Breaker ratings vs continuous loads

Strombea

Strombea

Senior Member
Location
Prescott, Arizona, USA
So I know that a standard breaker is rated for 80% of its rating and 100% rated breakers are rated with an approved assembly for 100% continuous load.

But my question is with Non Continuous loads on a standard breaker. Is it still only 80%? And if so does that mean that there are 2 different deratings for the same load?

20 amp breaker can be loaded to 16 amps ...

But continuous loads need to be calculated first.

So a 14 amp wall heater @ 125% = 17.5 amps (more than 16) Can NOT be on a 20 amp breaker even though the 125% has already been accounted for. (its only pulling 14 amps)


I think the answer to my question is that there are 2 different rules happening here and they both have to be done to comply.

And with a 100% rated breaker... do you still need to add the 125% to a continuous load first as it is a different rule than the breaker rule? (the heater is still seen as 17.5 amps... NOT 14 amps)


Side question: Is a 100% rated breaker the same thing as a fully rated breaker in a series/fully rated system, or are these different products, Meaning if you wanted to not series rate and it was a continous load, do they make that combination in a breaker?
 
Strombea said:
But my question is with Non Continuous loads on a standard breaker. Is it still only 80%? And if so does that mean that there are 2 different deratings for the same load?
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For a non-continuous load the circuit with a standard 80% breaker can be loaded to 100% of the breaker rating. So a non-continuous 20 amp load can be on a 20 amp individual branch circuit. For a continuous load the maximum rated current for the same 20 amp circuit would be 16 amps. (16*125%=20 amps)
 
I like to take it the other way, because TECHNICALLY, there is no such thing as an "80% rule" for circuit breakers. Here is the proper way to look at it:

  • CONDUCTORS for continuous loads must be sized at minimum 125% of the load.
  • For non-continuous loads, they do not, so you can size the conductors for 100% of the expected load.
  • BREAKERS must be sized to protect the conductors.
  • So if you size the breakers for the properly selected conductors, whether it is continuous or not, you are always correct.
So case in point: if you have a continuous load that is 100A, the conductors must be sized art 125A. So if you size the breaker at 125A, in EFFECT, the breaker is only be used at 80% of it's rating, because 80% is the inverse of 125%.

Conversely if you have a NON-continuous load of 100A, you can use conductors rated for 100A, ergo you can use a breaker rated 100A.
 
You do not make (2) 125% adjustments.
The adjustment is for the conductor size. Once you have picked a conductor, you select your protective device.

Jraef beat me to it.
 
Ok thats what I have always thought and it makes sense. But I have inspectors and engineers telling me that even for non continuous loads you would never load a breaker to 100%, you would always load it to 16 amps. So in the case of 20 amp branch circuits say for a vending machine, if it pulls 19 amps then there is no problem using a 20 amp breaker.



Any thought on if you have ever seen a fully rated breaker for short circuit purposes crossed with a 100% rated breaker for continuous load purposes
 
Strombea said:
Ok thats what I have always thought and it makes sense. But I have inspectors and engineers telling me that even for non continuous loads you would never load a breaker to 100%, you would always load it to 16 amps. So in the case of 20 amp branch circuits say for a vending machine, if it pulls 19 amps then there is no problem using a 20 amp breaker.
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Whoever is saying that is incorrect. In your example the 19 amp load of the vending machine which is not a continuous load can go on a 20 amp individual branch circuit. {210.22}

Regarding 100% rated breakers for continuous loads you wouldn't see them in small ampere rated breakers.

210.22 Permissible Loads, Individual Branch Circuits.
An individual branch circuit shall be permitted to supply any load for which it is rated, but in no case shall the load exceed the branch-circuit ampere rating.
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Strombea said:
Any thought on if you have ever seen a fully rated breaker for short circuit purposes crossed with a 100% rated breaker for continuous load purposes
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I am not sure what you are getting at.
Short circuit ratings use but the term 'fully rated', while loading uses "100% rated". These two factors are not mutually exclusive.

It is rare to find 100% rated breakers in less than 400A frames although some manufacturers may offer them smaller. I don't believe anyone has ever made one in a non-electronic molded case version.
 
Strombea said:
Side question: Is a 100% rated breaker the same thing as a fully rated breaker in a series/fully rated system, or are these different products, Meaning if you wanted to not series rate and it was a continous load, do they make that combination in a breaker?
Click to expand...

This refers to the KAIC rating, and is unrelated from the continuous duty vs standard rating.

Fully rated means each breaker requires a KAIC rating that meets or exceeds the available fault current at its position in the distribution network, independently of other OCPD's in the system.

Series rated means an upstream breaker can protect a downstream breaker, so that the downstream breaker doesn't require the full KAIC rating for its location. Could be a main breaker and the branch breaker in its own panel, or it could be the breakers in a subpanel protected by a breaker in the main panel. This requires listed combinations of breakers, or listed combinations of breakers and fuses, to take credit for it. Breakers may be series rated with other breakers of the same manufacturer, or with a given fuse class as the upstream device. You are unlikely to find a combination between two different brands, that allows series ratings, if any even exist at all.
 
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