80% rated breaker

[QES]

lets say you have a 300a 80% rated breaker, so that mean its will be good for 240a. My question is why the label 300a?

 

[iwire]

lets say you have a 300a 80% rated breaker, so that mean its will be good for 240a. My question is why the label 300a?

Because it is good for 300 amps non-continuous.

You can load a standard 300 amp breaker 240 amps forever and up to 300 amps for periods under 3 hours.

 

[jim dungar]

lets say you have a 300a 80% rated breaker, so that mean its will be good for 240a. My question is why the label 300a?

Breaker nameplate ratings and conductor ampacities are complimentary.

If the load is continuous, the NEC requires you to size the conductors at 125% of the load. So a 240A continuous load requires a 300A conductor. The 300A conductor requires a 300A breaker.

Also be aware that while "open air" fuses themselves are 100%, all fuse holders and switches are only 80% rated just like circuit breakers. That is why the NEC has the same continuous load overcurrent sizing requirements for breakers and fuses.

 

[brian john]

Standard molded case circuit breakers are thermal magnetic inverse times current devices.

So in addition that 300 amp breaker (depending on manufacture, type of CB ECT) can carry 6 to 14 times it rating for in rush currents, such as motor and transformers, this may or may not be adjustable (magnetic portion). It also can carry 300% of it's rating for 30-200 (or more) seconds, this is the thermal portion of the CB and this can be effected by ambient temperature, heat from adjacent sources particularly other CBs, multiple starting and stopping of high inrush devices, particularly motors.

So basically at 240 amps by design it a 300 never trip (assuming no load issues), but in a high ambient with adjacent CB's loaded to 80% of their FLA it may trip.

 

[iwire]

So basically at 240 amps by design it a 300 never trip (assuming no load issues), but in a high ambient with adjacent CB's loaded to 80% of their FLA it may trip.

I thought ambient was not supposed to be an issue until it reached above 40C?

And don't the breaker manufacturers have adjustment factors for high ambients?

 

[davidr43229]

Also be aware that while "open air" fuses themselves are 100%, all fuse holders and switches are only 80% rated just like circuit breakers

Fuses and Breakers are rated to continously carry 110% of their rated current at ambient. 25 degrees C for fuses, 40 degrees C for breakers. It's when the OCPD is operating beyond it's ambient that derating should be a consideration.
Just my $.02

 

[iwire]

Fuses and Breakers are rated to continously carry 110% of their rated current at ambient. 25 degrees C for fuses, 40 degrees C for breakers.

You lost me here, standard OCPDs are only rated 80% continuous at less then 40C. :-?

 

[brian john]

thought ambient was not supposed to be an issue until it reached above 40C?

40 C/104 F, not unusual for a mechanical room or electric closet with transformer and no ventilation to reach this temp. Additonally this ambient can be in the panel with the door closed adjacent CB's adding to the heat gain.

 

[davidr43229]

You lost me here, standard OCPDs are only rated 80% continuous at less then 40C.

Again, Fuses are designed to run 110% continously at 25 Degrees C. When you increase the ambient, say to 140 degrees F or 60 degrees C the fuse will only carry 90% of it rated current and 70% of it's opening time.
Just my $.02

 

[winnie]

Don't confuse the _trip rating_ of a device with the current which it is rated to carry on a continuous basis.

The 300A number is a trip rating. It is vaguely related to the current required to actually cause the device to open the circuit sometime at some temperature. Given the allowed manufacturing tolerance ranges, magnetic versus thermal trip, changes in ambient temperature, phase of the moon, etc, the actual time to trip at exactly 100% of the trip rating could be anything from pretty soon to never; the trip rating is a nominal value, that roughly describes a pretty complex subject.

The 80% value is the continuous current rating, which depends upon a number of factors including device temperature rise and life, chance of nuisance tripping, the copper development association, and presumably another host of factors

IMHO given current technology, it should be possible to require that all breakers be 100% devices, but I've not examined this in detail, and won't make such a claim with any more certainty than a rant

-Jon

 

[templdl]

Let's see, if you start with the caculated 125% of the continuous load plu s100% of the non-continuous load. The cable must carry that caclulated load.
Then the breaker is applied to protect the cable.
If in fact the cable was sized based upon a load that was continuous, 125% of the contintinuous load, then the breaker would be applied at 80% be default.

 

[brian john]

It is vaguely related to the current required to actually cause the device to open the circuit sometime at some temperature. Given the allowed manufacturing tolerance ranges, magnetic versus thermal trip, changes in ambient temperature, phase of the moon, etc, the actual time to trip at exactly 100% of the trip rating could be anything from pretty soon to never; the trip rating is a nominal value, that roughly describes a pretty complex subject.

Well said, need to stamp that on my tool case.

 

[Psychojohn]

So a 100A breaker will carry 100A continuously asumung adequate cooling, but the NEC requires a load to be calculated at 80% to eliminate nuisance tripping. Is that the case?

 

[don_resqcapt19]

Given that the UL standard says a breaker can take up to 1 hour to trip at 135% of rating and it is never requried to trip at 134%, breakers will often carry 100% forever even within a panel. Yes I know the code says otherwise.
Don

 

[davidr43229]

So a 100A breaker will carry 100A continuously asumung adequate cooling, but the NEC requires a load to be calculated at 80% to eliminate nuisance tripping. Is that the case

Exactly ! you "nailed" it.

 

[QES]

can we do 100% rated breaker but using 90c wire at 75c ampcity?

 

[templdl]

Regarding the application of 100% rated molded case circuit breakers:
It all starts with a 100% rated OCPD such as a molded case circuit breaker that is UL listed for application at 100% of its rating. These breaker will have a solid state trip unit and usually will be at least a 400A frame if not larger.
100% rated breakers AREN'T ANY BETTER than the its common 80%counterpart. The only difference if that they have been UL list for 100% application only when used in an enclosure of installation that is also listed with it for 100%.
When you are so inclined to apply a 100% breaker and read the instructions carefully you will find that it must be applied with 90degC rated cable, the cable of which is applied at 75degC. Remember that cable must be sized to carry the load and breakers then protect the cable.
Normally, the cable is sized 100% of the non-continuos load plus 125% of the continuous load. The cable size selected must carry that load. The breaker is sized to protect the cables rated ampacity that is often more than the calculated load because the breaker rating is based upon the cable rating and not the load.
When applying 100% rated breakers it is 100% plus 100% of the continuous and non-continuous loads. You select the cable size based upon 75degC and you will note that it gives you the opportunity to select a smaller cable. Smaller copper cable means lower cost. BUT, that cable MUST BE RATED 90degC and not 75degC. This may also give you the opportunity to step down a breaker frame size depending upon what cable size that you end up with and you get lucky. The next smaller frame size can also reduce cost.
So applying cable at 100% is a bit more than that.
I trust that this sheds some light on the application.