x amps = watts. or Watts divided by volts = amps.
If appliances are gas, then they do not count.
Look at labels on each item for watt rating.

Add up peak usage for 120Volt. Each electric appliance has rating label.  For example: microwave 1500 watts + big screen TV 500 watts + computer 300 watts + refrigerator 700 watts + lights 240 watts + dishwasher 800 watts + 1 Hp hot tub pump 750 watts + washing machine 350 watts etc = 5320 watts divided by 120 volts = 44 amps.
Add up peak usage of 240Volt. For example water heater 4500 watts + heat/ac 5000 watts + clothes dryer 1800 watts + 1 Hp water well pump 750 watts + top and bottom ovens 7500 watts etc = 19550 divided by 240 volts = 81 amps.

Total PEAK amp draw = 81 + 44 = 125 amps.
125 amp Main breaker
Main breaker
PEAK amp draw is STRESS test for main breaker

Example calculation above shows peak 125 amps.
If main breaker is 125 amps, then main breaker is too small to handle 125 amp PEAK load.
Each time PEAK amps are used, the main breaker gets warm.
Sooner or later, the main breaker has gotten warm too many times, and it stops functioning. This may take years before main stops functioning.

Electricians use 80% safe maximum rule when calculating capacity of electrical devices such as circuit breaker.
80% rule also applies to switches, outlets, dimmers, timers, etc.
If main breaker is rated 125 amps, then electricians multiply 125 x 80% to find maximum safe load for that breaker.

125 amps x .8 safe maximum = 100 amps
So safe maximum for 125Amp main breaker is 100 Amps.
water heater circuit breaker Calculate safe maximum for water heater circuit
If residential water heater is connected to 20 amp breaker and 12 gauge wire, then the same overheating problem occurs over time.
Circuit breaker will not last as long, and damage can occur to busbar.
See basic water heater circuit
20 amp breaker is subject to 80% safe-maximum rule.
20 amp circuit breaker x .8 = 16 amps. So safe maximum for 20 amp breaker is 16 amps.
As shown above, 4500 watt water heater draws 18.75 amps. And 5500 watt water heater draws 22.91 amps.
So 20 amp breaker with 12 gauge wire are rated too small for most water heaters. Over time the breaker can wear out sooner due to overheating.

Recommended that all water heaters be connected to 30 amp breaker and 10 gauge wire.
Safe maximum for 30 amp breaker x .8 = 24 amps.
30 amp breaker and 10 gauge wire run cooler, with less heat loss and more efficiency. Circuit breaker and main box last longer with less chance for heat damage.
Figure Volts Amps Watts for water heater
Volts x Amps = Watts
Watts divided by Volts = Amps
Most residential water heaters are non-simultaneous. This means both elements are not ON at same time.
How electric water heater works
How to wire thermostats

Calculate safe maximum for timers/ electrical circuits
Read label on side of water heater for element wattage. Element wattage is tank wattage.
For example: If water heater has 5500 watt elements: 5500 watts divided by 240Volts = 22.91 amps
4500 watt water heater divided by 240Volts = 18.75 amps.
Electricians use 80% rule for safe maximum. WH21 is rated for 25 amps x .8 = 20 amps.
Calculation shows that WH21 is ok for 4500 watt water heater, but beyond safe-maximum for 5500 watt water heater.
Exceeding safe maximum means WH21 will run hot and not last as long.

If electrical device is run at-or-near maximum amp rating, the device will not last as long because of heat.
Same as running a car at full throttle. Car will not last as long.
In contrast, 40-amp WH40 and T104 timers are better long term investment for water heater timer.
WH40 and T104 are rated for 40 amps, with safe maximum of 32 amps, so they withstand more amperage without heat loss, and will last longer.
How to wire WH40
How to wire T104
Intermatic T174 timer is identical to T104, except T174 has day-skipper feature
Main breaker is tripping
Circuit breaker is tripping
Circuit breaker is warm
Figure volts amps watts for home
Breaker must match amp draw of appliance, and wire size.
circuit analyzer
surge protection
- Some gas water heaters plug into 120Volt outlet that should also be on a dedicated breaker. This is to prevent electrical interference from affecting gas control valve. This type of installation should also include surge protector to prevent surge from knocking out gas control valve electronics.

Example Peak usage is 125 amps.
80% rule
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