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Wiring 3 gang switch, have 4 cables coming into box. (2) 14/3 cables coming from 3 way “primary” switch that also feeds to two different light fixtures. Also have a 12/2 cable from different circuit to feed switch and then exit to a different light.

Any code issues with using 2 different gauge in same box?

While the 14/3 3-way switches won’t have neutral (will mark white as black traveller) the box will have a neutral from other circuit. Is that up to code?

Should I combine all grounds or keep circuits separate?

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  • If the two wire gauges are connected on the same circuit, then you must use the breaker size for the smallest gauge. It sounds like just two separate circuits in the same box, which is okay as long as you remember to turn off the two breakers.
    – crip659
    Jun 15, 2023 at 20:06

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ObDisclaimer: I'm not talking about MWBCs where 2 hots share a neutral on purpose. MWBCs are one circuit even if you're accustomed to thinking otherwise, and MWBCs need to come off a double-pole or handle-tied breaker.

It's OK to have 2 separate circuits in the same box.

Grounds must be all tied together. This makes sense if you think about an installation with metal boxes (where all grounds must be tied to the metal box, and switches and some receptacles pick up ground via the mounting screws)... or a non-flex metal-conduit installation where the conduit shell is the ground and there is no ground wire.

Neutrals for each circuit must be carefully kept apart. You would be well advised to get some gray electrical tape so you can wrap some neutral to identify them as being from a different circuit than the first. That means no -- you cannot meet the "neutral on switch loop" requirement by saying "oh, there's neutral from a different circuit in there".

However - the "neutral on switch loop" requirement does not apply to every switch in a 3-way or 3/4-way switch group. To understand the rule, think of it from the perspective of a motion sensor. You only need neutral at enough switch locations to cover the whole room. That means in a rectangular room it's usually enough for one to have neutral.

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  • Also need to make sure the "box fill" is big enough for the number of conductors and the various connections needed, but odds are a 3-gang should be big enough.
    – Huesmann
    Jun 16, 2023 at 14:05

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