80% and 100% Rated Breakers... Help!

[dave5567]

I have a question regarding Branch Circuit O/C Protection as defined in the NEC, Article 210.20 Overcurrent Protection.

As per 210.20 (A) : If I have a branch circuit supplying a continuous load, I must ensure that the circuit breaker protecting that branch circuit has a rating not less than 125% of the continuous load.

Exception: If the entire assembly, including the breaker, is rated for 15A continuous, then I must ensure that the circuit breaker protecting that branch circuit has a rating not less than 100% of the continuous load.

What I am confused about is the following :

If I have a 15A circuit breaker rated for 80% this means it can handle 12A of current flowing continuously.
But by 210.20 (A) I must ensure that the rating of my breaker is not less than 125% of my continuous load.
Hence, by 210.20 (A) the maximum load I should design for seems to be 12A/125% = 9.6A max since its a 15A breaker (15A trip) but it's only 80% rated (12A).

This doesn't seem logical to me but I want to get the opinion of someone who is more familiar with O/C sizing in the USA.
Thank you,

 

[Strathead]

I have a question regarding Branch Circuit O/C Protection as defined in the NEC, Article 210.20 Overcurrent Protection.

As per 210.20 (A) : If I have a branch circuit supplying a continuous load, I must ensure that the circuit breaker protecting that branch circuit has a rating not less than 125% of the continuous load.

Exception: If the entire assembly, including the breaker, is rated for 15A continuous, then I must ensure that the circuit breaker protecting that branch circuit has a rating not less than 100% of the continuous load.

What I am confused about is the following :

If I have a 15A circuit breaker rated for 80% this means it can handle 12A of current flowing continuously.
But by 210.20 (A) I must ensure that the rating of my breaker is not less than 125% of my continuous load.
Hence, by 210.20 (A) the maximum load I should design for seems to be 12A/125% = 9.6A max since its a 15A breaker (15A trip) but it's only 80% rated (12A).

This doesn't seem logical to me but I want to get the opinion of someone who is more familiar with O/C sizing in the USA.
Thank you,

In my opinion, the reason for 210.20 (A) is that a standard breaker has a tolerance of plus or minus 20%. That said, the breaker is a 15 amp breaker, that is the "rating" referred to in 210.20 (A)

 

[infinity]

Excluding the exception:
15 amps*80%=12 amps
12 amps*125%=15 amps

 

[wwhitney]

If I have a 15A circuit breaker rated for 80% this means it can handle 12A of current flowing continuously.
But by 210.20 (A) I must ensure that the rating of my breaker is not less than 125% of my continuous load.
Hence, by 210.20 (A) the maximum load I should design for seems to be 12A/125% = 9.6A max since its a 15A breaker (15A trip) but it's only 80% rated (12A).

No. The rating 210.20(A) is referring to is the nominal rating of 15A. And 15A / 125% = 12A.

That 80% figure you are using doesn't appear anywhere in the NEC, it's just from 80% = 1/125%. Your last calculation is applying the factor twice instead of once.

Cheers, Wayne

 

[kwired]

The overcurrent device setting must be at least 125% of continuous load plus 100 % of non continuous load is how NEC describes it. Similar wording for conductor ampacity.

So if you had 8 amps of continuous load and 4 amps of non continuous load you need 8 x 1.25=10 plus the 4 amps = 14. That circuit could still be on a 15 amp breaker as well as use a 15 amp conductor.

 

[sgtsparky]

You have the idea but there are some devices like heaters once you hit 50% or more that is the max. It depends what the load is for example motors always 125% for the first,,, but you have the basics

 

[jim dungar]

Notice how conductors and breakers are both sized the same way, such as at 125% with continuous loads.

Overcurrent protective devices are tested in open air, but they are installed in enclosures with limited air flow. This difference in heat dissipation is part of the reason for the 125% factor. You will not find residential or commercial rated devices rated for 100% loading. Typically breakers need to be 600V 200A frame with electronic trip units before 100% loading is possible.

So, determine your load. Choose your conductors. Pick your breaker. Do not over think the process.