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I have a box with 3 switches in it and was replacing 1 switch for a smart switch. I switched off the breaker and verified none of the switches worked. The initial wiring had all the neutral lines in the box all bundled together with a wire nut. Nothing out of the ordinary as far as I know. As I was adding in the neutral wire for the smart switch I found out the fun way that at least one of the neutrals still had juice running in it.

After a bit of trial and error, I found that if I switched off a second breaker, then there was no juice in that box.

Is this okay, or should I get an electrician out here to figure out what is going on? As far as I know, it has been this way since I bought the house a few years back.

  • Hello, and welcome to Home Improvement. Great question: keep 'em coming! – Daniel Griscom Jan 27 at 11:56
  • Sounds like a multi wire branch circuit, many years ago they did not require handle ties, that requirement was added to code so this shocking experience won't happen. – Ed Beal Jan 27 at 18:33
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It depends

Now that you have identified both breakers, remove the panel cover (if you are comfortable doing that) and follow the hot wires from the two breakers. See if they converge onto the same cable. Typically one will be red and the other black.

If they do, then you have an intended thing called a Multi-wire Branch Circuit or MWBC. You are almost OK, but there's one more thing to check.

Current Code requires that these two breakers have common maintenance shutoff so if you shut off one, the other must shut off. I hardly need to explain why, eh?

There's one more rule that has always been absolutely essential. The two breakers must be on opposite poles of the panel, so that there is 240V between them. If they weren't, the neutral wire would overload!

Wouldn't it be great if there was one breaker type that guaranteed both this and common maintenance shutoff?

There is. It is called a double-pole or 240V breaker. It resembles a dual breaker, except its handles are factory tied, and it fits in a double breaker space. Use the amp rating of the existing breaker, but no more than 15A for #14 wire or #12 aluminum, 20A for #12 copper or #10 aluminum.

Where there's one, there,s two. Search the panel for every such case of a 3-wire cable going to 2 separate breakers, and do the same thing with them too.

Otherwise it's a big mess

Other than the situation above, neutrals must only interact with their own circuit. Unfortunately, some electrical workers believe "If it works, it's safe".

One helpful rule here is that "currents must be equal in every cable or conduit", meaning any power that comes out on a cable's "hot" must go back on that same cable's neutral or some other wire in that cable. And that means wiring must be done in a physical "tree" layout -- branches can come out of the panel, but they can't cross-connect to each other because that would be a "loop" and real trees don't do that.

By the way, ground wires are allowed to loop / cross-connect, and should. That is because ground does not flow any power except during an emergency. This confuses people who already think ground and neutral are the same thing. They're not. Not even close.

The #1 place this kind of cross-connection shows up is in switch boxes with more than one switch. However, this one isn't any smart switch! You would not notice the tiny trickle from a smart switch, still, Do not allow them to poach neutral from other circuits. This can get tricky where 3-way switch circuits are smarted.

  • I wonder if it would be practical to recognize a category of smart switch that uses its neutral wire primary as a voltage reference and, except under fault conditions which should force an emergency shutdown of the power feed, would put so little current through the neutral wire that a grounding conductor could be used instead without tripping a GFCI or creating a shock hazard? Such a thing would be safer than other neutral-wire bodges people might try to use when retrofitting smart switches. – supercat Jan 28 at 20:23
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Most Likely Not OK

You probably have neutrals mixed up. The one exception is an MWBC (multi-wire branch circuit). I doubt that is the case based on your description), but that is a possibility. Assuming that is not the case:

In addition to the (obvious) hazard when working on wiring, there is another problem. In normal US residential service, neutrals are not protected from an overcurrent condition. Normally this is not a problem as the current on neutral = the current on hot, and hot is protected. However, with two circuits sharing a neutral, there is no guarantee that the neutral currents will equal the hot currents, and this can be very dangerous.

For example, if you have two 20A circuits and each one has a 16A load and the neutral fails on one of them then you will now have 32A on one neutral wire, which is more than the standard 20A circuit wiring is designed to handle. The hots will still have 16A each, so the breakers won't trip. But the neutral wire with 32A will burn up.

In order to fix this, you need to find out where that neutral wire comes from and what hot wires it is paired with. Even though all the switches were off, you may have a hot for another circuit passing through this box that is on the other breaker. If that's the case, it just needs to have its neutral separated from the others. But you may find nothing from the other breaker (except this neutral) in which case you have to look elsewhere, starting with any junction boxes for anything (e.g., lights, receptacles) that is powered by the other breaker and see if you can find something that is connecting back to this box that shouldn't be there.

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