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If I run 12-3 NM cable from the circuit panel to a room can I use that cable to power both the lights and outlets?

Black and white to the light switches and outlets. Red from light switches to the lights, terminated at the panel and outlets where not needed.

enter image description here

Is this a problem? I'm not in the US, but would it violate US codes? I'm looking for local codes.

Local code does require 12-gauge wire for inside wiring, not 14-gauge.

Walls are exposed.

It's an off-grid installation with 120 volt max 50 amp power.

Edit: Corrected diagram that previously had the red wire coming from source.

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    @crip659 No, this cannot be an MWBC. Only one of the red or black wires can be connected to the supply. The other must be unconnected at the supply end. Otherwise closing the switch would create a short instead of turning on the lights. The first segment could just as well be a /2 cable, but leaving one wire in a /3 unused for that segment is fine if the OP doesn’t want to buy two different spools of cable.
    – nobody
    Commented Mar 27 at 13:55
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    @nobody You are right. I was thinking red as switched hot, but with it connected to a breaker/supply it will short.
    – crip659
    Commented Mar 27 at 14:04
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    Ok, so the little red tail off to the left in the pic is an error. There is one circuit. The red wire is just switched power. There's nothing wrong with what you're doing. It's not common because it seems a little wasteful of wire running the /3 to all the outlets where it isn't needed. But if the runs are short and you just want to buy one spool of cable, that's fine.
    – jay613
    Commented Mar 27 at 14:26
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    You've left earth/ground out for clarity, right ?
    – Criggie
    Commented Mar 28 at 10:02
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    @criggie yes, ground left out for clarity. Commented Mar 28 at 12:02

2 Answers 2

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With just a few changes, you can make this work. Your /3 wire has hot (black), switched hot (red) and neutral (white). Note that you can also use /2 wire on any run that doesn't carry switched power, which will be cheaper.

You can use 12 gauge wire on 15 A circuits if you want. It won't hurt anything (except maybe your fingers trying to manipulate the thicker wire).

Redesign of wiring from OP's post.

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    Thank you. Yes, I made that mistake in the diagram; red is not connected to supply. The entire house is only 400 square feet so I'd like to use the one spool of wire. 12 gauge is required by local code even though 14 gauge would be used in the US. Commented Mar 27 at 14:29
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    Makes sense. There's nothing wrong with running 12/3 and just not using the red, except for the extra cost. Make sure to cap any unconnected wires with a wire nut.
    – Matt S
    Commented Mar 27 at 14:30
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There is nothing wrong with doing this. There is however one reason why people usually have the lights and outlets separate.

Pretend it is dark outside you turn the lights on and plug a kettle or something similar into an outlet. You then turn the thing on, and sit in your chair... Suddenly your lights turn off because the thing you plugged in decided to short and your breaker just is now tripped... Now you have walk around in the dark to find that kettle, un plug it so it won't re-trip your breaker... and then you need to find your breaker box, in the dark.

This is why people usually put lights on a separate circuit. Because lights don't trip. And walking around in the dark stubbing your toes to flip a breaker switch is not a fun experience.

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