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I'm in a bit of a dilemma. The house we bought has a detached garage supplied by 12/3 UF with ground. The previous owner/handyman set this up as a multiwire branch circuit (MWBC) with a twist: one of the conductors is fed from the main panel in the basement to create a 20A circuit in the garage for plugs, but two other conductors in the 12/3 UF are used as switch legs for a 3-way switch, enabling the garage lights to be turned off either from the house or from the garage. This leaves the equipment grounding conductor to be used as a shared neutral for both the plugs and the lights in the garage - definitely not code compliant, probably a bad idea all around. The EGCs of the NM cables that feed the lights and plugs in the garage are tied together in a box in the garage, and not bonded to the makeshift neutral (EGC of the 12/3UF), as far as I can tell - so there is no true safety ground for the garage outlets.

I am not ready to dig up the yard and re-feed the garage, which will probably be the ultimate solution: reclaiming the 12/3 for only the lights, and adding thicker conductors in the trench for a proper subpanel in the garage. I also realize that the easiest short-term solution would most likely be to rearrange the existing wiring connections to make either a single 20A circuit on a 3-way switch for all lights and plugs in the garage, or a single always-on circuit in the garage, to which I could connect the plugs and the lights, with a switch in the garage for the lights. The first of these choices is not an option because the garage door opener needs to be always live for use with the car remote, and the second choice, though preferable, loses the significant advantage of the 3-way switch.

In general, I'm ok with using the ground of the 12/3 UF as neutral on a temporary basis because of the tough nature of UF cable, and because it hasn't caused any obvious problems yet. I'm thinking of driving a grounding electrode at the garage and connecting the equipment grounding conductors of the branches in the garage to that electrode, which wouldn't be bonded to the neutral. I know this would not be code-compliant, but I wonder if it would be an improvement. I'm also wondering if GFI protection of either/both of the branch circuits in the garage would be possible and preferable - though I know that using GFIs on MWBC creates new issues.

Again, I'm not yet ready to dig up the existing cable, and not willing to run anything overhead.

Thoughts?

Thanks! Benjamin

  • That is not a MWBC, unless the light is on a different breaker thsn the receptacles. There is no useful reason to do that,lights are not a big enough draw. – Harper Apr 24 '18 at 16:59
  • Can you check the in-ground wire - if its in a conduit then pulling fresh cable might be as simple as blowing/sucking a drawstring through then lube and grease a second cable. This fails if its a direct-bury cable. – Criggie Mar 7 at 19:00
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I'm thinking of driving a grounding electrode at the garage and connecting the equipment grounding conductors of the branches in the garage to that electrode, which wouldn't be bonded to the neutral.

This doesn't work.

Let's say you have a piece of equipment short the hot wire to the ground. The power tries to get back to the source. When it tries on your circuit it has to go through the ground to get there. Consequently, the breaker will not trip and it will sit there waiting for you or one of your family to get in the circuit. This is what kills people.

So, short answer is, this is a bad idea.

Spend the time and money and do it the right/safe way.

Good luck!

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    Exactly. Dirt is a terrible conductor. If it was a good conductor, we wouldn't bother mining copper! – Harper Apr 24 '18 at 16:45
  • Code requires the ground resistance to be 25 ohms or less or a second rod is required. You would not believe how high many homes ground resistance is even at code the ground is not low enough resistance to trip a breaker even at 1/3 the code value on a 20 amp circuit a ground fault on a 120v circuit may not trip the breaker, better and safer to do it right. – Ed Beal Apr 24 '18 at 17:44
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This needs to be addressed now, not at some hypothetical future when you acquire a Round Tuit.

Change the 3-way switches to any of a variety of smart switches that require only one traveler. The master goes near the light.

Now the wires return to their proper function:

  • ground is ground
  • white is neutral
  • black is always hot
  • red is 3way comms

If you didn't want to do it that way, the same can be done with a relay (SPDT). The house switch either energizes the control wire, or does not. At the garage, the relay plays the role of the other 3-way. Given the prevalance and eade of obtaining 1-wire (or even 0-wire!) Smart 3-ways, the relay solution is no longer worth the trouble unless money is very tight.

They also make 0-wire 3way smart switches which use the powerline for comms. This frees up red for use in a proper MWBC.

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    I am not familiar with smart switches that require only one traveler, nor with SPDT relays. Some quick googling hasn't helped much. Could you point me in the right direction? So far what I've found is still calling for 2 travelers. Thanks! – benjamin Apr 26 '18 at 10:51

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