Feeder Breakers in a distribution panel (Sizing Requirements)

[wwhitney]

From what I've gathered the 25% adder for conductor sizing with continuous loads is to provide more heat sinking to keep the temperature of breaker terminals and internal components within bounds.

Sure, but that's to compensate for the limitations of thermal-magnetic breakers. So in the hypothetical world where we had better breakers, that wouldn't be necessary. At least, as I understand it.

Cheers, Wayne

 

[synchro]

Sure, but that's to compensate for the limitations of thermal-magnetic breakers. So in the hypothetical world where we had better breakers, that wouldn't be necessary. At least, as I understand it.

Cheers, Wayne

I agree with that. I was mainly getting at the point that the "shape of the damage curve of a conductor" was not likely to be the driving reason for the 25% adder because a higher temp wire insulation could reduce that concern. I concur that the heat dissipation and temperature limitations of the breakers themselves are probably the main reason for the added 25%.

 

[wwhitney]

I agree with that. I was mainly getting at the point that the "shape of the damage curve of a conductor" was not likely to be the driving reason for the 25% adder because a higher temp wire insulation could reduce that concern. I concur that the heat dissipation and temperature limitations of the breakers themselves are probably the main reason for the added 25%.

OK, so maybe "damage curve of a conductor" isn't the correct term.

What I means is a curve that indicates, for a given current, how long it is OK for a conductor to experience that current before the OCPD activates. That curve clearly has a different shape than the trip curve of a thermal-magnetic breaker, in the usual thermal environment of a panelboard. Because if you select a trip curve that protects the conductor well for time scales on the order of minutes, you may get nuisance trips with time scales on the order of hours.

I don't see how increasing the insulation temperature helps with this, I would think the shape of the conductor allowable current-time curve would be similar across different insulation temperatures.

Cheers, Wayne