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ericmunyon12345 04-01-2013 04:05 PM

Shared neutral Question.
I have two circuits running to four switches to control some 120v plug-in fans in a shop. Can I (1) use 12-3 wire between the two, 2-gang boxes just to reduce the cluster in the boxes? (I know, why not do that from the panel to have a mwbc? I didn't run the original feeds; I'm adding the switches).
If I do use 12-3, effectively giving the circuits a shared neutral in the middle of the run, is a double-pole breaker required?
I'd like to think there is no problem with this as the neutrals are connected at the bus bar anyway, but I so appreciate the advice from the licensed pros who check in on this board. Thanks!

mpoulton 04-01-2013 04:58 PM

I can't tell exactly what you're describing, but it doesn't sound code compliant. It sounds like you have two separate circuits feeding two separate boxes, and you've made some kind of an interconnection between them, and that interconnection carries neutral current both ways depending on the circumstance. That's a problem on a couple levels. First, you probably have net current flow through that interconnect cable. In other words, not all current carrying conductors are in the same cable. This is a no-no. Second, if you are passing current between two independent circuits you are violating one of two rules depending on how you're doing it. Either you created a MWBC and need handle ties or a 2P breaker, or you have a situation where the power from one breaker may energize the load terminals of another breaker. Either way it's a problem. Exactly how do you have this connected? Can you post a diagram?

ericmunyon12345 04-01-2013 05:09 PM

Well, what's going on is I have each circuit in each switch box (one controlling one switch in each two gang box). The fans only pull about 2 amps each, but a bit over 6 for startup. With four fans on each circuit, I split it like this so that startup draw would be on different circuits when each pair of switches is flipped. (The fans were set up exactly the same before, we just used the pull chain on each fan to turn them on/off.) I'm not joining circuits as long as I keep my lines/loads straight. It would just eliminate one white and ground wire between the switch boxes if I do 3 wire. So at the switches, yes the two circuits would be sharing a neutral, but I'd think this should be fine though it may required a double pole (shared handle) breaker.

AllanJ 04-01-2013 05:27 PM

If you run a multiwire branch circuit, it must go all the way back to the panel.

That is, you may not combine two separate circuits (two 2 wire cables) up in the house to become a MWBC (one 3 wire cable) continuing further on (downstream) to additional switch boxes or outlet boxes.

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