Can I run a #12 outlet branch and a #14 light branch off a 30A breaker?

Question

Was wondering if i can feed an entire room with one 30 amp breaker, 10/2 to an attic junction box, then break out 12/2 for outlet daisy chain (four 15 amp-type outlets) and 14/2 for a light circuit.

Answer 1

If you really want that, you can use an MWBC

It sounds like you want to throw more than 20A into a room. That can be done. But you really, really need to bone up on the particulars of Code, because this is easy to dangerously botch.

You can use a multi-wire branch circuit, which uses a /3 cable to provide 2 hots and a common neutral. You are able to then fork this circuit, and either split it into two separate 120V circuits at its first stop, or carry it along for awhile as a dual circuit, serving each socket on alternating hot/neutrals. (don't split sockets; that used to be clever, but it doesn't work with GFCI receptacles, USB, smart receptacles etc. So you paint the next guy into a corner if you do that.)

The MWBC can be 15A with #14 wire, or 20A with #12 wire. Dual 20A in a bedroom is certainly going to take care of both the high-end gaming PC and the window A/C unit you'll need to remove the heat. Dual 15A is a cheap way to get that 30A.

There are some special rules for MWBC.

• Must be a 2-pole breaker (or handle-tied breaker configured as a 2-pole, to guarantee opposite poles: 240V between the hots). 240V between hots is a good thing and vitally important to not overloading the shared neutral.
• Neutral wires must be pigtailed at each outlet or switch, anywhere both sides are present -- you cannot use a device as a splice because if you removed the device, you'd sever neutral. Like using the 2 screws on a receptacle; that's right out.

Now, the 2-pole breaker requirement may have a funny intersection with Code. Nevada is the lone holdout still on 2011 NEC, probably because they don't like the "GFCI/AFCI on darn near everything" requirements of 2014 NEC. So here you go

210.12 Arc-Fault Circuit-Interrupter Protection.

(A) Dwelling Units. All 120-volt, single phase, 15- and 20-ampere branch circuits supplying outlets installed in dwelling unit family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, or similar rooms or areas shall be protected by a listed arc-fault circuit interrupter [AFCI breaker], combination-type, installed to provide protection of the branch circuit.

I pulled this off a forum post from 9/21/11, so I definitely didn't give you NEC 2014 by mistake.

So this requires a 2-pole breaker that is also AFCI, or some panel makers implement this as two handle-tied* breakers which are AFCI.

I hate stiff wire

If you don't mind some light pipefitting, there's a cure for that. It's called EMT conduit. Outside of a special bender, all you need is a hacksaw and some patience. Then you get to run individual wires (which are individually not nearly as stiff) -- and you also get to use stranded wire, which is not stiff at all, and a sheer pleasure to use. It's difficult to attach to screws, but simply pigtail those with solid wire - easy.

You can transition from Romex to EMT at any junction box. Simply enter the Romex into the junction box with a listed cable clamp, and splice it onto the EMT wires.

By literal Code, if the run between the service panel and the first outlet is metal conduit, you can use AFCI receptacles which are much cheaper than AFCI breakers. From there you can use /2 Romex. Because it's MWBC, you'd want a 2-gang junction box and put both AFCIs there. (I recommend a 4-11/16" square box with an offset mud-ring, since it has enough space for AFCI/GFCI, and the right face to mount AFCI/GFCI devices).

210.12(A) Exception No. 1: If RMC, IMC, EMT, Type MC, or steel armored Type AC cables meeting the requirements of 250.118 and metal outlet and junction boxes are installed for the portion of the branch circuit between the branch circuit overcurrent device and the first outlet, it shall be permitted to install an outlet branch-circuit type AFCI at the first outlet to provide protection for the remaining portion of the branch circuit.

Oh, but I really want to run 30A+

Subpanel. You run the feed size of your choice to a subpanel (in an allowable location), then branch off all the 15A/#14 circuits you please.

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* Literally, it says that a pair of breakers with handle-ties listed by the manufacturer will also suffice, but you must make sure this puts the breakers' outputs on opposite poles so there is 240V between the hots.

By the way, a 2-pole breaker is much more than two 1-poles with a factory assembled handle tie. It also has an internal mechanism to ensure common trip. That is not required for MWBCs.

Answer 2

You might get yourself a house wiring book and brush up before you have things smoldering. :)

You can't reduce wire size like that. #12 and #14 have no business on a 30A circuit. You can increase wire size from the panel and then reduce it downstream to prevent voltage drop over very long runs, but I doubt that's necessary here. If you do, be sure to label accordingly at the panel so no one assumes that the entire circuit is done with the heavier wire and upgrades the breaker.

Instead, run a 20A breaker with #12 wire throughout, even to the lights. It's no fun working with that heavier wire, but it's required. You can still use 15A outlets, which are usually rated for 20A pass-through.

20 amps is plenty for nearly any residential situation. If you need more it's probably for a device that requires a different receptacle anyway.