80% capacity

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elvis_931

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I have always been told that the actual load on a breaker should not exceed 80% of the breaker rating i.e. 100amp breaker- actual load not to exceed 80amps. Is this true? Where is it in the NEC?
 
It refers to continuous loads.

230.42(A)(1) + (2).
230.208(B).
422.10(A).
424.3(B).
426.4.
427.4.
550.12(D)(2).
552.46(B)(3)(b).
625.14.
625.21.
et al.
 
fireryan said:
Thanks 480
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That's what I came up with searching for 'continuous load'. I'm sure there's a lot more references if I had the time to look for both '80%' and '125%'. Both are used quite literally throughout the Good Book.
 
I believe that some breakers are rated for full load amps, which are allowed to be used at 100%. Otherwise 80% would apply. (see manufacturers documentation)
 
The only thing i can find in the NEC that says not to exceed 80% is 210.23(A)(1). I guess it is just a general practice not to exceed 80% of a breakers rating. Am I right?
 
The NEC uses the term 125% not 80% when describing loading of overcurrent protective devices.
 
elvis_931 said:
So does that mean for example that a 100amp breaker can have an actual load of 125amps?
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Just the opposite, if the rules apply. You need an OCD of 125% of the load, which works out to the load being 80% of the OCD.
 
elvis_931 said:
So does that mean for example that a 100amp breaker can have an actual load of 125amps?
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Ummmm, no. It means that the OCPD has to be sized at 125 amps for a continuous 100 amp load. Or, the 100 amp breaker has to be derated to 80 amps.

Continuous, in this case means more than 3 hours. I used to use this all the time when designing light plots for a local community theater. Rehersals typically used the floods and lekos for more than 3 hours at a time. My 1200 watt per channel dimmers could only be loaded to 960 watts per channel. I may have cheated by 40 watts from time to time :rolleyes: .
 
What you will also find is the UL's statement on circuit breakers will limit you to 80% of the continuous load on a breaker ( unless marked otherwise )but in fact it can be loaded to 100% as long as it is not a continuous load.

Here is a quote from the UL white book:
Unless otherwise marked, circuit breakers should not be loaded to exceed 80 percent of their current rating, where in normal operation the load will continue for three hours or more.
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I think since the recip of 125% is 80% most believe that the 80 percent applies to all breakers regardless but in fact it has to do with the continous loads.
 
480 and Radio mentioned it but I want it to be clear.

ALL breakers and fuses are designed to be used at 100% non-continuously, that is less then 3 hours straight.

For loads that are continuous the NEC requires the conductors and the overcurrent protection to be sized at 125% of the load. Which results in a breaker that is not loaded more then 80%.
 
iwire said:
ALL breakers and fuses are designed to be used at 100% non-continuously, that is less then 3 hours straight.

For loads that are continuous the NEC requires the conductors and the overcurrent protection to be sized at 125% of the load. Which results in a breaker that is not loaded more then 80%.
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Just so I'm clear on this, the only load in Resd. considered continous is the heating load. Is this correct?
 
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