Shared neutral
Two hot wires share one white wire back to the panel. You load each side, then let the white come loose and watch the voltage on each side swing away from 120 V.
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What to try
- 1On Wired right, drag a load from one side to the other. The white wire carries only the difference.
- 2Set White wire to Loose. The breakers go off and are tested dead first. Turn Black and Red back on and watch one side's voltage climb and the other's drop.
- 3Pick Both hots on one leg and watch the white wire carry both currents.
NEC 2026 sections it covers
- 210.4(A)
- A multiwire circuit shares one neutral between two hots.
- 210.4(B)
- Both hots shut off together.
- 240.15(B)(1)
- Single-pole breakers need a handle tie.
- 300.15(B)
- Pulling a receptacle must not open the white wire.
- 200.6(A)
- Two circuits on the same leg can't share one white wire.
- 250.148
- Grounds are spliced to each green screw and the box.
- 334.80(A)
- 12 AWG copper in NM cable is good for 20 A.
- Table 310.16
- The table of allowed amps for each wire size.
- 240.4(D)
- 12 AWG copper gets a 20 A breaker at most.
- 210.8(A)
- Kitchen counter receptacles need GFCI protection.
- 210.12(B)
- They also need AFCI protection.
Section numbers only. Read the full text in your code book. Temperatures and times in the visual are teaching models.
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More visuals
- Loose connectionLoosen one screw and watch heat build at that spot while the breaker reads normal.
- Crowded conduitAdd circuits to one pipe and watch the wires heat each other until each is allowed less current.
- Neutral and ground in a subpanelIn a subpanel, the neutral and ground stay apart. Leave the bonding screw in and normal current rides on the ground wire and the metal can.