80% of Breaker Rating - Main Disconnect

[designer82]

I know for branch ckt. breakers you can only load up to 80% of its rating but what about if your main disconnect is a circuit breaker, either standalone enclosed CB or main breaker in a panelboard, can you load up to 100% in that case?

 

[infinity]

You can load any breaker to 100% unless the load is continuous. A main breaker in a service is sized according to the load calculation. If the load calc says 100 amps then a 100 amp OCPD can be used.

 

[charlie b]

I agree with Rob. The load calculation takes continuous loads into consideration, so you don't have to apply any 80% factor. That said, my clients have always required spare capacity (usually 20%) for future load growth. But it's not a code requirement.

 

[manojaus]

We keep a 15%-25% buffer in the Circuit Breaker for temperature derating. This is a standard practice is Australia.

 

[Jpflex]

I know for branch ckt. breakers you can only load up to 80% of its rating but what about if your main disconnect is a circuit breaker, either standalone enclosed CB or main breaker in a panelboard, can you load up to 100% in that case?

I believe this rule is only for continuous loads but not non continuous loads?

I believe you can load a circuit breaker to 100% as long as it’s not continuous 3 hours or more run time generally)?

 

[jim dungar]

I believe this rule is only for continuous loads but not non continuous loads?

I believe you can load a circuit breaker to 100% as long as it’s not continuous 3 hours or more run time generally)?

Feeder and branch OCPD, both fuses and breakers, are rated the same as conductors.

If your loading requires you to up size your conductors by 125% then you also need to up size your OCPD.

 

[Carultch]

I believe this rule is only for continuous loads but not non continuous loads?

I believe you can load a circuit breaker to 100% as long as it’s not continuous 3 hours or more run time generally)?

That's correct.

If you have a mix of continuous and non-continuous loads, then it is 125% of continuous plus 100% of non-continuous that sizes a standard breaker or other OCPD that isn't specifically rated as continuous duty. A continuous breaker in a listed assembly rated for continuous duty is can be sized for 100% of continuous + 100% of non-continuous.