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Change 120 volt into 240 volt
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.
Buy:
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
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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
Push on wire connectors

Push-on wire connectors in place of twist-on connectors

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Push on wire connectors Amazon
Red 3P connector for 10-14 gauge wire
Lever connectors for stranded wire at Amazon
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.
Buy tools:
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

Basic plan if you have access to breaker box and your 120V circuit supplies power to 1 receptacle
Convert 120 volt circuit to 240 volt ...
This conversion works if the 120 volt circuit supplies power to only 1 receptacle. This would be 120 volt dedicated circuit typically found with microwave oven, small appliance circuits, power vented or flue dampered water heaters.
If the 120 volt circuit has numerous boxes with outlets, fan, refrigerator etc, then this conversion is not possible.
If you change 120 volt circuit into 240 volt, and circuit supplies power to outlets and appliances that are rated for 120 volt, then 120 volt appliances will burn out as soon as they are turned on.

OK, so safety first. Big wires on main breaker are still Hot when main breaker is OFF.
Caution: When working inside main breaker box ... even with main breaker OFF, the wires on the main breaker are HOT.

Basic plan if you have access to breaker box:
120V circuit has 1 Black Hot wire plus 1 White Neutral wire that go to the 120V receptacle.

Install a double breaker. Connect the Black wire to either terminal on the double breaker. Move the White wire off the Neutral busbar and connect to other terminal on double breaker. 

After that the double 240V breaker will supply 2 Hot wires
going to receptacle.
This will convert the receptacle to 20 amp, 240 Volt.

For receptacles or outlets, by code, you must also change the receptacle to standard 240V outlet, and if you're on a 20 amp breaker, it is code to use either 15 or 20 amp receptacle.
Cautions and general instructions for changing 120 volt to 240 volt.

When main breaker is OFF, then all small breakers are OFF and breakers are safe to work on, but wires on main breaker are Hot at all times.
Only a meter pull by power company will shut off power to main breaker.
 Use only 600 volt solid copper for #12 and #10 and #8 wires ... if stranded wire is used for 12, 14, then connect stranded to short piece of solid copper and cover with twist on wire connector ... and put solid copper wire under screw plate on breaker and Neutral busbar. Always tighten screws and twist on connectors very tight.

Stranded wire for small wires like 14-12 increase chance of wire splaying apart under heat caused by amperage ... the wire starts to get loose ... and begins to arc ... eventually overheating and melting connection point ... causing breaker to trip ... or burning out timer or outlet etc.

Never use extension cord for permanent household wiring.
straight blade plugs
Dedicated outlet for power vented heater.
A dedicated 120 volt circuit can be converted to 240 volt circuit. Use diagrams shown above and replace single breaker with double breaker, and do the rewire. If needed, the converted dedicated outlet can be converted to 240 volt and still provide 120 volt using an inustrial transformer.

cannot use two hots from different circuit unless from opposite breakers connected double breaker
If garage has 120 volt circuits from two different breakers, it is possible to create a 240 volt circuit and keep the 120 volt circuits intact.
Move breakers for each of the circuits next to each other and install double breaker. Then connect each hot wire to breaker just as before. In the garage you will still have two circuits of 120 volt just as before. Take one hot wire from each of the 120 volt circuits. These two hot wires will test 240 volt potential across both wires. Why? Because the two hot wires come from a double breaker that is pulling hot wires from two separate busbars in the breaker box. This will meet code. Why? Because both wires come from a double breaker. A double breaker is required so if one side of the breaker trips, the other will trip also.
You cannot get 30 amp 240 volt circuit from this arrangement ... unless each of the 120 volt circuits have 10 gauge wire. If each circuit has 12 gauge wire, then 20 amp 240 volt circuit is possible ... but not 30 amp. Why? The maximum amperage of 12 gauge wire is 20 amp. Do NOT install double 30 amp breaker and connect 12 gauge wire.30 amp breaker requires 10 gauge wire.



