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

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

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

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.

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).

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.

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

This answered a couple questions that have been nagging me for a while now -- hugely appreciated, thank you! :)

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.

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.

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!!!

WOW to you! I'm learning about running wire in my bathroom for an additional light which has 3 LED lights. You are the only one video I've watched that really explained things to me in the way I understand. Thanks for sharing

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?

First and foremost I want to thank you for taking your time to post these videos. I appreciate them deeper than you know. I'm one of those "ADHD" guys and I went to the electrician's apprenticeship at 37 years old. I learn best by hands on. And the apprenticeship I went through was well over 90% book work or computer. I went through it in the middle of the pandemic too so a lot of stuff was on zoom as well. Completely horrible for an ADHD brain like mine. I learn best by physically doing the work and then going back and seeing how it works because that makes it more interesting for my brain. I passed my apprenticeship a little above average but not by much and half of that was because I found the answers to a lot of my homework so I didn't end up doing a lot of it. Which also didn't help retaining any kind of knowledge. But now that I've been in the field working, I'm going back and watching these videos. It's like the puzzle pieces of my brain are finally falling into place. I'm a solid electrician when it comes to my work. It's always clean and it is always correct. But to have the understanding behind everything allows my troubleshooting skills to get that much better and to explain things a little bit better to the people around me when I'm doing my work. So again thank you for these posts.

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.

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?

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.

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

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?

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.

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.