OCPD manufacturer standard test setting vs real application setting

[unsaint34]

NEC 21.19 enforces circuit rate of 125% for continuous loads (if the breaker is not listed to operate at 100% of its rating). I understand that a breaker rate is determined at a standard test setting in free air and 40degree C, and that has to do with the reason for the 125%. But what if the environment of my application happens to be quite similar to the standard test setting? In that case, am I not over sizing the OCPD by 25%? I just want to understand why NEC is willing to take that kind of liability, or why not make an exception for applications like my hypothetical situation.

 

[G._S._Ohm]

The liability would be to somewhat shorten the life of the wiring insulation because it may/will run hotter?

 

[iwire]

NEC 21.19 enforces circuit rate of 125% for continuous loads (if the breaker is not listed to operate at 100% of its rating). I understand that a breaker rate is determined at a standard test setting in free air and 40degree C, and that has to do with the reason for the 125%. But what if the environment of my application happens to be quite similar to the standard test setting? In that case, am I not over sizing the OCPD by 25%? I just want to understand why NEC is willing to take that kind of liability, or why not make an exception for applications like my hypothetical situation.

You can get information from the breaker manufacturers that spell out how the temperature will effect their breakers.

 

[templdl]

NEC 21.19 enforces circuit rate of 125% for continuous loads (if the breaker is not listed to operate at 100% of its rating). I understand that a breaker rate is determined at a standard test setting in free air and 40degree C, and that has to do with the reason for the 125%. But what if the environment of my application happens to be quite similar to the standard test setting? In that case, am I not over sizing the OCPD by 25%? I just want to understand why NEC is willing to take that kind of liability, or why not make an exception for applications like my hypothetical situation.

If the wire is sizede to carry the calculated load and the beaker is then sized to protect the wire where willl be no reason to consider the breaker as being sized to carry 125% of the continuous load. I most cases the breaker willl be sized more than 125% if the continuous load if you size the wire which will be at least 125% of the lad and then the breaker will be sized at least the rated ampere rating of the wire (not the load.)

I have als included what is basical done in UL489 testing. The breakers are calibrated in a 40degC environment. However it is not uncommen for breakers to be installed an applied in a 25degC environment and as such a thermal magnetic breaker will carry more current before tripping. Trip curve will reflect how the 40degC trip curve will shift to the right at 25degC.