downsizing wire size to a light fixture

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Stevenfyeager

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United States, Indiana
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electrical contractor
We have some very stubborn LED step lights the customer insisted using. Came with their own shallow boxes that have to be used. #12 gauge wire coming in on a 20 amp circuit. (I know, I should have run #14 but I didn't) Is is ok to pigtail off of the #12 using a #14 or smaller in each box to each fixture ? I realize most fixtures have smaller gauge wires on them. Thank you.
 
Stevenfyeager said:
What about the small gauge wires we see on most light fixtures ? They seem to be about 14 or 16 gauge stranded.
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There is an entire article that answers your question... Article 402 Fixture Wires, 2 pages.
Note to self: Code proposal for 2026 (yikes) change title to Luminaire Wires.
 
I realize now that I'm quite ignorant of the proper application of 402 Fixture wires.
I thought they were wires inside listed devices, I guess. Or maybe flexible plug-in cords.
I can't think of having seen these on jobs: I think they would have stood out as too-small conductors...
For example, 402 points back to 240.5 which describes required OCPDs for fixture wires, and 240.5(B)(2) says you can have 50' of 18 AWG on a 20A breaker!!!!! 100' of 16 AWG on a 20A breaker!
I'm sort of dumbfounded that I'd never noticed this before. 402 mentions having it installed in conduit, so this is not about plug-in cords or flexibility.
I wonder how often people see this used, and in what kinds of circumstances.
 
CodeQuandary said:
I realize now that I'm quite ignorant of the proper application of 402 Fixture wires.
I thought they were wires inside listed devices, I guess. Or maybe flexible plug-in cords.
I can't think of having seen these on jobs: I think they would have stood out as too-small conductors...
For example, 402 points back to 240.5 which describes required OCPDs for fixture wires, and 240.5(B)(2) says you can have 50' of 18 AWG on a 20A breaker!!!!! 100' of 16 AWG on a 20A breaker!
I'm sort of dumbfounded that I'd never noticed this before. 402 mentions having it installed in conduit, so this is not about plug-in cords or flexibility.
I wonder how often people see this used, and in what kinds of circumstances.
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The rule in 240.5 is why factory made fixture whips can have 14 or 16 AWG conductors, as long at the wire is fixture wire. If you make your own using THHN, the wire size would have to have the same ampacity as the OCPD. If you make your own using fixture wire, you can use the rule in 240.5.
 
IIRC, I "think" you can use 18 AWG fixture wire up to fifty feet with a 20 amp OCP. What I am unclear about is if it has to be bought as an assembly (fixture whip).

Is it allowable to put three (or two even?) #18 fixture wires (TFFN?) in a 50' length of 3/8" FMC in the field?
 
Russs57 said:
Is it allowable to put three (or two even?) #18 fixture wires (TFFN?) in a 50' length of 3/8" FMC in the field?
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No, 3/8" FMC is limited to 6' in length.

It's typical when you have long rows of fixtures connected end to end to run the fixture wire through the row. For a 20 amp circuit the #18 would be limited to 50'. For large conductors (#16) the maximum length increases to 100'.
 
I've often wondered about this myself too. When installed something like can lights and wiring the #14 to the fixture wire, I've always just felt like something was wrong and that fixtures should come with #14 or #12 to match branch circuit OCPD's. I know it's unlikely, but I just envision a partial short in the fixture, pulling enough amperage to burn out the smaller wires and start a fire, but not enough to trip the breaker.

I know it's acceptable, but I don't even like putting 15A receptacles on a 20A circuit. Something about everything being 20A all the way through from breaker, to wire, to final device, satisfies my OCD just right, lol.
 
Jerramundi said:
I know it's acceptable, but I don't even like putting 15A receptacles on a 20A circuit. Something about everything being 20A all the way through from breaker, to wire, to final device, satisfies my OCD just right, lol.
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For a given grade of receptacle, the conductive parts of both are the same; only the slot shape is different.
 
