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I am looking at replacing a couple of old, porcelain style single bulb fixtures in my basement with some 4 lamp fluorescent fixtures. I am actually running new wire to the fluorescent fixtures from a 100amp sub panel in the basement, and not reusing then existing wiring that goes to the current light.

As I was taking the porcelain fixture down I discovered that there is 14 gauge wire going to it - ok. There are 3 such fixtures in the basement. There are also other things on this same breaker upstairs in the main living part of the house - 3 or 4 outlets that are on 12 gauge wire. The breaker is a 20 amp breaker for all of these. Is this ok to mix 14 & gauge wire like this? If not, why did they do it?

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It is generally NOT okay to have a 20A circuit breaker on a circuit that has any 14 gauge wire -- and most homeowners won't run into the exceptions

If there's any #14 wire anywhere downstream, you must use a 15A breaker to protect that wire. It's about fire prevention. #14 wire is rated for 15A. Sure, it will carry more, but the N.E.C. ampacity ratings take into account the resistance of the wire insulation to heat and other factors. Bottom line, #14 wire equals 15A breaker.

It's fine (or even required) to oversize the conductors, but not to oversize the breaker (except under rare circumstances that you'll never need to know about unless you become a licensed electrician or wind up doing funny things with motors that invoke Article 430). #12 wire on a 15A breaker is fine. The wire is protected. For longer runs, for example using #12 wire on a 15A breaker or #10 wire on a 20A breaker will reduce voltage drop. There are calculations for wire size related to voltage drop over distance that tell you when you need bigger conductors.

Why was this done?

Hazarding a guess, the person who did it might have had some #14 Romex lying around the garage. Maybe they didn't want to run out and buy some #12, or felt they couldn't afford it. Maybe they bought #14 on purpose just because it was cheaper and they didn't understand the requirements. Or maybe they reasoned that it was okay since the only load on those #14 wires would be the lights, which would never draw more than about an Amp per fixture. That's true enough, technically speaking, as long as nothing ever goes wrong, but it still illegal (actually illegal, because it violates the N.E.C. rules).

But stuff does go wrong, so you never intentionally create a situation that has the potential to start fires or electrocute people.

A few (far from exhaustive) examples of things that could go wrong include; somebody comes along and replaces one of those fixtures with a bigger fixture, or with one of those fixtures with a receptacle on it, or adds a convenient receptacle elsewhere on the 14 gauge stretch of the same circuit, or both. Or something goes wrong with one of the light fixtures that causes it to steadily draw more current than it was meant to, and all of a sudden you have a circuit breaker that will happily feed 20A onto #14 wire for long periods of time.

  • Here's another good explanation, regarding the fixtures being safe on the #14 wire segment as long as nothing ever goes wrong, but code still prohibits this: diy.stackexchange.com/a/11497/24137 – Craig Jan 18 '15 at 18:41
  • I'd be careful using words like "any" and "never". There's exceptions to the max breaker size for a given wire size; they might not be common in homes but accuracy's nice. – ChiefTwoPencils Jan 19 '15 at 10:23
  • @ChiefTwoPencils fair enough. It does seem pretty unlikely, though, that a DIY'er needs to do ampacity calcs on their THHN conductors (or whatever) in conduit under limited circumstances in order to enable them to oversize the breaker, doesn't it? If they don't oversize the breaker, then the wire remains protected. The light fixtures in this particular case won't draw too much for the #14 wire, true, unless there is damage or a defect of some kind, in which case a resulting electrical fire would NOT be constrained within metal conduit, etc. I was just trying to consider the context. :-) – Craig Jan 19 '15 at 18:05
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    True. My concern wasn't with diy-er doing calcs, but asking about what they come across in the process. FE, I have a workshop on my property with motor circuits; the next owner may see some untypical stuff and be confused by statements made about a specific context. As I spend more time here I see that there's a lot of localized q&a where some may come across a similar question but with different circumstances and be sent in the wrong direction or otherwise made to believe their wiring is dangerous or wrong when it's not. I prefer to point out the exceptions just to be clear. – ChiefTwoPencils Jan 19 '15 at 20:40
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    Should we have a Q&A on here about what the exceptions are? – ThreePhaseEel Jun 8 '16 at 23:09
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Keep in mind that every electrical circuit generates heat, and the smaller the wire (i.e. the higher the gauge numter) the more heat is generated. Heat generated by electrical circuits can be deceptive, because it's not related directly to temperature, only the rate of temperature increase. The more insulating material that's around wire, a bulb, or other component generating heat, the less heat is removed from the heat-generating component, and if the ability of heat to leave a component can't keep up with the heat generated, then you'll end up with a very dangerous situation.

We had this happen a few years ago when a down sleeping bag stored on the top shelf in a closet came out of it's stuff-sack and expanded around a 25W incandescent lightbulb. A 25W bulb generally runs quite cool, but in this case there was no place for the heat to go, so the temperature rose until we smelled smoke and discovered the sleeping bag smoldering and smoking in the closet! The sleeping bag was ruined, but it could have been much worse.

The same goes for putting AWG 14 wire in a 20A circuit. Nine times out of ten you'll have no problem, but if the circuit is carrying any current, the wire will generate heat, and if the heat can't escape, you've got a potential fire on your hands. There are doubtless many situations under which a AWG 14 wire carrying close to 20A will generate heat faster than it can be dissipated through the surrounding materials. Using AWG 12 wire reduces the rate at which heat is generated by the wire pretty much below the ability of any construction that meets code to dissipate the heat.

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there is a tap rule in the NEC that allows the you to run 12awg and tap off it using 14awg. for example from breaker to outlet #1 12awg, from outlet #1 to outlet #2 12awg, but while in outlet #1 you may tie the wire from the breaker and the wire to outlet #2 to a 14awg that would go to the recep.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Oct 1 at 18:57
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    Not to a receptacle, no. In a residence, only for spurs to luminaires, and only in certain lengths. And this is an obscure bit of Code, and I would not be surprised one bit if the inspector didn't know it (or prohibited it in a local amendment) and nixed it. Then you have a Code argument. – Harper - Reinstate Monica Oct 1 at 20:06

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