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.

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.

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

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.

Oops sorry, 20 amp was mentioned, Thanks, Strat

You guys are great. Thanks much. Shortround3

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

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!

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?

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!

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?

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.


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.

...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.

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.

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.

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.

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.

You are welcome for the information!

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

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

Interesting old thread.

Seems to be some confusion about the 80% rating, in general.

To begin with, NEC carry's over the 80% rule from Underwriters Laboratories as applies to circuit breakers.

From the 2008 edition of the NEC:

Article 100- Definitions
Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.

II. Branch-Circuit Ratings
210.19 Conductors — Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.

(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any
Adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

210.20 Overcurrent Protection.
Branch-circuit conductors and equipment shall be protected by overcurrent protective devices that have a rating or setting that complies with 10.20(A) through (D).

(A) Continuous and Noncontinuous Loads. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.

From a UL white paper, without purchasing the actual UL 489 :

http://www.ul.com/global/documents/offerings/perspectives/regulators/electrical/newsletters/MoldedCaseCircuitBreakersMG.pdf


See Page 14:

38. 100 Percent Continuous Rated — Unless otherwise marked for continuous use at 100 percent of its current rating, a circuit breaker is intended for use at no more than 80 percent of its rated current where in normal operation the load will continue for three hours or more.


Some leeway is given by the NEC with this statement:

"the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load"


Next, how does this relate to residential. The NEC does not have any reference to if a circuit is in a home or anything that is not a dwelling.

A circuit is a circuit, no matter where it is located. The issue is, can the load on this circuit operate for more than three hours, if so, the breaker cannot be loaded more than 80%.

Now, I don't know where the concept that any circuit in a home will never be used more than three hours. I know I live in my home more than three hours.

I can easily have a general purpose lighting circuit, that can serve lighting fixtures and some receptacles. I can easily have a 15 amp general purpose lighting circuit with 18- 100 watt light bulbs in permanently installed lighting fixtures, plus any amount of table lamps and any amount of appliances, such as TV, stereo, etc. plugged in.

After wiring this circuit, and I leave the home, I would need to be very sure that all the 18 fixtures are never on more than 3 hours, otherwise this circuit is not Code compliant.

In a real situation, sure most homes would not use all 18 lights plus the appliances and table lamps all at the same time, let alone, for more that three hours.

But, could it? I chose not to take that chance, and would arrange the circuit so this amount of load would could not be on one circuit.

I don't get too excited by a true general purpose lighting circuit, with a combination of a few light fixtures and some receptacles.

But if there is a certain room, such as a den or family room with a large amount of lighting, I will treat that as a continuous load and assume it will be all on for more than 3 hours, and that circuit will not be loaded more than 80%.

We have to remember the true definition of "Continuous Load" from above.

Article 100- Definitions
Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.


The word "expected" is a very important one that is often ignored or overlooked.
Some folks think if a load might be, or will ever be, on for 3 hours or more it should be considered a continuous load. This is not at all true.

I must say, erring on the side of caution is personal preference though that we all do for certain things.