Jerramundi said:
I've often wondered about this myself too. When installed something like can lights and wiring the #14 to the fixture wire, I've always just felt like something was wrong and that fixtures should come with #14 or #12 to match branch circuit OCPD's. I know it's unlikely, but I just envision a partial short in the fixture, pulling enough amperage to burn out the smaller wires and start a fire, but not enough to trip the breaker.

I know it's acceptable, but I don't even like putting 15A receptacles on a 20A circuit. Something about everything being 20A all the way through from breaker, to wire, to final device, satisfies my OCD just right, lol.
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You're overthinking this. The fixture wires, even if #18, are large enough to open the OCPD if there is a fault. Nothing will burn up, if this were an issue the NEC wouldn't allow a #18 fixture wire on a 20 amp circuit. Nothing wrong with a 15 amp duplex on a 20 amp circuit either.
 
infinity said:
You're overthinking this. The fixture wires, even if #18, are large enough to open the OCPD if there is a fault. Nothing will burn up, if this were an issue the NEC wouldn't allow a #18 fixture wire on a 20 amp circuit. Nothing wrong with a 15 amp duplex on a 20 amp circuit either.
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I know. I overthink everything, haha. Sometimes it works to my benefit. Other times it throws me into an anxious tail spin.
Just humor me for a second, because I like to question everything.

Question #1) If the #18's are large enough to open the OCPD during a fault, that is to say, large enough to briefly handle an amount of current that exceeds the rating of the OCPD, why are they not considered large enough to handle the branch circuit current from say the switch to a single fixture? Obviously it's different if that branch circuit section is ahead of multiple loads.

Question #2) Isn't there a type of fault that doesn't pull enough current to trip the breaker? I want to say "partial short," but I'm not sure if that's the correct terminology. Perhaps I'm wrong and confusing this with the idea of a fault occurring on an improperly bonded/grounded raceway.

I know per code there's nothing wrong with a 15A receptacle on a 20A circuit, I'm just saying it bothers my OCD. My mind just conjures up the image of someone plugging in extensions to the point of pulling say 18A and burning up the receptacle.

I think the same of pulling a #14 switch leg from a switch fed by a 20A branch circuit and 20A OCPD. I know I'll catch flak for this, but if the OCPD is 20A, I typically pull #12's all the way to the fixture.

And before you ask, yes.. tap conductors bug me, lol.
 
LarryFine said:
Read post #14 again.
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As far as the 15A receptacles on a #12 wire and 20A OCPD is concerned, I was just giving an example about my OCD. I do it knowing its' code approved. I'd feel better if I had official manufacturer's specs identifying that the internal components on a 15A receptacle are rated to handle 20A, but I've never bothered to hunt them down and instead just relied on the NEC as most of us do. Also, you state that this is a only for a specific grade of receptacle, something the code that allows us to put 15A receptacles on a 20A circuit does not dictate as far as I'm aware. But again, the receptacle issue was just an example of my OCD.

The main issue I was touching on was the size of fixture wires. To reiterate, my question in post #16 was...

Question #1) If the #18's are large enough to open the OCPD during a fault, that is to say, large enough to briefly handle an amount of current that exceeds the rating of the OCPD, why are they not considered large enough to handle the branch circuit current from say the switch to a single fixture? Obviously it's different if that branch circuit section is ahead of multiple loads.

As with the 15A / 20A receptacle example, I'd just feel better if it was #14 or #12 all the way through from breaker up to and including internal fixture components... unless the fixture is on a ballast/driver that contains, or the fixture itself contains, some sort of additional OCPD.
 
Jerramundi said:
Question #1) If the #18's are large enough to open the OCPD during a fault, that is to say, large enough to briefly handle an amount of current that exceeds the rating of the OCPD, why are they not considered large enough to handle the branch circuit current from say the switch to a single fixture? Obviously it's different if that branch circuit section is ahead of multiple loads.
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A short-circuit fault is a short-duration event, so there's little time for insulation damage.

As with the 15A / 20A receptacle example, I'd just feel better if it was #14 or #12 all the way through from breaker up to and including internal fixture components... unless the fixture is on a ballast/driver that contains, or the fixture itself contains, some sort of additional OCPD.
Click to expand...
The rules that cover our work are different from those that cover manufactured assemblies.
 
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