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In a 2000 era home in Georgia, I replaced an overhead lighting fixture with a combo ceiling fan and light. (Yes, I reinforced the junction box appropriately.)

The wiring was simple enough, the power supply is via the switch box (now two switches). The question is: For the wiring that runs between the switch box and the fan & light, can I use a single 3-conductor cable instead of two 2-conductor cables? Electrically, it works. Safety and code issues?

Ground and neutral connect through the switch box to both fan and light. I split the incoming hot (black) to supply both switches, then on the outbound side I ran one switched hot for the fan (via the red), and the switched hot for the light (via the black).

It's not just that I'm cheap (I am). The rationale for doing this is that it was easier to pull a replacement single 3-conductor wire rather than pull a second 2-conductor cable.

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Lights are usually wired with 14awg. Are you sure yours isn't? I wouldn't upsize wire. It can give the impression of a larger current capacity downstream. – isherwood Mar 28 at 19:35
    
I wouldn't worry about that, the guy replacing the breaker should be going by the wire size at the breaker, which should be correct or labeled. – Harper Mar 28 at 22:46
    
Heh, if you ever need a job as an electrician ;) – Kris Mar 29 at 0:42
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@isherwood - Yes, it's overkill to use 12 gauge for lighting, but it's on a 20 A circuit - so that's what is needed. – jbbenni Mar 29 at 2:20
up vote 9 down vote accepted

There's no problem at all with what you've done. In fact, that's the way it's supposed to be done. If you were to use two /2 cables, you'd either be paralleling the neutral (310.4), and/or not running all the circuit conductors together (300.3(B)). Parallel neutrals are not allowed in this situation, so you've actually avoided a code violation by doing it with /3 cable. You've also kept all the circuit conductors in a single cable, and avoided violating that code section.

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Yep, nearly all light/fan combos are wired this way during construction. I wasn't aware that parallel neutrals were a violation. What's the potential danger? – isherwood Mar 28 at 19:34
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Electromagnetic heating is the main problem to avoid by running wires together. From the 2014 NEC "300.3(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding con- ductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4)." – ArchonOSX Mar 28 at 20:12
    
310.4 Conductors in Parallel: Aluminum, copper-clad aluminum, or copper conductors of size 1/0 AWG and larger, comprising each phase, polarity, neutral, or grounded circuit conductor shall be permitted to be connected in parallel (electrically joined at both ends). – Ben Welborn Mar 28 at 20:16
    
@BenWelborn are you just quoting the code section, or suggesting that the OP might use 1/0 or larger conductors to feed the fan/light? – Tester101 Mar 28 at 20:18
    
@Tester101 310.4 makes few exceptions; in a nutshell, wire smaller than 1/0 cannot be run parallel. – Ben Welborn Mar 28 at 20:35

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