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enter image description hereenter image description herepicture of junction boxI was looking for hopefully a quick answer. I have a multi wire circuit coming out of main breaker box. It is a 30 amp double breaker with 10 gauge wire that divides into the double breaker . I concluded it is indeed a multi wire branch circuit. My question is how do I transition the 10 gauge wire to smaller wire and add outlets. The 10 gauge wire runs about 40 feet or so to a garage that is attached. Do you use wire nut in a junction box or...? Here is a decent pic of the junction

  • Are you replacing the breaker with a smaller breaker? Because you can't put smaller (translation: regular) outlets on a 30A circuit...only 30A ones. – ThreePhaseEel Jan 6 at 3:11
  • It is a split wire branch. Circuit. It has been in place for some time now. And yes it is a 30 amp double in the main box. So you saying that I can only use 30 amp receptacles? No reducing it at all? – Brian Jan 6 at 3:20
  • It used to be for several different power tools running at once . I believe it is transitioned by wire nuts to a 12 gauge wire – Brian Jan 6 at 3:22
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    If this is no good how do I do this to make it safe. Sub box or?.... – Brian Jan 6 at 3:23
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    @Brian, so it's no longer a shop, and you're not anticipating any high draw on the new outlets you add? In that case, yes, I'd just downbreaker to 20A -- that'll protect the existing wiring, and leave you enough room for some expansion. However, I don't see a ground wire in there -- are you able to add a ground wire from the junction box back to the panel? Or are you okay with all your new outlets being ungrounded (i.e. GFCIs with the NO EQUIPMENT GROUND sticker)? – Nate S. Jan 7 at 1:02
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Subpanel Time!

You can't run any 15A or 20A devices (e.g, 15A or 20A receptacles or light fixtures designed for 15A or 20A circuits) on the 30A breakers. But you can install a subpanel. This will let you put as many things as you want on the circuit. From a practical standpoint, you could definitely put a subpanel with 4 x 15A circuits (15A x 2 = 30A on each leg) but you could actually install more - e.g., 6 x 15A or 4 x 20A or some other combination. That is because the 30A limit is based on the total simultaneous usage. Since most of the time most of the smaller circuits will have very little use (e.g., a lighting circuit might pull only 1A for LEDs to light up the whole garage), this can work very well. About the only thing you wouldn't want to put on this subpanel is a heavy 240V load like a water heater or air conditioner.

The subpanel can be any size 30A or larger. As I understand it from other Q&A, in an attached garage you generally don't need a main breaker in the subpanel, but it is OK (and often convenient) to have one, and it can be any size - larger than 30A is perfectly fine as it functions here as a switch and protection for the subpanel but not for the wires back to the main panel. Ground and neutral not bonded in the subpanel. And don't forget GFCI - most garage circuits will need to have GFCI protection.

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    This is a code compliant answer and a good one. A sub would be needed unless downsizing the breaker in the main panel. + the only quick way or code compliant is to downsize to a 20 amp or have a sub panel then you can have several 15 and 20 amp circuits and they would be code compliant. – Ed Beal Jan 6 at 5:56
  • A lot of willing helpful folks here. Greatly appreciated! Now that I got a good look at it again today. Here is how it is set up. A thirty amp double breaker in main box with one black and one red and one white wire , which is 10 gauge wire, plugged into it. The black plugs into one half of the 30 amp double breaker and the red wire plugs into other half and the whitet goes to neutral and ground bus in the main box. It isn't run through any conduit . It is just old three wire 10 gauge with the fabric type of outermost insulation. Now . This is where I saw something noteworthy. – Brian Jan 6 at 19:28
  • The 10 gauge wire is connected to black and white only , the red is not used , it's taped off.. So 10 gauge wire is in a nut with multiple 12 gauge wire . Both black and white. Like I said before it has been like this for years. And more advise would be great. Thanks in advance – Brian Jan 6 at 19:32
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    So what to do about ground then . How would I run a wire from a ground in new sub panel back to main box. The wire that is in place is older 3 wire 10 guage – Brian Jan 6 at 19:37
  • @Brian, that's a very different situation that you first described, and will need some more investigation. If there's any #12 wire in the circuit, the breaker can't be larger than 20A, so your current installation isn't legal or safe. Can you edit the question with these new details, and ideally, a picture of your breaker box + wherever box the #12 splice is happening in? – Nate S. Jan 6 at 19:59
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I'm posting this partly to verify that I understand the alternative to manassehkatz's answer (also mentioned by Ed Beal), which would be to:

  1. Downsize to a handle-tied, 20A breaker (which doesn't trip together but would be manually switched off together)
  2. Diverge the MWBC to two #12/2 cables in a junction box, where the neutrals split from the single source neutral
  3. Feed outlets and lights as desired from the #12/2 cables

Feedback and clarification welcome.

