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