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I am buying a very old house with fairly new looking breaker box in Ohio. When updating my bathroom,(installing a bathroom exhaust fan, new lights, ect.) I found that the wire that runs the bathroom is a 12-3/ground wire on a 30 amp 240v double pole breaker. The red and black wires are running two separate circuits sharing the neutral. I pulled the 240v breaker and put in two 120v, 20 amp breakers to see what all is on the circuit with the bathroom. (It's running the bathroom, an outdoor light and a couple plugs on the other side of the house.)
Ok, my questions are; Is it ok (safe) to just run a new 12-2 wire on a 20amp breaker for everything in the bathroom and leave the rest alone? Should I just change the 30 amp double pole to a 20 amp, or put in two 20 amp single pole breakers using the 12-3 with ground wire? (Sorry about the long question) Thanks!

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    You cannot use two separate breakers (without at least a listed handle tie) if it's still a MWBC (multiwire branch circuit). You do need to scale back to 20 amps with 12 GA wire - that was a clear violation your home inspector should have caught. If you change it from being a multiwire branch circuit (by running new wire to half of it) you could go to separate single breakers, but there's really no need - MWBCs are safe and code legal if done correctly. 30A on 12Ga wire is not correct, of course. If things on both legs are on, the neutral current is reduced, which is why one neutral is fine. – Ecnerwal Dec 26 '16 at 4:50
  • What make and model is this panel? – ThreePhaseEel Dec 26 '16 at 4:57
  • @Ecnerwal Sounds like an answer. – bib Dec 26 '16 at 14:58
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The last guy got the 2-pole breaker thing right, the amperage wrong, and the GFCI wrong. Here's what you need.

The breaker needs to be correct for the smallest wire in the circuit. If it's all 12 AWG (no 14 AWG extensions) then the breaker must be 20 amps or smaller. A 30A breaker is wrong and should be removed from service immediately.

Since it's a multi-wire branch circuit (MWBC), the circuit needs common shut-off, and with circuit breakers, that means either a 240V/2-pole breaker, or single breakers with handle-ties. A handle-tie is a bit of plastic that sits in between the handles so they jointly shut-off. It's sold by proper electrical supply houses; Big Box won't stock it.

Since it's a bathroom, at least part of the circuit needs to be GFCI protected. Single-pole GFCI breakers with handle ties won't work. The only options are:

  • A 2-pole, 120/240V GFCI breaker (expensive)
  • GFCI receptacles, making sure if you connect any additional loads on the "LOAD" side of the GFCI receptacle, that they use a dedicated hot-neutral pair separate from the MWBC shared neutral.

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