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    No you'd have the full 20A on each. The hots each get their own breaker. – batsplatsterson Jan 6 at 19:09
  • So not common-trip? – isherwood Jan 6 at 19:59
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    They don't actually have to be common trip, just handle tied so when turned off they turn off together. But with common trip you could still have 20A on circuit A and 20A on circuit B and neither would trip. But with common trip an overload on circuit A would at the same time trip circuit B. The handle tie is a recent requirement, used to be you could use any two breakers from different legs, or three breakers on three legs in three phase. – batsplatsterson Jan 6 at 20:08
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    @batsplatsterson You'll probably get common trip anyway. The vast majority of handle-tied breakers will knock each other out; they're just not guaranteed to do so, so you're not allowed to use them in cases common trip is required. The Code reg is giving freedom to use, say, a disconnect switch with fuses. Which OP could do, in fact. – Harper - Reinstate Monica Jan 6 at 21:50
  • @Harper-ReinstateMonica -- yeah, a 30A fused A/C disconnect costs all of $10 or so – ThreePhaseEel Jan 7 at 1:32
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Note: this answer assumes all your wiring is copper. If anything is aluminum, treat it as two AWG smaller, and use alumiconns instead of wire nuts. So if your 10 AWG is aluminum, treat it as 12 AWG copper, and put a 20A breaker on it.

Step 1: Retrofit a ground wire

If you want any of your new circuits to be three-prong grounded circuits instead of two-prong circuits, you're going to need a ground wire. Fortunately, the most recent NEC allows you to retrofit this pretty easily -- simply run a 10 AWG bare or green wire from your junction box to your panel by any route that is convenient (it doesn't have to follow the original cable, but it can), or to your grounding electrode system if that's easier (not to a water pipe though). You may optionally also run #12 bare or green wires from the junction box to the things that the black half of the circuit is currently powering, to upgrade them from two-prong to three-prong outlets as well.

Step 2, Option 1: Downbreaker to 20A and add new circuits

Replace your 30A breaker with a 20A breaker. This is required since some of the wires on the black half of the MWBC is 12 AWG, and a 20A breaker is required so that 12AWG doesn't melt. Then, on your red wire, you have another, additional 20A available -- this is where we will add the new outlets. With the power off, simply wire nut the hot wires that go to your new outlets to this red wire, unbundle the white wires and add the neutral from your new outlets to the bundle, and connect them all back together with a (large) wire nut. Wire nut the ground to the new ground wire you just added. I can't tell if the black wires have a wire nut under the tape or not, but if not, this is a good time to fix that. Note that on your new outlets, since this is a MWBC, you must pigtail the neutrals, rather than using the outlet as a splicing block if you're taking power on further to other outlets.

Step 2, Option 2: Subpanel

With the power off, unbundle all wires from the junction box, and remove it. Install a small subpanel in its place. Connect the incoming black wire to one of hot busses, the red to the other, all whites to the neutral bar, and ground to the ground bar. Assuming your incoming 10 AWG wire is copper, you can leave the 30A breaker in place. For all outgoing wires, install circuit breakers of the appropriate size (i.e. 20A for your 12 AWG circuits), and land their hot wire on the breaker. You can either leave the hot wires currently bundled to the black wire as one circuit, or even better, split it up and land each one on its own breaker. The new circuits you're puting in also get their own breaker. One advantage of this solution is that nothing is an MWBC anymore, which allows you a lot more freedom to use GFCI and/or AFCI breakers or outlets as appropriate, and you don't need to bother pigtailing neutrals at your outlets.

  • Hey , thanks a million. Superb answer. – Brian Jan 8 at 22:23
  • Thanks again for the response . This may seem a silly question. How would i leave the black wires that are currently bundled together onto one breaker in the sub panel – Brian Jan 9 at 0:14
  • And when you say all whites, do you mean ALL . I.e. the 10 gauge coming from main box and the bundled ones in the picture. Do I attach these all to neutral bar? – Brian Jan 9 at 0:26
  • Last thing ...I saw mentioned earlier that I can actually use a larger box that can handle more amps. For example a sixty amp. Sub panel. Not that i need more amps , but for the sake of having more breakers to use for individual circuits – Brian Jan 9 at 0:29
  • So the 2, two wire splits coming out of junction will have to have gfci anyway, correct? – Brian Jan 9 at 1:17

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