When (if ever) can 14ga wire be used with 20A circuit breaker?

Can wiring to a light off a 20A circuit be done in 14ga wire? What about wiring to an upstream 15A receptacle? Or, does all wiring on a 20A circuit breaker need to be at least 12ga?
Thanks
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That is correct; with the slight exception if the #14 wire has a separate 15a breaker on it. Some people will use #14 on a lighting switch leg, on the theory that it couldn't possibly see more than a couple amps; but it is a code violation.
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You simply cannot use 14ga wire on a 20 amp circuit.
Any electical inspector that sees that in a box will simply flag it and make you change the line or the breaker. (just as an FYI)
If you pull too much near the 15amp side and exceed the limits it wont stop for another 5amps. In effect you can be overloading the line. It may not happen on your lighting idea, but why bother with the risk for a few bucks more on getting the correct wire.
Using 12ga on a 15amp is perfectly fine. Its nice that down the road if you need to upgrade you can just change the outlets and breaker and you got a 20amp line.
Tom
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blueman wrote:

It will work just fine for a switched light but is technically a code violation, even if the wire ampacity is 20A (which 14 gauge THHN-2 is.) You could argue that you were using the 10 foot tap rule (or 25 foot, I don't remember which would apply better here), but you'll get shot down.

No. What if somebody plugs a 10A load into both halves of the duplex receptacle, or extends the circuit someday?

Yes. 12 ga is not that much more expensive than 14 ga, and your time is worth something; do the job right.
Best regards, Bob
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blueman wrote:

I would say never. I hear folks say it can be used between switch and bulb, but that just does not seem right. Breaker protects the wiring, nothing more nothing less.
Perhaps someone puts in a heavy duty bulb of some sort that is broken and begins to draw lots of current. The circuitry wont be protected by the breaker. An all around bad idea.
To put it simply, the wiring should _always_ match the breaker.
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This depends on the situation. If the box has #12 and your UL listed light fixture is #14 probably #16 then it is fine.
If your adding the #14 yourself no must be #12.
Open your electric water heater some day, you will have at least #10 feeding #12. It is inside a UL listed and labeled appliance, there for ok doky.
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Huh? What are you saying?

Yeah, I noticed mine is. I presumed that was because the #12 wires are THHN and between two things rated for higher temperatures. No? Still, it seemed like a foolish place to save $0.02.
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Toller wrote:

If the wires melt and burn within the water heater housing, what is the end result?
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wrote:

A 4500w water heater only pulls 18.75a. There is plenty of safety factor in a #12 as a "fixture wire" but if it is protected by a 30a breaker the "branch circuit" conductors must be #10.
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snipped-for-privacy@aol.com wrote:

if it only pulls 18.75a, why not put 12 everywhere? Protection is not tied to the loading of the appliance, its tied to what happens under failure conditions. If that appliance fails with a resistive short, it can pull up to the load allowed by the circuit protection.
More likely they ensure that melted and burned wires wont start a fire within the appliance. And that the appliance is obviously bad anyway so loosing some wiring is not a significant penalty. You dont want to loose wiring in your wall due to a bad appliance.
So 12 is not 'safe' in terns of it not failing, but in that if it does fail, its not goign to cause any additional significant damage.
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wrote:

You notice I said the "branch circuit" conductors ARE required to be #10 (when using a 30a breaker). If you are not happy with what a recognized testing lab says is OK under the covers of listed equipment, you will have to take it up with them. I'm sure a testing lab would point you to facts about exactly how much temperature rise you actually get in 12ga wire with a 30a load. It is far from being enough to damage a 90c conductor.
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snipped-for-privacy@aol.com wrote:

You misread my post. I didn't say it was not OK within the appliance. What I said was that it was OK, *not* due to the fact that it wont burn, but to the fact that it wont cause a fire when/if it does.
Just because you can put a 12Ga wire on a circuit within an appliance does not mean you can do that outside of the appliance.
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wrote:

10-4 ... peace
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Three scenarios that are legal come to mind. "Fixture wires" in a luminaire can be as small as 18ga on a 20a circuit. "Taps" can use smaller wires but they need the appropriate protection at the load end and be enclosed in conduit the whole way. Dedicated motor circuits can really blow your mind. You could have 14ga wire on a 40a breaker if it served a 120v 1hp motor. The same might be true of a small AC compressor. This is not something a weekend handyman should be doing. You have to understand article 430 of the NEC to know what is legal. For most folks you should just use the fail safe rule 240.4(D) that has the familiar 14ga= 15a, 12ga = 20a, 10ga = 30a.
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