80% breaker rating

[steve66]

Only the demand load matters for sizing the breaker and feeders, not the total connected load.

And if continuous loads are multiplied by 125%, then we don't need to consider the breakers 80% limit. As long as your demand load is less than 225 amps, you can use the 225 amp breaker.

I think its that simple, but I'm interested in hearing opinions if anyone thinks its more complicated than that.

 

[jrohe]

I am in complete agreement with everyone who has essentially said, "Ignore to 80%/100% hysteria, calculate the demand load, size the conductor for the demand load, and size the breaker for the demand load." If using a 100% breaker, the continuous loads are calculated at 100 percent. If using a standard breaker, the continuous loads are calculated at 125 percent. If using optional calculations for dwelling units, schools, restaurants, whatever, use that demand load. There is no need to, nor is it Code-required to, multiply that value by 125 percent to offset the phantom 80% rating of a standard circuit breaker.

There is a benefit to 100% rated breaker assemblies insofar as when you use them, you can potentially reduce conductor sizes. Take, for example, a facility with 1200 amps of continuous load and 350 amps of noncontinuous load.

Using a standard circuit breaker, the conductors would need to be sized at 1200 * 1.25 + 350 = 1850 amps. The next standard size OCPD is 2000 amps, so you would need to increase conductors up to 2000 amps. Use 6 sets of #400 kcmil copper 75 degree C conductors or whatever else you want to get 2000 amps.

With the exact same load profile but using a 100% rated breaker assembly, the conductors would need to be sized at 1200 + 350 = 1550 amps. The next standard size OCPD is 1600 amps, so you would only need 1600 amp conductors to feed this identical load just by changing the type of breaker. So instead of using 6 sets of #400 kcmil copper 75 degree C conductors you can now use 5 sets of #400 kcmil copper 75 degree C conductors.

Now whether the savings afforded by reducing one set of conductors will offset the additional cost of a 100% rated breaker assembly is debatable.

Note that I kept referring to a 100% rated breaker assembly. It is not only the breaker that needs to be 100% rated; it is the entire assembly. As such, you can not simply purchase a 100% rated breaker, slap it in a standard piece of equipment, and apply that breaker at 100 percent.

 

[iceworm]

Charlie -
Using UL489 listed CBs for an example: Per the UL spec, the CB is required to never trip at 100% rated load, but must trip within 1 minute at 130% (I don't have the reference available today, but the numbers, if not correct, are close)

Both of the following meet UL spec:​

CB holds forever at 100%. Trips instantly at 101%
CB holds forever at 129%. Trips instantly at 130%​

One might say that the UL rates all CBs at 100% continuous load.

I think UL 489 say the breaker must trip at 135% in 60 minutes or less.

Yes, you are correct. (With the reference in front of me)
current ........ Rating .......................... Time
200% .... frame rating 0 to 225A .... 2 min to 10 minutes

135% ........... 0 - 50A ........................within 1 hour
135% ........... >50A ...........................within 2 hours

100% never trip

So I need to modify my post:

Both of the following meet UL spec:​

CB holds forever at 100%. Trips instantly at 101%​

CB holds forever at 134%. Trips instantly at 135%​

My reasoning still stands

ice