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How to trace electric cables behind walls
main breaker box
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120Volt and 240Volt breakers
Image shows 120Volt line and 240volt circuits.
Both circuits have ground wire.
Each 120 and 240 require two wires to complete circuit.
120V requires 1 hot and 1 neutral
240V requires 2 hots
Ground wires are for safety.
Read more about 240 volt

Some 240Volt lines have additional neutral wire. However, neutral not necessary to complete 240Volt circuit.
Some 240Volt lines have white and black instead of red and black

Most service boxes today have separate Neutral and Ground busbars, but these busbars are bonded together electrically, and are the same.
Illustration on left shows both Neutrals and Grounds on same side of box. In actual box, the Neutral is on right side and ground on left. Illustration shows separated neutral and ground so it is easier to illustrate wires leaving breakers.

Color code and wire sizes
Basic 120 volt and 240 volt water heater circuits
How to wire 240 volt outlets
How to wire 3-phase outlets and timers
How to convert 240Volt  water heater to 120volts

Never assume power is OFF
Use non-contact voltage tester on timer, on wire, on outlet to see if power is present.
Press button on tester, if single beep then no power is present. If continuous beep then power is present.
Never stand on bare ground, always stand on dry boards, do not hold or touch anything metal when working on timer or water heater that has power, tape tester leads to wood sticks, never touch wet water heater, or flooded water heater, or timer that had a short or fire without turning power OFF.

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With 120Volt circuit, 1 cable leaves main breaker box... this cable has 1 Black Hot and 1 White Neutral and 1 Ground.
The cable usually goes to a ceiling junction box first .... where other cables connect.
From there a cable travels to each successive box.. switch box, outlet box, ceiling box etc.
Maximum 16 boxes per breaker
Each box on circuit breaker is linked to the previous box by 1 cable that can be traced back to original cable from breaker box.
Ultimately each box has only 1 Hot wire arriving from the breaker. (excluding 3-and-4-way switches)
To find source of problem, the Hot wire must be identified.

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Following test steps apply to all boxes except those with 3-way and 4-way switches
1) Turn breaker OFF
2) Identify all boxes on breaker. Make a list or mark with tape.
3) Remove switch or outlet from box where you think problem is located.
4) Separate all Black and White wires in box. Leave ground wires connected.
5) Make sure no wires are touching each other.
6) Tape tester leads to wood sticks to keep hands away from live Hot wires. Stand on dry boards. Never stand on bare ground
7) Turn breaker ON
8) Test each Black wire to bare ground wires. Test each White wire to bare ground wires
9) The wire that lights up is the Hot wire... and this wire should be Black.
10) The cable carrying the Black Hot wire also has the White Neutral
11) Leave breaker ON
12) Identify which boxes on the list from step #1 are OFF and which are ON

14) Turn breaker OFF
15) You know the Hot Black and White Neutral cable from steps #9-10.... connect these wires to one of the other cables in switch box... Black to Black and White to White. Keep other cables separated.
16) Turn breaker ON.
17) Test which boxes or lights on the list from step #1 are ON
18) Compare this with the boxes that were OFF from step #12
19) This test will correctly identify the cable supplying electricity to those boxes
Gang switches
Gang switches.
This means 2 or more switches or outlets 'ganged' together in one box.

Hot wire will connect to each switch, starting at first switch and jumpering to each other switch or outlet

With 120V, each gang box has 1 Hot wire by code.
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