UL508a versus NEC

[petersonra]

Something I had not previously noted.

It appears that the restriction on rounding up to the next size OCPD allowed in 240.4 above 800A is not in UL508a. At least I could not find it.

 

[Jraef]

I'm not sure what you mean.

UL508a just parses it out more and in a way is more restrictive. The rules for motor circuits are basically the same as the NEC, and for motor circuits, Article 430 trumps 240, allowing for breakers in motor circuits to be significantly larger than the conductors, because the OL relay will be protecting the conductors. UL508a, being for "industrial control panels" then has other restrictions for other types of circuits that limit the size to well below 800A anyway, leaving only the rules for "Feeders" in 32.3 that might apply, which is based on the use of other devices anyway. So indirectly you are covered.

 

[petersonra]

I'm not sure what you mean.

UL508a just parses it out more and in a way is more restrictive. The rules for motor circuits are basically the same as the NEC, and for motor circuits, Article 430 trumps 240, allowing for breakers in motor circuits to be significantly larger than the conductors, because the OL relay will be protecting the conductors. UL508a, being for "industrial control panels" then has other restrictions for other types of circuits that limit the size to well below 800A anyway, leaving only the rules for "Feeders" in 32.3 that might apply, which is based on the use of other devices anyway. So indirectly you are covered.

My head is starting to spin.

Section 32 deals with feeders of all types in a UL508a panel. this is what 32.3 says.

32.3 Sizing of overcurrent protection
32.3.1 The size of the overcurrent protection shall not exceed:
a) The rating of the largest branch circuit protective device in the circuit plus the full-load
currents of all other motors or other loads in the group; or
b) The ampacity of the conductors or bus bars on the load side of the overcurrent device.

If one is forced to accept option b) it is certainly more restrictive.

So a) would be less restrictive.

Say I have 5 100 HP motors.

Typically I would have 200 A breakers.

So my max CB for the feeder would have to be 4*124 plus 200? 696A?