How many breakers can feed a single box?

Question

I was replacing old light switches in a 2-gang box (~1950 home) and discovered it was fed from two different 15A breakers... and not only from two different breakers, but two different legs so 240V existed across the input between the switches!

There are two switches (ie, two yokes). I don't remember if the neutrals are tied together, but certainly only the hot is switched on each. Each switch is serviced by ancient 14/2 groundless cloth woven NM cable.

For this question please assume that the neutral for each switch is unbroken; it just passes through the box and continues to the light.

The breakers are on opposite ends of the panel, its just a coincidence that they ended up on different legs. So far I've found two different 2-gang (2-yoke) boxes with this issue and both boxes are on the same "pair" of breakers.

These are just lighting circuits (I think!) and all the lights are low-power LEDs, so I plan to fix it by retiring one of the breakers and splicing both at the panel run from the same breaker.... but it generated a few questions on the subject:

1. Does this meet current-day code?
2. How many breakers (or hots? legs?) can feed a single box?
   • Are there exceptions for the content of the boxes, such as:
     • Outlets and Switchs?
     • Blank-plate-covered boxes used for splicing (ie, conduit intersections)?
     • others?
3. Are there labeling requirements for things like this?
4. Other considerations?

Answer 1

Does this meet current-day code?

Yes, provided they have separate neutrals. Code requires careful separation of the neutral groups. Neutral is NOT common and you do NOT tie them all together as you do with grounds.

However it can also be legitimate for them to share a neutral, but that isn't Code as it is done, since the breakers must be handle-tied in that case.

Wait, Harper, what did you mean about them sharing a neutral?

That's possible. These critters are called *Multi-Wire Branch Circuits" or MWBC or shared neutral and there's nothing really wrong with them, except work awkwardly with AFCI and GFCI, and with those increasing in requirement, MWBCs have fallen out of style.

On a MWBC, The breakers must be handle-tied which usually makes them pretty obvious.

Also on MWBC, the neutrals must be pigtailed. Unlike plain circuits, they are not allowed to rely on a device for splicing, because the other side of the circuit needs to work if the device is removed. The observant installer will notice that this neutral serves the same devices as those hot wires.

MWBCs must be identified so you know which hot wires go with which neutral. In cable installations, this is usually obvious and no further marking is required.

How many hots can feed a single box?

No limit.

However, any breakers (MWBC or not) must be handle-tied if they serve the same yoke (device/receptacle/switch).

Are there labeling requirements for things like this?

Like 2 circuits in 1 box? No. You are expected to watch for that, and check for 1 circuit per yoke.

Expecting you to check for 2 circuits per yoke is unreasonable, so handle-ties are required in that case.

And MWBCs need their own markings in the form of groupings and handle ties.

Answer 2

The breakers should be handle-tied (common shutoff, located adjacent) if they feed things in the same box. Code may be more lax and only require that for things on the same yoke, I can't recall. I'd apply the "if you shut off this box, this box should be shut off" rule and make a handle tie if maintaining two breakers.

Additional things apply if it's actually a MultiWire Branch Circuit (where one /3 cable can carry two circuits without overloading the single neutral wire) but for just "two breakers serve one box" that's not sharing a neutral, those don't apply.