Fig-2 shows 120V
Move black wire
from 240V breaker to 120Volt circuit breaker rated for correct amps.
30 amp breakers and 10 gauge wire run cooler with less heat loss. Circuit is more efficient, and breaker box lasts longer ... so you can use a 20 amp single breaker, or a 30 amp single breaker because the orange wire is 10 gauge. If the wire is yellow, then wire is for 20 amp breaker only... do use 30 amp breaker and yellow 20 amp wire..

Move white wire from 240V breaker to Neutral busbar where ground wire connects. If White is not long enough, then buy short length of 10-2 or 12-12 sheathed cable, and take out length of white wire. Join white wires with twist on wire connector. Use only solid copper for #12 and #10 wires.

Modern breaker boxes have separate Neutral busbar where white wires are connected ... while illustration shows both Neutral and ground busbars on same side of box for purpose of illustration, the modern breaker box has Neutral and ground busbars on separate sides of box.
Always connect ground wire onto the ground busbar ... and white Neutral on the Neutral busbar. Water heater must be grounded for safety.
Resources:
Basic 240Volt water heater circuit

240 volt water heater circuit
Change wiring from 240 volt to 120 volt
Fig-1 shows typical 240V water heater circuit.

Fig-2 shows 240V:
Two Hot wires connect to circuit breaker
Ground wire connects to Ground busbar


How to replace circuit breaker
How to install subpanel
120 Volt subpanelLarger image
Change 120 volt panel to 240 volt
Step 1)
Basic 120 Volt subpanel can supply power for 120 volt single pole breakers but not 240 volt double pole.

Step 2) Inside the main circuit breaker box, identify single pole breaker supplying power to 120V subpanel. Note the amperage of breaker and wire size. Example illustration shows 30 amp 120 volt single pole breaker.

Step 3) Important:
Identify if other 120Volt outlets and switches are on same circuit ... does the 30 amp single breaker inside main panel supply power to anything other than the subpanel?
Turn breaker OFF, and check which outlets and switches are on that breaker. It is against code if 20 amp switches, outlets etc are connected to 30 amp breaker. A 30 amp breaker can only be used for 30 amp outlets, switches, and other 30 amp loads like the subpanel, water heater etc.
This means that single-pole breakers inside subpanel must be 20 amp, if they serve 15-20 amp outlets, switches etc

Step 4) The change-over to 240Volt will NOT work if other 120 volt outlets and switches are on same breaker ... why? Because the circuit is going to be changed from 120 to 240 volt ... and those other 120 volt outlets and switches will be energized with wrong voltage.

Step 5) If the breaker serves more than the subpanel, then a dedicated breaker and wire must be installed for the subpanel.

120 volt and 240 volt
Non-electricians and quasi-electricians in Americas often say 110 and 220 volt.

In Americas, it is 240 volt, and not 220 volt. It is 120 volt, not 110 volt.
Check label on your 240 volt appliances such as AC unit, window air conditioner, oven, dryer, motor, pump etc. Check manuals from Intermatic, Square D, JEA etc. Standard voltages in the Americas are 120, 208, 240, 277, 415, 480, 347, 600 volt. Residential voltages can be 120, 208, 240 volt.
Electrical devices that are made for International voltages, like Siemens industrial transformers will also show 110-220 since these are standard voltages found in some countries
In Americas, when transformer is rated for 240 volt, then 110-220 volt is minimum voltage that can be supplied by power company before voltage is considered out of range. Reference
Low voltage will damage motors unless motor is rated 208-240 volt. Some electrical appliances have dual 208-240 volt rating, since some businesses and  residential apartments are supplied with 208 volt. Residential homes typically receive 120-240 volt.
This websuite uses 120 and 240 since most readers of this website are in Americas, and the single-phase breaker box illustrations are made specifically for Americas and not  for international electrical practices.
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Install and troubleshoot garbage disposal Waste King disposal
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surge protection
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box timers Resource:
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Control water heater w/ Z-wave
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