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

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

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.

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. 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

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

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

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

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

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

You are welcome for the information!