Three-way switch circuit between house and barn - how to wire

Question

I am in the process of wiring a new barn. We have an existing 4 conductor feed powering a sub-panel in the barn from the house.

I ran into a question while figuring out how to wire a three-way circuit between the buildings - a three way switch in the house, pulling power from an existing lighting circuit, connected to a second three way switch in the barn, which in turn feeds outdoor lights on the barn.

We have three individual 12GA conductors run along with the sub panel conductors. Ideally, I would like to use two of them for the two switched legs of the three-way circuit and use the third as an additional single pole switched circuit, also fed from the house.

Is it allowable to connect these circuits back to the neutral and ground buses in the sub panel, or do I need to use the third conductor to bring a separate neutral all the way from the house just for the three-way circuit?

If the later is the case, is it legal to have the barn three-way switch in a multi-gang box with other circuits fed from the sub panel? I realize I would need to keep the neutral for the three-way isolated from the others in the box to avoid paralleling the sub panel feed back to the house.

Answer 1

Big problem: no ground

NEC 2014 gives you broad latitude to share grounds among circuits. The one thing it does not allow you to do is share grounds among panels. Circuits served from the main house cannot use the grounding system from the subpanel, or vice versa.

Sharing neutral is out of the question

What has never been allowed is sharing a neutral among different circuits. This causes a lot of problems. It breaks GFCIs and AFCIs. It causes potential overloads, since neutrals do not have overcurrent protection (breakers) and depend on being mated with only one single hot which does have appropriate overcurrent protection.

What's more, neutrals have to be white. No tagging a colored wire. So if your three 12AWG wires do not have a white among them, you'd need to pull one - and might as well pull a ground too.

Of course... If the circuit was 240V, none of the wires would be neutral, would they?

Relays?

Given 3 colored wires, no neutral or ground, here's what makes sense to me. Power both circuits (the 3-way and the simple switched circuit) from the subpanel at the barn. Use the 3 wires to control the coils of two relays. I think if you put the relays immediately off the subpanel, e.g. in knockouts on the subpanel itself, you would effectively be inside the steel shielding of the subpanel and dodge the the grounding issue. It's a stretch, but it's all you got short of pulling more wires.

At the house, you bring 120 or 240V to your two switches. Plain 1-way switches. You send two switched wires and a common to the barn. Each relay coil takes 1 switched wire and shares the common.

Then you feed power off a breaker in the barn subpanel, which goes to the relay contact marked common on each relay. The relay to control the simple load is a plain SPST relay (any other kind would also do) and is wired like a plain switch. The common contact takes always-hot from the barn subpanel, the NO contact is switched-hot for that load.

The other relay is a SPDT type. Note that this is exactly the same layout as a 3-way switch, and you do the standard 3-way switch layout. You wire always-hot to the common relay contact. Each of the NO and NC contacts go to the two messengers, which go to the barn 3-way switch. The common of the barn 3-way switch goes to the light. Standard 3-way switch layout, except one switch is a relay.

That's how I would do it with the wires present.

Answer 2

More wires needed (for the regular route)

You'll need a separate neutral for the circuit from the house. This is 300.3(B)/310.10(H) of the code, and exists to keep massive loops from emitting magnetic fields which can interfere with sensitive devices and overheat ferrous metal objects inside them.

However, it's legal to commingle multiple circuits with separate neutrals in the same box as long as the hots and neutrals are kept separated, electrically speaking.

Provided there's room in the conduit, you can also stuff a 3rd switched hot in with the existing travelers and neutral and have it switched off the same circuit as the travelers -- this gives you your single pole switched circuit for well, whatever, along with the three-way circuit for the outside lights.

If you don't have a white wire -- you'll have to stuff a white in for the neutral (as it can't be colored or tagged). If there isn't a ground in there, you'll have to stuff a bare wire down the conduit for the ground as well.

Relays can save you if you can't pull wire

Using a couple of devices called relays can save you if you can't pull any more wire through the conduit (which is unlikely, but possible). You'd mount these on a box in the barn and power their coils from switched circuits from the house, then wire their contacts as if they're the appropriate kind of switch in the barn circuit (SPST = regular switch, SPDT = 3-way switch).

This also means that you won't need a white wire for the common, as the circuit from the house to the barn is a Class 1 remote control circuit now instead of an ordinary lighting circuit, and Article 725 doesn't specify colors. Note that a Class 1 remote-control circuit does not need a power-limited source (unlike a Class 2, Class 3, or power-limited Class 1 circuit) -- an ordinary AC branch circuit suffices provided the Class 1 control circuit is wired using 14AWG.