How many 20 watt lights on a 20 amp circuit?

shortround3 · #1 Feb 11, 2008, 01:03 PM

I have a 20 amp 120 volt circuit. I have been told that load for a breaker should be 85% of rated capacity and that the formula for determination of required amperage is A=W/V. If this is correct it appears the 14 20 watt undercabinet lights I need would total 2.33 amps. Is this correct? If not, how is the problem worked out? Thanks.

Stratmando · Feb 12, 2008, 08:37 AM

TK and CeilingFan meant 1920 watts on a 20 amp circuit. They provided info for 15 Amp circuit.
Volts X Amps = Watts(120X20=2400 Watts)--------2400 X .8= 1920 Watts(80%)
1920/20(bulb)=96 bulbs can be used on 20 amp circuit.
They both provide good info, was probably late.

shortround3 · Feb 13, 2008, 07:09 PM

You guys are great. Thanks much. Shortround3

Washington1 · Feb 13, 2008, 09:50 PM

Quote:

A circuit can have no more than 80% for a lighting circuit than can run more than 3 hours

Please read what TK said in quote<-----------

Short,

If you are installing your lights in a residential setting, then the 80% does not apply. You can use 100% of supplied power. Yet it doesn't hurt to apply the the 80% rule.

So:
120v(20A)=2400w
2400w/20w=120 lamps

EPMiller · Feb 13, 2008, 10:05 PM

Quote:

Originally Posted by Washington1

If you are installing your lights in a residential setting, then the 80% does not apply. You can use 100% of supplied power....

Actually the 80% does apply if you have circuit breakers. Breakers are manufactured to trip out above 80% load after about 3 hours. Less time if you are closer to the max rating. If you load it more than 80% you will be running to the panel regularly, sometimes they will not even stay reset until they cool. Certain older (thankfully no longer made) brands were notorious for not tripping out unless there was a dead short. I prefer the old style fuses over those panels!

Washington1 · Feb 13, 2008, 11:02 PM

EP,

I'll get back to the breakers after you respond to this question.

Where in the NEC does it say you must consider a residential general lighting load continuous?

stanfortyman · Feb 14, 2008, 04:01 PM

Quote:

Originally Posted by EPMiller

Actually the 80% does apply if you have circuit breakers. Breakers are manufactured to trip out above 80% load after about 3 hours. Less time if you are closer to the max rating. If you load it more than 80% you will be running to the panel regularly, sometimes they will not even stay reset until they cool.

WOW! This is SO wrong it's not funny!



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