Sizing of Feeder Protection UL508A vs NEC

[Edonlon]

I have been working through UL508A but I am having one sticking point where it feels as if UL508A and the NEC conflict.

Specifically, the following excerpts:
NEC:

430.63 Rating or Setting - Motor Load and Other Load(s)
Where a feeder supplies a motor load and other load(s), the feeder protective device shall have a rating not less than that required for the sum of the other load(s) plus the following:
(1) For a single motor, the rating permitted by 430.52
(2) For a single ... (blah, not applicable)
(3) For two or more motors, the rating permitted by 430.62

However in UL508A, section 32.3 on sizing of overcurrent protection, it says

32.3.1 The size of the overcurrent protection shall not exceed the ampere value determined from (a) and (b), whichever is larger.
a) The rating of the largest branch circuit protective device in the circuit plus 125 percent of all heater loads plus the full-load currents of all other motors or other loads in the group that could be in operation at the same time; or
b) The ampacity of the conductors or bus bars on the load side of the overcurrent device.

A couple of things to unpack here, it seems like UL is saying "shall not exceed" and NEC is saying "shall have a rating not less than" when selecting overcurrent protection devices for the feeder.

1. This seems to conflict? Does anyone have any clarification for this apparent contradiction?
2. In 32.3.1, it seems like "shall not exceed the ampere value determined from (a) and (b), whichever is larger" would allow for someone to have an OCPD that is smaller than a), if a) is larger, but larger than what would protect the conductors. This would be violations elsewhere in the standard. What is the point of b)?

Bonus question, the panel I am designing uses 2 VFDs, each on their own branch, and 3 AC/DC converters. How would I determine the supply conductor size according to UL508A? I am specifically referring to section 28. I believe it falls under 28.3.3 because it is a group of more than one load, whereas 28.3.5 is worded for only a single VFD. 28.3.3 sounds as if I can size it based on 125% the FLA of the motor, not the VFD, which would be preferable as having to size extra copper for a motor that will not change based on the VFD would bump me up an extra breaker size and cable gauge, and sounds like gallivanting over-provisioning.

 

[jaggedben]

The NEC and UL508A don't conflict here because their scopes don't overlap. If the wires are covered by one, they are not covered by the other, and vice versa.

 

[wwhitney]

I didn't look closely at the UL 508 information in the OP, but I will say that I have never understood the rationale behind NEC 430.63. As far as I can see, it should read parallel to 430.62 and say:

"A feeder supplying a specific fixed motor load(s) and other loads consisting of conductor sizes based on 430.24" shall have an OCPD that does not exceed the value specified.

And I think 430.63's deviation from that approach is simply a mistake. As it currently reads, I think you'd be allowed to size the conductors per 430.24, but protect them at an arbitrarily high value, which is obviously crazy. Or if the intention is that when you use 430.63, you may not size the conductors per 430.24 but need to size them fully (in accordance with 240.4 less 240.4(G)) for the OCPD size selected, further language in 430.63 is required to convey that.

Cheers, Wayne

 

[Edonlon]

Wayne, I’m inclined to think the same way. I agree that it seems like having arbitrarily large feeder protection seems unsafe and counterintuitive. Driving the point home, choosing a smaller OCPD makes sense, especially since the calculated number in both UL and NEC is a worst case scenario.