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I've been searching for answers but most of the questions involve using two separate gauges for the same circuit.

I'm in the process of upgrading to a 200 amp service and I changed out all the receptacles for a 12 gauge circuit with a 15 amp breaker as a preventative measure to upgrade to 20A later in the future.

The problem is the cost of 12/3 is double the cost of 12/2. My light fixtures are going to be run using 14/3 on a separate 15A circuit.

I've read the NEC2017 book and understand that you need at least 14 gauge minimum to run anything with 15A. So my question is, will I fail inspection if I am using 14/3 gauge wire on a separate 15A breaker for my light fixtures/switches but a 12/2 gauge wire on another separate 15A for my receptacles?

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  • Using 14 AWG copper for lighting through a 15 A breaker is fine for lighting circuits. But why are you using 14/3? Do you mean 3 insulated wires in the cable -- black, red, white plus uninsulated ground? Is this NM cable?
    – Jim Stewart
    Commented Jul 6, 2021 at 16:27
  • I'm trying to pin down your primary concern. Is it just about using #12 wire on a 15A circuit? Nothing about what you're doing sounds like a problem. The fact that some devices are lights and some outlets isn't relevant.
    – isherwood
    Commented Jul 6, 2021 at 16:27
  • Correct black, red, white plus ground for my light switches. Am I doing something wrong?
    – lemonpledge
    Commented Jul 6, 2021 at 16:30
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    @JimStewart Simple switches, no smart switches. Never been a fan of smart switches. I am a software engineer and it upsets me that it takes longer to open the app than it would for me to get up and just turn off the switch.
    – lemonpledge
    Commented Jul 6, 2021 at 16:50
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    Your concern about changing the breakers to 20 A even though your wire for the receptacle circuits will be all 12 AWG probably comes from a wrong belief on your part that you cannot feed 15 A receptacles through a 20 A breaker. This is not true. You can keep all your old 15 A receptacles even if you upgrade your wire to #12 copper and upgrade the breaker to 20 A.
    – Jim Stewart
    Commented Jul 6, 2021 at 17:11

3 Answers 3

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Since the OP is a software engineer he might well be interested in the fact that there is an alternative way to wire a pair of what we in the US call "3-way" switches. The traditional way in the US is called 2-wire control, and the new alternative is called 3-wire control.

To implement 2-wire control there needs to be a /3 cable between the two switches and to implement 3-wire control there needs to be a /4 cable between the two switches.

AFIK the advantage of the 3-wire control is that there is a continuing line hot in both switch boxes whereas with 2-wire control the line hot in one switch box is interrupted during the process of switching at the other box. AFIK this would only be important if there was an active device in that box that required a constant line hot.

One thing though would that if later you wanted to put a receptacle near that switch box, you would be able to get an unswitched line hot and neutral from that switch box.

EDIT

Another feature of 3-wire control is that the same wire between the switches is always the line hot in both boxes. With 2-wire control the hot switches from one traveler to the other when the "line switch" is flipped.

With 2-wire control one switch is the "line switch" (common connected to the line hot) and the other switch is the "load switch" (common connected to the load).

But with 3-wire control both switches are hard connected to the line hot, and both switches are hard connected to the load. Any active device that will work in one switch box will work in the other. The arrangement is symmetrical.

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    ...assuming you wanted to put a receptacle on your 15A light circuit, at all, rather than keeping the lights and receptacles separate, and the receptacle circuits at 20A. I like not having the lights go out when a receptacle circuit is overloaded, perhaps because I grew up in a place where "stumbling through the dark to find a flashlight to get to the fuse box" was normal, and left a lasting impression.
    – Ecnerwal
    Commented Jul 6, 2021 at 17:56
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  • 15A (duplex) receptacles are fine on a 15A or 20A circuit. The only place you MUST have a 20A outlet is if it's the only outlet (simplex, and only one of it) on a 20A circuit. You can have more 20A outlets, but you don't need them. They are rated for the use (and usually if you see some sort of 15A only language molded in, you will see that it's about using the backstab connections, which you should not use anyway, and they don't fit 12Ga wire)
  • 12Ga wire is fine on a 15 or 20A circuit.
  • 14Ga wire is ONLY fine on a 15A circuit.

In modern practice where you must supply neutral to switch locations:

Typically, you only need /3 wire for "what would have been a switch loop" (I suppose it still is, just less confusing to neophytes than the hot white version) and between 3-way switches unless doing a Multi-Wire Branch Circuit (MWBC) - which you don't mention.

So, if power runs to the light switch, and then power runs to the lights, /2 is fine everywhere on that light circuit. If power runs to a light, and then a single cable runs to a switch location, that cable needs to be /3. Or you can run two cables to the switch location (depending on the relative price of buying /3 cable .vs. the hassle of running two cables.) If you have a 3-way switch setup, you need /3 between the two switches.

Or perhaps you are confused about wire gauges? 12 Ga is LARGER than 14Ga. so 14Ga minimum, means not a larger number (smaller wire) than 14Ga.

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  • Is the modern practice to have lighting and receptacles on separate circuits and have all 120 V receptacles on 20 A breakers (wire either 12 AWG copper or 10 AWG aluminum)? My house originally (circa 1970) had ten 120 V circuits, six were 15 A and four 20 A (clothes and dish washers plus two more in kitchen). All the receptacles in the bathrooms, bedrooms, living rooms, garage, exterior are on 15 A circuits. Lighting and receptacles are mixed on the same circuits.
    – Jim Stewart
    Commented Jul 6, 2021 at 19:28
  • @JimStewart That would probably be best practice, but I don't know about typical practice in modern times...
    – TylerH
    Commented Jul 6, 2021 at 19:33
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    There is no consistent modern practice. What you describe is typical of code-minimum save every penny mindset. In modern times, the cost differential of the breakers is usually nothing, the labor costs the same unless you are the sleazy builder that actually uses backstabs for low quality failure prone connections to save 30 seconds, and against those fixed costs, the cost of the wire to go to 20A on all receptacle circuits is a small matter. Likewise, the cost of separating the lights from the receptacles is trivial with planning, but some don't plan.
    – Ecnerwal
    Commented Jul 6, 2021 at 21:18
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    Would modern "reasonable" best practice have all 120 V circuits, lighting and receptacles, wired in 12 AWG copper all fed through 20 A breakers or would that be wasteful extravagance?
    – Jim Stewart
    Commented Jul 7, 2021 at 0:23
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    20A is generally overkill for modern lighting.
    – Ecnerwal
    Commented Jul 7, 2021 at 0:29
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Cost wise I would be more worried about having to buy many spools of wire. Cheaper to buy 250’ of #12 than 100’ of #12 and 150’ of #14. Price it.

All your 15A circuits must be laid with wire that is at least 14 AWG copper. You are always allowed to use bigger wire anywhere you want. (although it can be confusing if the wire going into the panel is #12 and wires farther down the circuit are #14).

All your 20A circuits must use 12 AWG wire or larger. You can use larger wire anywhere you want.

15A receptacles are allowed on 20A circuits as long as the circuit has at least 2 sockets (any duplex recep will suffice).

A circuit with all #12 wire and all 15A receps, can be characterized as either a 15A circuit or a 20A circuit at your discretion.

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