UL 508A Sizing feeder Circuit

[paulohpm]

Hello,

I am sizing a feeder circuit for a panel according to UL 508A and came into a question:

My input current is of 76.5 A (considering all the loads), thus, it is fine for me to use a cable of 4 AWG (max. 85A @ 75 c).
For the circuit breaker I did 76.5 A x 1.25 = 96 A and then I selected a breaker of 100A.

My question is:
Is it ok for me to keep this configuration according to UL 508A? 100A breaker + 4 AWG load side cable
Or do I need to level up the cables?


Thank you very much in advance!

 

[Dennis Alwon]

Welcome to the forum. I don't know what UL508A is but the nec does not allow a #4 on a 100 amp breaker as a feeder.

Load x 1.25 if the loads are continuous =96A thus you need a conductor good for 96 a which would be #2

 

[infinity]

I agree with Dennis but if your using a 75 degree C conductor with 75 degree C terminals you can use #3. You could use the #4 with a 90 amp OCPD.

 

[Jraef]

...
My input current is of 76.5 A (considering all the loads), thus, it is fine for me to use a cable of 4 AWG (max. 85A @ 75 c).
...

There's the source of your dilemma... this statement is incorrect with respect to the FEEDER. The feeder conductors must be sized per the NEC, not UL508A and thus at 125% of the load. The INTERNAL conductors are what UL508A is concerned with and INTERNALLY the conductors can be sized per the 508A rules that don't require the 1.25 value.

 

[paulohpm]

Thanks for the responses but I think I was not so clear on my question

I have sized only the breaker with the 1.25 factor, but not the cable

so, having a conductor fitting the 76.5 A (4 AWG) which is the load without any factor and a 100A breaker protecting 96 A (continuous load - 76.5 x 1.25) would it be allowed as per UL 508A / NEC as the conductor withstands only up to 85 A?

 

[infinity]

Thanks for the responses but I think I was not so clear on my question

I have sized only the breaker with the 1.25 factor, but not the cable

so, having a conductor fitting the 76.5 A (4 AWG) which is the load without any factor and a 100A breaker protecting 96 A (continuous load - 76.5 x 1.25) would it be allowed as per UL 508A / NEC as the conductor withstands only up to 85 A?

No this is not NEC compliant. First you calculate the load, then the conductor size and finally the OCPD. The OCPD cannot be larger than the conductor ampacity.

Are you sure that this is a continuous load? If so you need a conductor with a minimum ampacity of 96 amps.

 

[Jraef]

What “cable” are you talking about? You prefaced this talking about the FEEDER, which by definition would be the wires going INTO the control panel. Those would be field wiring and must comply with NEC rules. UL508A only applies to the conductors used INSIDE of the control panel and there are different rules from what the the NEC requires. You cannot use the UL508A conductor sizing rules for the FIELD wiring, UL508A even explicitly states this.