I just bought a house and am changing around some outlets in the garage, and I want to make sure the electricity is wired properly. There are 6 wires in a conduit going to the garage, three hot (red, purple, and blue) 2 neutrals (white), and one ground (green). Each of the hot wires has their own circuit breaker. Red and blue were wired to share a common neutral, and purple had it's own neutral. When I used a voltmeter and put one lead on red and one on blue it reads roughly 0 volts, when I do the same for blue and purple it reads roughly 240 and purple and red also 240.

Should red and blue share a common neutral? My guess is that I should rewire it so that red and purple or purple and blue share a neutral, but I want to confirm.

Thanks a lot!

  • Add location, not everybody uses 120 per hot,some use 220/240.
    – crip659
    Mar 17 at 22:26
  • @crip659 -- if they were on Eurostyle power, then they'd be seeing 400V L-L Mar 17 at 22:36
  • @ThreePhaseEel You are right, not thinking right, red blue purple throwing me.
    – crip659
    Mar 17 at 22:40

You're correct that red and blue need to be on different neutrals

You are correct that the original wiring was wrong; having red and blue share a neutral would subject that neutral wire to the sum of the currents from the red and blue wires as they're on the same leg of the service. Which wire you pick to go with the purple wire depends on the position of the corresponding breakers in your panel; ideally, either red's or blue's breaker would be next to purple's breaker, so you could whack the appropriate $2 handle-tie for your panel on the two breaker handles to provide a common maintenance shutoff for both circuits on that neutral. However, if neither red's breaker nor blue's breaker is next to purple's breaker, then you may need to rearrange your panel to accommodate the common-shutoff requirement. (If you plan to put red and blue on the same yoke, by the way, you'll need a three-pole handle-tie as different circuits on the same yoke need common maintenance shutoff as well.)

  • Thanks! That's very helpful. None of the three breakers are next to each other, but I'm going to get a panel upgrade soon and will make sure two breakers with a shared neutral will be next to each other so that I can run a handle-tie between them. Mar 17 at 22:48

Having had to rewire a factory after someone stole all the breakers and all the wires up to the edge of a panel, I see it in the converse manner as ThreePhaseEel.

I don't say red and blue need to be on different neutrals. I say red and blue breakers need to be on opposite phases. I.E. the breakers are mispositioned. I wouldn't do anything to the garage wiring, other than understand it.

I suspect it was wired correctly originally (albeit lacking the handle-ties now required) and then some years ago, a previous maintainer rearranged some breakers and did not realize these were MWBCs. .

So I would reposition the red/blue breakers back to where they belong: on opposite phases... and declare victory, myself.

In other words, I'm saying "fix it at the breaker panel by correctly phasing red and blue"... and don't "try to rearrange the garage wiring to change how the MWBCs are grouped".

For right now, swap the 3 breakers around so red/blue are opposite phased. It's not a handle-tie, but it's what can be done right now. (I assume you have a panel full of tandems/double-stuffs, and those get VERY confusing).

However, separate from that, it's a Code requirement that the neutrals be identified in association with their partner hot(s). Things like that are why my electric toolbox has 10 colors of tape. I would simply tape the white wires with the colors of the hot(s) they support; there's nothing wrong with that *when it's individual wires in conduit; as remarking individual white or gray wires does not convert them into a hot.

So wrap them or tape them as pairs/triplets, at both ends. The ground is shared and doesn't need to be associated with one or the other.

  • Thanks, but could you explain why you think that red and blue can share a neutral? As far as I can tell of a MWBC, the hots are out of phase so the neutral carries the difference in current instead of the sum. If there are 3 hots, 2 of which are one phase and one of which is the other, then of the 3 combinations there should be 2 combinations of 2 hots with opposite phases, and 1 combination of hots with like phases. In this case the blue/red combination seemed like the odd one out, which leads me to think they're on the same phase. I could be thinking about this the wrong way though. Mar 19 at 16:57
  • @hustlerwhite69 Because in OP you said "Red and blue were wired to share a common neutral". Mar 19 at 17:13
  • @hustlerwhite69 I edited my answer to clarify my point, your goal of getting them opposite phased is correct and essential... I'm saying just swap breakers, instead of trying to rewire the garage circuit. Mar 19 at 19:10
  • Ah okay I understand! Thank you Mar 19 at 22:39

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