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I am currently in the middle of finishing a portion of my basement to become an office. I am in NY State in the US. I plan to have two groups of lights, each controlled by its own switch - two light switches in total. My plan is to run supply wiring for the lights up to an initial junction box in the joist cavity, and then have them go out to their appropriate locations from that point. I plan on installing a drop ceiling, so everything going in joist cavities will be accessible.

Given that two 12/2 cables will be nominally required, one for each group, my plan is for the initial supply wire coming from the light switch box to be a 12/3 cable. The hot of each 12/2 would connect to the appropriate hot in the 12/3, and then both would share the ground and neutral. Is this considered acceptable, or will I need to run two 12/2 cables from the light switch box directly?

Thanks.

Edit: Apparently what I'm describing is a Multi Wire Branch Circuit, which has additional requirements associated with it that I have not considered. I'll just run two 12/2s.

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  • Does this answer your question? Can I run two 12/2 cables from one 12/3 cable?
    – isherwood
    Nov 17, 2022 at 19:32
  • (possibly ignorant) Q: Why do you need separate hot lines coming from the breaker box? If both could fit on a single breaker, couldn't you use drop the 12/3 entirely, and use 12/2 throughout? Nov 17, 2022 at 20:28
  • I don't think this would necessarily be a MWBC. Can you provide more detail about how exactly power is getting to the switches? A full diagram showing your intended wires between your breaker box, switch box, junction boxes, and lights, would be helpful.
    – nobody
    Nov 17, 2022 at 23:14
  • I'm mistaken, it's not a MWBC. For some reason I thought the 12/3 was coming from the panel itself. Nov 18, 2022 at 0:32

2 Answers 2

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That is called a Multi-Wire Branch Circuit

or MWBC for short. And it is 1 circuit containing 2 hots. You can call it 2 half-circuits; I do.

Even though you're only running it to a split-point, and not having any outlets inline in the 14/3 section... it's still a MWBC and still must follow MWBC rules. Read up on what those are. A few points:

  • Breaker handles must be handle-tied. 2-pole breakers are acceptable. Handle-ties don't necessarily cause common trip but 2-pole breakers do.
  • Neutral must be pigtailed up until the final split. (once the two half-circuits are 12/2 cable no more pigtailing is required).
  • AFCI and often GFCI breakers are typically required on any modern circuit; good luck with that. There's a reason MWBCs have fallen out of fashion.

Since MWBCs are allowed to serve both 120V and 240V outlets... and the biggest gaming PC power supplies are now too big for a 120V circuit... that might be a thing to think about. Supporting both 120V and 240V requires a 2-pole breaker due to common trip being required. That makes AFCI more interesting still.

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  • Those huge power supplies are silly. Even dual 4090s in a Threadripper system maxed out will fit within the 1600W of a normal wall outlet.
    – KMJ
    Nov 17, 2022 at 20:13
  • @KMJ wait, you play with only two 4090s? How do you expect to get 500 FPS that way? ;) :) :) :) :) Of course you know the wall socket draw is considerably more than the advertised watts. "850W" power supply (on the DC side) is 10A/1200VA out in AC provisioning land. Nov 17, 2022 at 20:22
  • Of course. That hypothetical system would be about 1400W of DC and still pretty safely fit on a 15A circuit. The rise in gaming power supplies is all about bigger number = better, plus of course crypto mining.
    – KMJ
    Nov 17, 2022 at 20:28
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    I don't see any evidence that this is a MWBC. It could just as easily be the case that the two light switches are fed by one 120V circuit (/2 cable) and for whatever reason the OP wants to run a single /3 cable towards the two banks of lights instead of two /2 cables the whole way.
    – nobody
    Nov 17, 2022 at 23:15
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    @nobody oh, you're right. Deleting Nov 18, 2022 at 0:32
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If I understand this correctly, it sounds good:

  • Panel to Switch Box - 12/2 - hot and neutral.
  • Switch Box to Junction Box - 12/3 - two switched hots and neutral.
  • Junction Box to Light Set 1 - 12/2 - switched hot and neutral.
  • Junction Box to Light Set 2 - 12/2 - switched hot and neutral.

The key is that all wires in each cable sum to 0. Each neutral will match the corresponding hot, switched hot or switched hots.

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