Should You Be ONLY Using 12/2? When Can You Use 14/2?

When it comes to using NM cable to wire a house, there are 2 general camps that contractors fall into. Those that wire the house in 14/2 and those that will wire the house entirely in 12/2. But which is the correct way to do it? In today’s episode of Electrician U, Dustin dives into this topic and gives us some explanations and tips on doing it correctly.

00:00 - Intro
00:15 - Which one do I use?
02:05 - Parallel Circuits
08:25 - Conclusion

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So, the initial response to this question is to check with your Local AHJ (electrical inspector) to find out their requirements and the requirements of the jurisdiction you are working in. The inspector will have the final say of what they are willing to inspect. The current electrical code (2023 NEC is the most recent, however, many jurisdictions are still using the 2020 NEC) is also a resource to be referenced. Articles 240.4, 210.21 and 310.16 are great articles governing circuitry and wire sizing. Something to keep in mind is that type NM cable falls under the 60-degree rating, and not the 75-degree rating used in most commercial wiring.
The next, and most practical, explanation of what to use to wire a house would be the circuitry type itself. In recent discussions on Electrician U, we have covered Series wiring and Parallel wiring. From those discussions, we have deduced that (for amperage) on a series circuit, amperage is the total of ALL the loads, and that amperage is throughout the entire circuit. This is because we must travel THROUGH one load to get to the next load. But, on a parallel circuit, the amperage is in direct relation to the resistance of the particular load within each leg, and the combined amperage of everything that is on the entire circuit is only on the conductors that are feeding. This is where we will focus on some options we have (again, depending on what our local AHJ will accept).
LED lighting draws considerably less amperage than its incandescent/fluorescent counterparts. So, while installing 12/2 NM cable from the panel throughout the power portion of the circuit, once we leave the load side of the switch, that can be run in 14/2 since amperage draw is less. Consider this circuit- let’s say we have a parallel circuit with one receptacle feeding a 5a load, a second circuit feeding an 8a load, a light switch that was controlling 4 LED can lights at 1a, and a 3rd receptacle at the end completing our circuit. With the first 5a load running, the amperage draw would be 5a from the breaker through our load. Once we started our second 8a load, the conductors around that load would be drawing 8a but where they enter the circuit conductors “feeding” the other loads, the amperage draw is 13a. If we added the 4 can lights, those feeding conductors are drawing 14a now, but those on the load side of the switch are only drawing 1a! So, as long as the local AHJ accepts it, many contractors will run 12/2 conductors throughout the receptacle circuit and up to the line side of the switch, and swap over to 14/2 for the load side of the switch throughout the lighting portion. Over the course of a large home (or many smaller homes added together), the cost/labor savings can be substantial!

[oddtech]

I always ran separate 15 amp circuits to the lights and 20 amps to receptacle circuits. Blow a receptacle breaker the lights stay on

[joelboutier]

IN THEORY that is correct… the switch leg will never carry the load the home run will carry. BUT, I’m pretty sure that code wise, the conductor size must match the rating of the breaker ampacity ALL THROUGHOUT THE CIRCUIT… meaning a #14 AWG switch leg on a 20 amp circuit still violates code.

I think the new standard method of new construction home wiring is to run 15 amp circuits for lighting & smoke detectors because 14/2 & 14/3 is so much cheaper… especially for travelers & smoke detector interconnect… than 12/2 & 12/3, and LEDs are so efficient anyways. LEDs just don’t require much current like old incandescent bulbs.

For circuits supplying receptacles & other power, electricians use #12 AWG/20 amp circuits in new construction to accommodate appliances that draw more current. A vacuum might draw 7 amps & a space heater might draw 12 so #14 AWG/15 amp circuits can be inadequate for powering receptacles.

I still don’t think that #14 AWG switch leg on a 20 amp circuit would would pass with our inspectors even if in theory what you’re saying is totally correct. There are many portions of that circuit that will not carry the full load.

[austinluepkes]

I own a duplex where they basically did this on some of the receptacles. 20A breaker with some 14awg feeding the last few receptacles. Tenant had his gaming PC setup (multiple monitors, high power pc stuff etc) and nearby baseboard heat going. The 14awg was probably drawing over the 12 amps it really ever should have been drawing. With the combination of the heat from the pc and baseboard and the overloaded wire, it melted off the receptacle. Luckily didn’t start a fire but that’s how I found out about it. I imagine a 15 amp breaker may have prevented the problem which is what I put on instead.

Moral of the story, DONT MIX AND MATCH WIRE!!!

[benchociej]

NEC 240.4(D)(3) - if you have any 14 AWG copper wire, the overcurrent protection cannot exceed 15A

[allenshepard]

Great explanation.
After working with cheap #18 luminary wire behind a fixture that breaks, I'd rather stick with something larger.
Plus, if they repurpose the circuit for a vent fan or warming light they will need the current.
Never cheat code, always go above the minimum.

