Shared neutral on the same leg

Question

I have a two bedroom cape with 14-3 running to the second floor where it splits. I noticed at the panel some time much later that the two 15 amp breakers that power up the individual rooms, are separated by a 20 amp breaker that feeds a designated circuit. The 14-3 neutral is carrying the return load of both 15 amp circuits from the same leg.

The electrician was qualified and licensed. Shouldn't he have moved the two 15 amp circuit breakers together so that they would be pulling from separate legs? I am trying to better understand this.

Answer 1

Absolutely right, this is super bad for exactly the reason you say! Turn off both circuits and get yourself a 2-pole breaker of that same type. Put the two hots on that breaker and install it, moving other breakers as needed.

The use of a 2-pole breaker will guarantee they are correctly on opposite poles, and is now required by Code. (well, handle ties are, but it works out the same. 30 places near hear stock 2-pole breakers, only 1 stocks handle ties.)

Be careful not to split up another multi-wire branch circuit. In fact, watch for more - where there's one, there's five.

Also, make sure you're not dealing with double-stuff or the GE Q-line "half width breakers". In a Q-line, an intermediate breaker would be correct, but a 2-pole breaker would be a great deal more correct.

Answer 2

Yes. The two breakers for a multiwire branch circuit would normally be right next to each other even if it was wired 50 years ago. Verify that your panel bus is staggered and the breakers next to each other come from different legs of the service.

Also, recent National Electrical Code changes require that the two breakers be tied with an approved handle tie or you need to install a two pole breaker.

The easiest and cheapest thing to do is swap one of the 15 amp breakers with the 20 that is in between. Then attach an approved handle tie to the two 15 amp breakers.

Good luck and stay safe!

Answer 3

Make sure your breakers are on opposite phase. Do that by putting a volt meter between the two breakers. If you read 220v you are good. If it reads 0 you are on the same phase and the neutral can carry twice the intended current.

I have a house wired with the ge q-line half width breakers. Those go a-a-b-b-a-a-b-b etc. so it’s easy to get two adjacent breakers on the same phase. I found one split circuit in the box wired wrong.

Answer 4

You do want the two breakers for a multiwire branch circuit to be right next to each other so the handles can be tied together, but in terms of overload protection and the effect on the neutral, it should not be a problem.

The slots on a breaker panel go +-+-, so a two pole breaker will draw from +- and the maximum load on the neutral will never be double the breaker rating on a MWBC, but only the breaker rating.

Suppose you have a 15A single pole (breaker 1), 20A double pole (breaker 2), and a 15A single pole (breaker 3) as you describe. We'll number the slots as +1, -1, +2, -2. Then the breakers would connect as follows: +1 (breaker 1), -1, +2 (breaker 2), -2 (breaker 3).

The ideal would be +1 (breaker 1), -1 (breaker 3), +2, -2 (breaker 2).

But in both cases, breaker 1 and 3 are on + and - which is safe for the neutral. The situation you want to avoid is ++ or --. Putting a double pole breaker between two single poles will not cause this.