I'm in the U.S. I was working on a light that is part of a 4-way switch. The particular box I was working on was a 3-way switch with line coming in from the panel. In the box was the black line wire, two travelers, ground, and 4 white wires all bundled up. I turned off the circuit breaker for that particular light. As expected the light turned off. The black line wire was no longer hot but the 4 neutral wires were hot. I am new to this stuff but I did not expect that. What is going on here? Is this normal?
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2Is the branch circuit in question part of a multiwire branch circuit?– ThreePhaseEelCommented Jun 17, 2021 at 11:42
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1 Answer
It sounds like a multiwire branch circuit.
A multi wire branch circuit is when 2 breakers share a neutral.
current code requires the breakers to be handle tied or use a double pole breaker.
To find out if it is a MWBC trace the conductor back to the cable entering the box (service panel or breaker box) if a red black white and green, follow the other wire back to its breaker and turn it off to do the work (both off). These breakers should be on opposite legs or one a even number slot and the other an odd number slot and not a single tandem breaker that would overload the neutral.
Installing a handle tie or a double pole breaker eliminates this issue and the NEC has required ties for ~ 4 code cycles individual states 20 or more years.
If you have conduit there could be 2 hots for 1 white, and current code requires the wires to be bundled to make identification of the associated wires possible as you could have 4 hot and 2 white going to 4 breakers. There is no requirement on using different colored hots in this case but most electricians do use colors to keep things straight.
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