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There are two live 2-wire 12 gauge cables entering a junction box, and one 3-wire 12 gauge cable traveler exiting said junction box. Can the two 2-wire cables on separate breakers be connected to the 3-wire cable, sharing the neutral and ground?

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2 Answers 2

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TL;DR No

You are not allowed to do this, and there are 3 potential problems if you did it anyway:

  • GFCI

While the hots would stay the same, the neutral would be merged. Which means that on the way back to the panel (so to speak) it would split and likely not split the same way, resulting in one hot having more current than its partner neutral and the other hot having less current. Which means instant GFCI trip. Most people don't have GFCI breakers, but if you do have GFCI breakers then this just won't work.

  • Hots on Same Leg = Neutral Overload

If the hots are on the same leg (i.e., 0V between them instead of 240V) then the neutral could overload easily. Run a 16A heater on each hot and now you've got 32A on the neutral (instead of the 0A that you'd have if they were on opposite legs) and an overloaded/overheated neutral wire. The solution to that for a proper MWBC is a double-breaker or a handle-tie to make sure the breakers are side-by-side (normally meaning opposite legs). But with two separate 2-wire cables it won't be obvious that the breakers need to be installed that way.

  • Hot/Neutral Balance

Aside from the GFCI (instant trip) problem, in general every pair or group of wires have to be equal/balanced. That typically means:

  • hot + neutral
  • hot + switched hot (switch loop)
  • hot + travelers (3-way)
  • switched hot + travelers (3-way)
  • neutral + travelers (3-way)

but with this setup the neutrals may not match their partner hots in the 2-wire cables.

Note that you could connect one of the two-wire cables to 2 of the 3 wires in a three-wire cable. One wire stays unused, but still better than running a new two-wire cable since it is already paid for.

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You must join grounds when cables enter a box, weather on the same circuit otherwise. Neutrals may not joining not on the same circuit

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