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shortround3 · Feb 11, 2008, 03:03 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.

tkrussell · Feb 11, 2008, 05:31 PM

A circuit can have no more than 80% for a lighting circuit than can run more than 3 hours. Or 1440 watts or 12 amps for a 15 amp circuit.

1440 / 20 watts = 72- 20 watt lamps.

ceilingfanrepair · Feb 11, 2008, 05:32 PM

And your calculation is correct, the 14 bulbs should draw 2.3 amps.

Stratmando · Feb 12, 2008, 06:37 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.

tkrussell · Feb 12, 2008, 01:13 PM

Oops sorry, 20 amp was mentioned, Thanks, Strat

shortround3 · Feb 13, 2008, 05:09 PM

You guys are great. Thanks much. Shortround3

Washington1 · Feb 13, 2008, 07:50 PM

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 80% rule.

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

EPMiller · Feb 13, 2008, 08:05 PM

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, 09:02 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, 02:01 PM

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!

Stratmando · Feb 14, 2008, 05:22 PM

Washington, If you went to work and left the lights on(2300 Watts) or other cuircuit on, Would it be closer to 3 or 8 hours?

EPMiller · Feb 14, 2008, 08:12 PM

Originally Posted by Washington1

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

It's not NEC, it is just a fact of the way circuit breakers are made. They will trip out after a certain time interval when there is more than 80% rated current draw.

Originally Posted by stanfortyman

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

Are you saying that manufacturing breakers that way is wrong or that I am way off base with my explanation? If you don't like being limited to 80% continuous then use a plug fuse panel with type W fuses, but I don't think they are manufactured any more. All plug fuses that I can find now are time delay, I don't know if they will blow after continuous use at less than full load. Can someone enlighten me on that? I have never installed a fuse panel.

EPMiller · Feb 14, 2008, 08:20 PM

Originally Posted by shortround3

...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.

That is the correct formula and you did the math right. However if these are 20w fluorescent tubes you will have to add the ballast losses into the equation to get the total current draw.

Washington1 · Feb 14, 2008, 08:59 PM

Originally Posted by Stratmando

Washington, If you went to work and left the lights on(2300 Watts) or other cuircuit on, Would it be closer to 3 or 8 hours?.

Strat,

Come on, are you serious!
To say a residential "General Lighting Load" falls under the 80% rule would be a mis-understanding of code. It's more a design issue, not a code issue. If you want to apply the 80 percent rule to your design, then that's fine.

EP,

Circuit breakers are designed to carry 100% of their rated current while the NEC dictates an 80% application.

stanfortyman · Feb 15, 2008, 02:55 PM

Originally Posted by EPMiller

Are you saying that manufacturing breakers that way is wrong or that I am way off base with my explaination? If you don't like being limited to 80% continuous then use a plug fuse panel with type W fuses, but I don't think they are manufactured any more.

You sir are way off base. This has nothing to do with what I like or dislike.
The 80% rule has NOTHING to do with fuses or breakers.
The NEC dictates the 80% rule under certain circumstances. This applies to fuses or breakers.
A breaker can hold 100% of it's rating FOREVER. This is one reason for the NEC restriction.

You really can't be serious when you say a breaker will trip at 80% of it's rating.

If you like I can drum up some NEC text for you to read up on.

EPMiller · Feb 15, 2008, 04:11 PM

Originally Posted by stanfortyman

A breaker can hold 100% of it's rating FOREVER. This is one reason for the NEC restriction.

stanfortyman,

I stand corrected. I got the 80% information from an electrician that I got some of my training from and took it as gospel. I went to SquareD and did some research on trip curves. See document 0730CT9801.pdf on their QO line. I also found elsewhere that breakers must be able to open or close at 80% or less of their rating, which would explain why I have found breakers that will not stay ON even though they were working below (but close to) their rating. Once they cooled and the load was changed things were fine.

So we can theoretically run 120 20w incandescent lamps on a 20 amp circuit. Of course the inrush would be around 100 amps, hope you have a GOOD switch. And with manufacturing tolerances what they are, I still would use the 80% rule for design. Don't use these numbers for fluorescent lights or any ballasted light (think CFLs) for that matter.

Handyman2007 · Feb 15, 2008, 09:38 PM

I agree,, I have been doing this a long time and have yet to see a breaker trip at ONLY 80% for ANY length of time. Although it may be a safe thing to have happen, I would suspect a bad breaker if this actually did happen.

Washington1 · Feb 16, 2008, 10:36 AM

You are welcome for the information!

whitehallmike · Apr 22, 2011, 06:49 PM

And you are correct here breakers are able to withstand 100% of their rated load and in casess more as inverse time circuit breakers are based of the time/current table

whitehallmike · Apr 22, 2011, 06:50 PM

Article 210... 125% which is th inverse of 80%