80% rated main breaker of a residential 100A panel

[fandi]

Hello All,
Would the 100A main breaker (not 100% rated) of a residential 100A panel trip when the total amp usage is more than 80A (let's say 90A)?
Thanks.

 

[JFletcher]

No. 80 percent is continuous load, and has nothing to do with trip characteristics or curves. A 100a breaker under normal ambient temperature should carry 90a indefinitely... It's just that you are only supposed to load it to 80a for continuous loads.

I haven't look real closely at trip curves recently, however I believe a properly operating 100 amp breaker could carry as much as 110 (or at least 105) amps indefinitely, assuming ambient temperature was under 40 degrees Celsius.

Eta: a 100% rated breaker just allows you to use the full capacity of the breaker even under continuous loads. You can load an 80% rated breaker at 100% capacity, just not continuously.

 

[augie47]

good explanation :thumbsup:

 

[infinity]

Hello All,
Would the 100A main breaker (not 100% rated) of a residential 100A panel trip when the total amp usage is more than 80A (let's say 90A)?
Thanks.

That breaker would be an inverse time CB meaning that as the current exceeds it's rating the time it will take to trip decreases. At 100 amps of load it will probably go for a long time before tripping, maybe it won't trip at all. As you go over 100 amps the amount of time it takes to trip will decrease as the current above its rating increases. Maybe someone will post the trip curve info.

 

[fandi]

No. 80 percent is continuous load, and has nothing to do with trip characteristics or curves. A 100a breaker under normal ambient temperature should carry 90a indefinitely... It's just that you are only supposed to load it to 80a for continuous loads.

I haven't look real closely at trip curves recently, however I believe a properly operating 100 amp breaker could carry as much as 110 (or at least 105) amps indefinitely, assuming ambient temperature was under 40 degrees Celsius.

Eta: a 100% rated breaker just allows you to use the full capacity of the breaker even under continuous loads. You can load an 80% rated breaker at 100% capacity, just not continuously.

And by 'continuously' you mean more than 3 hours? I don't have continuous loads other than lighting and EV charger (future) so I guess the main breaker would not trip even the total non-continuous load is 90A for more than 3 hours?

 

[Jraef]

There is no such thing as an “80% rated breaker”. All breakers are rated at 100% of their stated load. A 100A breaker will never trip when loaded to 100A.

The reason we THINK of them being “80% rated” is because of the INDIRECT rules of how they must be used, resulting it how the mfrs use them and test them. It’s indirect based on this:

1) CONDUCTORS must be sized for 125% of the CONTINUOUS load, based on 3 hours being considered continuous.

2) BREAKERS are sized to protect the conductors.

3) the INVERSE of 125% is 80%, meaning if I know a conductor is rated for 100A and I want to size it for a continuous load, 100 divided by 1.25 = 80A, ergo 80%.

4) So if you have a load that is 80 A continuous, you size the conductors at 125% of that, which will be 100A and since a breaker protects the conductors, you use a 100A breaker.

5) then because the breaker mfr KNOWS this, when they rate how you use a breaker in a panel, it is based on it never being loaded at more than 80% continuously, because you would not be able to do that to the conductors. So the heating of the breaker, when crammed into a panel with other breakers, is accounted for in the listing of them in that panel.

A ”100% rated breaker” is the same breaker, but listed for use under the same restrictions in which you can use and terminate (another issue) the cables at 100%. What you will find is that the only way to use a 100% rated breaker is as a stand-alone enclosed breaker or as a main breaker in switchgear, MCCs etc. where there are no other breakers adjacent to them and usually connecting to bus bar.