Ceiling fan and light on separate three way switches, code compliant?

Question

I am in a new-to-me house built in the 1970s. We are on the 2014 NEC.

I have two long living rooms, one with the ceiling fan improperly wired, and one without a fan at all.

I would like to have in each room one ceiling fan with the fan and light controls separate. This is a common request and easy to find plans for. What I would like to do in addition is to have three way switches for both the fan and light. Imagine a long room with the fan in the middle and at each end a two gang box with one switch for fan and one for the light. This is not easy to find plans for.

I have drawn up a simple plan as to how to do this like simple 3-way switch wiring but don't know if this is acceptable.

Most of the things I have read to this point state that this is not OK as when only the fan or only the light is in use, some current would be flowing through the neutral wires in each cable between the three way switches, which is not allowed.

I am wondering if this wiring setup is compliant with the 2014 NEC. If not, I ask why not, and what would be an appropriate way of wiring this.

The current source cable is at a wall switch box but could be moved to the ceiling fan box if necessary.

Answer 1

You have a rather...neutral problem on your hands

In your diagram, the neutrals in the two inter-box cables are connected in parallel. This can be interpreted as a violation of 300.3(B) and 310.10(H) regarding current-carrying conductors in parallel outside the same sheathe or raceway.

In practice, what will happen is the parallel neutrals will create a magnetic field between them, which will heat any ferrous metals in that field to at least some degree. This can make for a bad day when it's a bit of junction box between two knockouts that's getting toasty inside your wall!

Fixing this might be actually rather easy, though. One option would be to use a 14-2-2 from the fan/light back to the switch -- this allows the fan and light to be wired to their own neutrals, although you'd probably have to modify your fan to bring the light's neutral out separately to do this. In this case, the fan-travelers and fan-neutral are in one 14-3 between the switches, while the light-travelers and light-neutral are in the other 14-3.

Of course, if you can run a conduit or listed surface raceway between the two boxes, then you can simply pull in whatever wires you need provided the conduit fill isn't exceeded. There are also multi-wire metal clad cables available (with 6 wires or even more) that could be applied here if a conduit run is impractical, although you'd need to order them through an electrical supply house.

Answer 2

Good eye.

The only way to do this is conduit between switches

Unless you can find 14/5 with also ground.

The neutral and all four travelers need to run in the same cable or raceway (conduit). That's because the neutral is the return for all the travelers.

You can't parallel neutral as ThreePhaseEel discusses.

Or, keep them separate all the way

Your other option is separate them at the first switch, then, keep them separate after the second, with two 1x/2 cables going to the fan/light.

But since the neutrals can never rejoin, the two neutrals must be carried separately all the way to each device. so if the fan/light only has a common neutral wire, you cannot use this option.

Alternatives

You could use relays, as long as you keep your neutrals separate up to the relay coil. Or you could use 24V relays, either with combo relay-transformers, or by poaching 24V off your doorbell or furnace. Then your switch wiring could be 24V and use common thermostat wire.

You could use smart switches, so the power switching only happens at one switch, and the request from the other switch is handled by data rather than multiple travelers.

Answer 3

Two 3-ways switches to control a light and fan separately.