Electrical – use a two-pole single handle breaker for mwbc

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.

The Eaton type CH is typical among modern breakers in that multipole common tripping is performed by an internal mechanism, not by the handle tie. This is noted in the Eaton catalog in the spec description of the type CH breaker (page V1-T1-31 of the Eaton catalog, emphasis mine):

Product Description

Quick-make, quick-break switch mechanism combined with inverse time element tripping operation and trip-free handle design. Type CH circuit breakers trip to the OFF position, eliminating nuisance callbacks. The CHF family also includes a trip flag to differentiate between a trip and the breaker being turned off. The thermal-magnetic trip curve avoids nuisance tripping on mild overloads while reacting almost instantaneously to severe short-circuit conditions. Multipole breakers have internal common trip connection to operate all poles simultaneously. Handles are marked with ON-OFF indication and ampere rating of the breaker.

and in footnote 2 on the accessories table (page V1-T1-38):

2. Handle ties: typically used to join two similar independent single-pole breakers to form a two-pole noncommon trip breaker.

This means that the combination of 2 single pole type CH breakers and a handle tie is only usable for 240V only or 120V only circuits as per NEC 240.15(B)(1) and 240.15(B)(2):

(B) Circuit Breaker as Overcurrent Device. Circuit breakers shall open all ungrounded conductors of the circuit both manually and automatically unless otherwise permitted in 240.15(B)(1), (B)(2), (B)(3), and (B)(4).

(1) Multiwire Branch Circuits. Individual single-pole circuit breakers, with identified handle ties, shall be permitted as the protection for each ungrounded conductor of multi-wire branch circuits that serve only single-phase line-to-neutral loads.

(2) Grounded Single-Phase Alternating-Current Circuits. In grounded systems, individual single-pole circuit breakers rated 120/240 volts ac, with identified handle ties, shall be permitted as the protection for each ungrounded conductor for line-to-line connected loads for single-phase circuits.

As to breaker ratings, the CH line has you covered as well. All standard (i.e. no GF or AF functions) single pole CH, CHF, CHT, CH-HID, CHP, CH-M50, CH-HM and CHP-HM breakers are rated 120/240VAC, so they are listed for handle tied service on 240VAC only split phase circuits.

Furthermore, the UL Circuit Breaker Marking Guide states in paragraph 40 that

An external handle tie alone does not qualify as a common trip mechanism — a breaker of this type is marked to indicate it is an independent trip breaker.