Thin stranded wiring within light fixtures and 12 ga. ci

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hardworker

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I have a theoretical question on the subject of wire gage, breaker amp rating and code. Why is it permissible by code to wire lights with 12ga., use a 20amp breaker, when everybody knows the wire within the light fixtures are probably 20ga. stranded or lighter? How does this make sense?
 
Re: Thin stranded wiring within light fixtures and 12 ga. ci

The fixture itself is load limited by the bulb(s). Under normal circumstances it can't draw enough to overload the fixture wire. In a dead short situation, the (relatively short length) fixture wire would hold long enough for the breaker/fuse to pop.
 
Re: Thin stranded wiring within light fixtures and 12 ga. ci

Good answer, Tonyi. Let me add another 4 cents worth (it?s ?double coupon Thursday?): The purpose in life for the 20 amp breaker is to protect the #12 conductors from the breaker panel to the fixture. So what protects the #18 wires inside the fixture (code does not allow #20 for fixtures)? My answer is the sticker that the manufacturer puts on the fixture, saying ?Max 60 watt bulb,? or ?Max 100 watt bulb.? How so? Because (I believe) that sticker is not there for electrical reasons.

Even if the homeowner installed a 500 watt bulb, it would draw under 5 amps on a 120 volt circuit. So it would not overload the #18 wire. But the heat generated by the bulb itself (ever tried to unscrew a 500 watt bulb just moments after you turn it off?) might create a risk of setting the nearby materials on fire.
 
Re: Thin stranded wiring within light fixtures and 12 ga. ci

I get your answer and agree. Tell me this. In an earlier code question I had for the forum about a 14ga branch on a 12 ga. circuit, I was advised to step down the circuit from 20amp breaker to a 15, because of the lighter wire.

Why is it any different with the lighter wire in the fixture on a 12 ga circuit?
 
Re: Thin stranded wiring within light fixtures and 12 ga. ci

First you can not protect 14-2 with 20 amp breaker.
The fixture will limit the load
Your 14-2 could easily be added to or load changed
If the fixture wires burn up the repair will be no problem,they are acesable
 
Re: Thin stranded wiring within light fixtures and 12 ga. ci

What goes first during a mishap? The circuit is 12ga w/ a 20amp breaker and the fixture has 18ga. If the problem took place within the fixture would the 20 amp react before the 18ga. did a melt down?
 
Re: Thin stranded wiring within light fixtures and 12 ga. ci

Hardworker, the rules for fixture taps are in Article 402 Fixture Wires, with overcurrent protection per section 240.5
Take a minute to review 240.5...this is not branch circuit wiring but an allowance to use smaller conductors than 240.4 requires.

[ February 19, 2004, 01:47 PM: Message edited by: tom baker ]
 
Re: Thin stranded wiring within light fixtures and 12 ga. ci

Take a minute to review 240.5...this is not branch circuit wiring but an allowance to use smaller conductors than 240.4 requires
Click to expand...
Take Tom's advice, you could run 18 AWG in a fixture up to 50' on a 20 amp breaker, it does not have to be factory wiring. :)
 
Re: Thin stranded wiring within light fixtures and 12 ga. ci

I understand the code and I always follow it. I just want someone to explain to me from an engineering point of view, what the real difference is. Code allows it. Ok, but I want to understand it.
 
Re: Thin stranded wiring within light fixtures and 12 ga. ci

Along with Tony's good info I would add the 18 AWG is sufficient to carry the load, the load will not ever increase.

If a line to line or a line to ground fault happens the 20 amp breaker or fuse will still open fast enough to prevent the 18 AWG from lighting up.

That is why when you look at 240.5 you see 18 AWG up to 50' and 16 AWG up to 100'.

The smaller the wire the more impedance, the longer the length the more impedance.

By limiting the lengths the NEC has made sure the impedance is still low enough to operate the breaker or fuse.

This is the same area of the code that allows listed appliance cords to be smaller than 12 AWG and still be plugged into a 20 amp outlet. :)

[ February 19, 2004, 09:05 PM: Message edited by: iwire ]
 
Re: Thin stranded wiring within light fixtures and 12 ga. ci

Thanks, you made it very clear. I now understand what is behind the code.
 
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