[dtemp]

What you were describing at 1:45 to run 14AWG wire on a 20A circuit but just for the lighting legs of the circuit, I believe that is against code essentially everywhere, although I'd like to be proven wrong.

[gantmj]

Wire is sized for the breaker it's on, regardless of the load that may be on it.
If the 14 AWG romex somehow gets cut on a can light housing or the LED fixture goes bad, and that draws a heavy load without shorting, the wire will get too hot before a 20A breaker trips at 27+ amps.

[philibertperusse]

I completely disagree. If you have a 20 amps breaker, code does not allow you to reduce conductor size based on actual load except in very very specific situations. The idea being that if you have an overload (say a high impedance short circuit) in the circuit and you have reduced the conductor, it may melt before the cb trips. By sizing all conductors according to upstream protection you prevent this situation from happening

[phihead]

By me, the only time 12 gets used on a 15A circuit is on really long homeruns to guard against voltage drop. And I never use 14 on a 20A circuit. We use 12 gauge(20A), for kitchen receptacles, bath receptacles, laundry receptacles, dining receptacles, and garage receptacles. Everything else gets 14 gauge(15A).

[miket]

The only thing I see wrong here is if you put 14/2 onto a circuit that has a 12/2 home run and is fed by a 20-amp breaker, you’re actually not in compliance because all the cabling that comes off a 20-amp breaker needs to be rated for 20 amps.

[rty]

I was always told NEVER to mix gauges of wire. For example, if I run a 12 gauge to an outlet off your light circuit, the 14 gauge wire would get hot in the wall, with the breaker never going to trip. A big no-no. I was told to protect the circuit for the lowest wire gauge and in your example. if you use #14 for your lights, the circuit breaker should be a 15a. Not 20a
Correct?

[kimowilliams]

It's the same thing for us down here in the Caribbean. We use 14/2 for the light leg & 12/2 for everything coming out of the main breaker panel

[Nick-zgym]

Woah. But if the homeowner goes screwing in a bunch of old incandescent bulbs, or one of those screw-in adapters that turns a light socket into a receptacle, or a cheap light fails in a weird way that draws too much power but isn't a dead short, then it's entirely possible to overload that 14AWG wire if it's on a 20A breaker. Is the breaker meant to protect the wiring or isn't it?

[robertthompson]

Back in the days of incandescent lights and plug in the vacuum cleaners, bigger wires meant brighter lights and stronger vacuums. With LEDs and cordless electric, the demands on those circuits are much lower. Still remember to oversize the car charger, dryer, and range circuits. Reducing the voltage drop will save the customer thousands of dollars over the life of the home.

[UrgoMeister]

Something seems off in the presentation. I thought the NEC forbade running cables with smaller gauge wires than the breaker’s rating for the branch. E.g., can’t use 20 amp circuit breaker to protect a 14 gage (15 amp rated) cable. Otherwise, a shorted current of say 19 amps wouldn’t trip the breaker but it would overheat the 15 amp rated cable and potentially cause a fire. Do you have the NEC reference for this?

[OtisPlunk]

The drain pump on my Whirlpool washing machine is fed by 21AWG from the factory with a factory vampire wire piercing connection for termination. The bean-counters probably got a nice Christmas Bonus for sizing the wire "just right". The vampire connection caused the wire to corrode and fall off the pump. That's how I found this ingenious bit of engineering.

[davidkahler]

I don't think this is what you intended, but this video comes across as claiming you can run 14ga on a 20a breaker depending on the intended load. Which would be really bad if the intended load changed to something not intended. You should clarify.

[ronlovell]

In Canada, 14 gauge (15 amps) is commonly used for majority of homes outlet and lighting circuits, usually combind for individual rooms. 12 gauge(20 amps) is required for kitchens/dinning rooms, bathrooms, utility rooms and garages.

[yeroca]

Personally, if I was building a house right now, I'd go with 12/2 everywhere, just because you don't know how you're going to use the house in the future. A little extra headroom feels like a good idea. I'd bet that upgrading even just one branch at a later date is going to cost more than the difference of using 12/2 versus 14/2. 12/2 is harder to work with, though.

[jessequentin]

Glad to see a lot of concern in the comments, reinforces my confusion watching the video. I am not an electrician but I've done a little research for DIY purposes. My understanding: 12-gauge wire needs 20-amp breaker protection, 14-gauge wire needs 15-amp breaker protection. Running 14-gauge wire anywhere in a circuit that is connected to a 20-amp breaker is a no-no because 14-gauge wire is only rated for 15 amps, so if there is some sort of fault condition in the circuit that draws more than 15 amps but less than 20 amps you could have a potential fire hazard.