electrical
Has it ever been acceptable for a multi wire branch circuit to be on 2 separate breakers? My house, which was built in the mid 90s, has the dishwasher and the disposal on the same 12/3 cable, but not on a common trip breaker.
I'm going to replace the breaker, but I'm just curious if this was ever allowed.
I'm guessing the circuits are sharing a neutral, and so are considered a multiwire branch circuit.
Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.
In the 2008 version of the National Electrical Code (NEC), they introduced sub section (b) to section 210.4. This sub section covers the disconnecting means of a multiwire branch circuit, and requires a simultaneous disconnect of all ungrounded conductors.
NEC 2008
210.4 Multiwire Branch Circuits.
(B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.
This subsection can be satisfied by using a multipole breaker, which provides both simultaneous trip and simultaneous disconnect. The subsection can also be satisfied by simply attaching handle ties between multiple single pole breakers, which would provide simultaneous disconnect (and may or may not provide simultaneous trip).
Use a multimeter to measure voltage between the two hots (black to red). You should get somewhere around 240 volts. If you get 0 volts, the breakers are on the same leg.
An empty service panel looks like this
I've drawn rectangles where breakers would go, and labeled the legs A and B. As you can see, breakers across from each other (left/right) are on the same leg, while breakers next to each other (up/down) are on different legs.
If you measured between breaker 1 and 2, you'd get 0 Volts. This is because they're on the same leg, so they're at the same potential. If you measured between 1 and 3, you'd get ~240 Volts. These breakers are on separate legs, so they're not at the same potential.
Best Answer
Yes it is allowed, up through the 2020 NEC section 240.15(B)(1) single pole 120/240v breakers with handle ties are still allowed when the MWBC only feeds line to neutral loads.
I don't have my old books, but it wasn't until the 2000's that the common trip requirements entered the code, but even then your local authority may have not have immediately adopted the code or that section immediately upon publication. For instance 3 states are still using the 2008, four cycles behind.
Free access to the NEC at https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards/detail?code=70