Bus Bar ratings 1

[hogdawg]

I appreciate you folks response to my questions. I didn't get the answer so I must have ask it wrong. Here goes again:
Bus bar tables reflect imessity factors and degree rises in centigrade. Wonderful! Now I have to size bus bar (been doing it for years via IEEE table A27 but when evaluating other tables, questions arise. I deal with UL891 with their outlandish equations (1 sq in per 1000 amps) and I just want to understand why:
The higher the temp. rise, the higher the ampacity. Whats up?

 

[davidbeach]

Asked and answered. thread238-157256 No need for a new thread.

 

[waross]

Take a look at the ampacity tables for cables in the NEC.
You will see that the hotter a conductor is allowed to run, the higher the allowable ampacity.
Conductors, whether cables or bus bars, have resistance and carry current. The more current, the more heat is generated. The more heat that is generated the hotter the conductor becomes. The actual temperature depends on the the ability of the conductor to reject the heat through radiation, conduction, and/or convection.
When you use a higher temperature rating for a given conductor it will have a higher ampacity.
yours

 

[LionelHutz]

The current in the bussbar produces heat in the bussbar.

The higher the temp difference between the bussbar and the surrounding air the more heat it can dissipate. Or, the higher the temp difference the higher the heat transfer rate.

So, if you let the bussbar run hotter it can dissipate more heat which means it can handle more current.