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I have an existing junction box in my attic which is powering my exhaust system in the summer. The wire is 12-2 and it is a dedicated line, nothing else on it. I am putting a bathroom on the 3rd floor and have a fan/heater/light combo which I will run 12-3 from the unit to the switch. Can I connect the 12/3 wire in the junction box to the 12-2 wire for my power so as to not have to take the 12-3 all the way to the breaker box? The unit will have 3 switches.

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  • You need 12/4 for a 3-switch unit... Jan 20, 2018 at 17:38
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    @ThreePhaseEel 12/5 if it's a switch loop. Neutral which he'll definitely need for the digital timers everyone wants now. At that point might as well lay some EMT. Jan 20, 2018 at 21:39
  • @Harper EMT in residential - lot of extra work and pipe bending. The op does not state if his junction box is grounded either - he states 12-2 hot and neutral but does he really have a ground...? If the box isn't grounded running EMT from it will not provide that ground either. I have seen 12-2 run and that copper wire well - you know it was "EXTRA".. you never know a DIY might have done that Attic Fan.
    – Ken
    Jan 22, 2018 at 8:58

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It is not about the number of conductors it is about the load. Under the best of conditions the #12 can carry about 15-amps for an extended period of time. Check the power requirements of the heat-fan-light. Then get the amp draw from the attic ventilator. Add those together. If you can stay under the 15-amps your OK. Wire on. You will need a 12-4 between the control switch and the new heat-fan-light or one 12-3 and one 12-2. You need only one 12-2 between the junction box and the Heat-fan-light.

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  • #12 gauge wire is rated for 20 amps. Are you confusing it with #14 rated for 15 amps? There is no “about" involved either...
    – Tyson
    Jan 20, 2018 at 19:42
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    #12 is good for a 20A circuit which is good for 16A of continuous load. However a thing that is on a twist timer (that does not have a "remain on indefinitely" detent) is not a continuous load, it can support 20A of loads like that. Jan 20, 2018 at 21:40

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