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If either of these breakers are on, both of these wires have power. I believe that makes this a multi-wire branch circuit and that, configured thusly, this is a multifeed and that that is dangerous. Is that right?

Background:

I bought this house three years ago and am trying to untangle some electrical problems that came with it. Learning as I go. I did not wire this panel -- I bought it this way.

Explanation:

Using my voltage detector pen, I can determine that if either of these breakers is on, both of these wires have power. Also, I traced the wires up to where they exit the box in a cable, and although it's hard to see so I can't be 100% sure, I'm 99% sure that they are part of the same 3-wire cable. Using the pen again, if either of these breakers is on, the cable registers voltage.

My research suggests that this is a multi-wire branch circuit and would be OK except that the two breakers should be on different buses, which these are not. I'm just hoping to confirm that understanding so I can see about fixing it.

Thank you for any insights!

electrical panel

Alaska Man
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J. Matthew Turmer
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2 Answers2

18

The evidence would suggest that it is an improperly wired multi-wire branch circuit. The two legs of the circuit are erroneously and dangerously shorted together somewhere. When a MWBC is properly wired, the only path from one leg to the other is through the loads.

The reason the two breakers are on the same pole is some delirious spark monkey discovered that if installed properly they trip instantly when main power is restored. He was unable to discover the real problem but by moving stuff around at random he effected a miracle cure.

The quickest test for a real short circuit, as opposed to a phantom voltage transmitted through the loads, is to properly position the breakers on opposite poles and see if they trip immediately.

A. I. Breveleri
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11

Just to be clear it sounds like turning on either of these breakers energizes the receptacles, if this is correct then you should leave one off until you correct the actual problem. Actually you should leave one breaker off until resolved even if they aren't joined, having both of those on the same leg you could get 40 amps on the "neutral" without tripping and it could melt the neutral insulation.

You are right, the two wires of a MWBC do need to be on separate legs, when properly wired the neutral would only carry the difference between the two legs, but as wired it will carry the sum of the two.

The requirement for the breakers being handle tied only dates back to he 2008 version of the NEC. Before that usually electricians tried to put the circuits on adjacent breakers, but it wasn't required and rarely did we use handle ties.

If either of those breakers energizes all the same stuff then my first step would be to turn off both breakers and take apart every involved outlet box to locate where the two legs are joined, and separate them. Then I would rearrange the circuits so they would be adjacent and install handle ties.

NoSparksPlease
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