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I am trying to wire a small 12v (ac to dc converter used) composting toilet fan in a room where the closest wiring is a light fixture. The light fixture is switched however, and the fan needs to be always on.

So I will need to re-run wire from the switch so that there is 'always hot' wire to power the fan. Instead of pulling another wire through the wall alongside the existing, I thought I could use the old wire to pull in a new single 3-wire cable (two hots, neutral, and EGC) if this is allowed.

I would then switch one of the hots, while the other hot will feed the fan. In this situation, the neutrals are shared, however I would pigtail the lighting neutral so that it's not linear with the light itself. Is this allowed or advisable?

This is old work, and therefore the wall is not open, hence this work-around. My concern is that the holes in the studs may not easily accommodate pulling two cables through. Either way, I plan to use the existing wire to pull what's needed through, whether it's one 3-wire or two 2-wire; the single 3-wire just seemed like an easier pull.

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    Have you confirmed that the switch is fed by a hot and neutral, not wired wired as a switch-loop with the hot and neutral feeding directly to the light junction box? Jun 14 at 15:02
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    It's the same circuit, which means it's just the neutral, not a "shared neutral" which has a particular meaning in the context of "multi-wire-branch circuits" which this is NOT. Unless, as may well be the case, it's unswitched hot for a switch loop, and not neutral at all, which you need to check as suggested by @NoSparksPlease
    – Ecnerwal
    Jun 14 at 15:21
  • Is there accessible crawl space below or attic space above the water closet? That may be the only way you will be successful fishing new cables. Jun 14 at 15:41
  • Smart fan/light with smart switch
    – Kris
    Jun 14 at 15:44
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It is allowed to use the two hots as one switched and one unswitched. If using the switched and unswitched on separate circuit breakers sharing a neutral then NEC 210.4 Multi Wire Branch Circuit (MWBC) rules about marking and common disconnecting means must be followed.

If the existing cable was properly installed to begin with it will be "supported and secured" within 12" of the junction boxes. The code does allow fished cables to be unsupported but even if you successfully defeat the supports you will likely damage the insulation when changing direction or when pulling through more than one stud.

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