This is perhaps one of the most misunderstood aspects of circuit breakers.

The so called "80% rule" is actually not stated anywhere in circuit breaker listing and labeling rules. UL489, the standard for Molded Case Circuit Breakers, requires that all MCCBs are tested at 100% load.

YOU must size CONDUCTORS at 125% of the continuous load you are feeding per the NEC. You then size the breakers for protecting those CONDUCTORS. So INDIRECTLY the breaker will never carry more than 80% of the load that you selected the conductors for. Ergo people refer to them as being rated for 80%, but that's just because of the way the NEC is worded.

You cannot use a 100% rated breaker unless you have also met all of the conditions for the CONDUCTORS that allow you to size the CONDUCTOR at 100% of the load. One of those conditions will end up being that the conductors are rated for 90C. In order for you to use conductors rated and sized for 90C, all components in the circuit must also be rated for 90C. Lugs on standard circuit breakers are NOT rated for 90C, they are rated at best, 75C. So one thing you get when you buy a 100% rated breaker is that the lugs will be 90C (usually copper, not Al). The other thing is that panelboards are NEVER rated for 90C conductor use. So the only way to use a 100% rated breaker is to buy it as a separately enclosed stand-alone breaker, or as an open breaker that YOU will build into an assembly, such as a switchboard or MCC, that you will have listed for use with 90C rated conductors at 100% load.

So for example where you see this done most often is that the MAIN breaker in a switchboard or MCC is selected to be rated 100%, because the load side of that breaker is going to bolt directly to the bus bar, not cables, so that connection can be rated for 90C. The incoming cables to that breaker can then be sized, selected and rated based on 100% load as well.

But if you have a breaker in a panelboard and you think you can simply upgrade it to 100% rated to get more current out of it, you are going to be disappointed because you will find that the 100% rated breaker cannot be used that way.

Last edited by jraef; 01/14/17 07:14 PM.