NEC 215.3 vs UL 508A 32.3.1 - Feeder Overcurrent Protection Conflict?

[JCWA_Controls]

Hi everyone, first post here. I'm not an electrical engineer/electrician and I'm relatively new to this field, so please bear with me if some of my terminology or foundational knowledge is lacking.

I'm working on designing some panels for a project and am at the stage where I need to size the circuit breakers. I have a panel with a feeder going to several branch circuits, for which the circuit breaker ratings have been selected. When I looked into sizing the circuit breaker for the feeder, I found NEC and UL 508A information that appears to be in conflict, so I'm hoping someone can help point out what I'm missing here.

Excerpt from NEC 215.3 - Overcurrent Protection (2023):
"Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load."

UL 508A 32.3.1 - Sizing of overcurrent protection (of feeder) (2022):
"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."

I've tested some scenarios and the result you get from UL 508A 32.3.1 is lower than the result you get from NEC 215.3 in some cases, particularly if using a conductor with the lowest ampacity possible that can handle the load. So if I calculate, for example, 30 A from UL 508A 32.3.1 and 34.5 A from NEC 215.3, I have a contradiction: UL 508A is telling me to not exceed 30A, while NEC is telling me not to go lower than 34.5 A, so one method results in a 30 A circuit breaker and the other results in a 40 A circuit breaker.

Am I missing something here? I found a question similar to this on this forum a couple years back, but the answers didn't provide any clarification for me. Any help would be greatly appreciated, thanks for bearing with me as I try to understand a completely new field of knowledge.

 

[tortuga]

Hi everyone, first post here

Welcome

if I calculate, for example, 30 A from UL 508A 32.3.1 and 34.5 A from NEC 215.3, I have a contradiction:

Can you show your full calculation ?

 

[petersonra]

there is no contradiction. UL508a determines how the control panel is designed. The NEC only determines how the installation is done. They are separate issues.

 

[JCWA_Controls]

Welcome

Can you show your full calculation ?

That was just an example of a possible set of values, but I can do a real one demonstrating the same thing.

Let's say I have the feeder CB-1 going to 9 branch CBs, CB-2 through CB-10 (my actual panel). 24V DC power. Each component consumes 75 W (not my actual panel, but demonstrates my point), all continuous loads. To size the CBs for these (NEC 210.20):
(75 W)/(24 V) * 125% = 3.9 A
CB-2 through CB-10 = 4 A

Following UL 508A 32.3:

For (a):
Rating of largest branch circuit + load of all other branch CBs = 4 A + 8*(75 W)/(24 V) = 29 A
Note that nothing is multiplied by 125% because there are no heater loads.

For (b):
The ampacity is 30 A - 10 AWG copper wire (if I'm not doing this wrong, this would always be bigger than (a) unless there are heater loads)

The larger of (a) and (b) is 30 A.

Following NEC 215.3:

Continuous Load * 125% = 9*(75 W)/(24 V) * 125% = 35.2 A

Thus the contradiction, cannot go lower than 35.2 A according to NEC, cannot go above 30 A according to UL 508A.

there is no contradiction. UL508a determines how the control panel is designed. The NEC only determines how the installation is done. They are separate issues.

I'm aware my design is governed by UL 508A instead of the NEC. But what do you even mean, practically? Let's go with this example where the UL 508A result indicates you need a 30 A circuit breaker and the NEC result indicates you need 40 A. If I design according to UL 508A knowing it conflicts with the NEC, then when whoever it is goes to install it, are they supposed to go against my design and use a 40 A CB? If so, what was the point in me designing it? I simply don't understand how these are entirely separate issues, it seems like they go hand in hand. Again, this is all very new to me and I may be totally wrong, just trying to explain my perspective so you understand where I'm coming from.

 

[don_resqcapt19]

....


I'm aware my design is governed by UL 508A instead of the NEC. But what do you even mean, practically? Let's go with this example where the UL 508A result indicates you need a 30 A circuit breaker and the NEC result indicates you need 40 A. If I design according to UL 508A knowing it conflicts with the NEC, then when whoever it is goes to install it, are they supposed to go against my design and use a 40 A CB? If so, what was the point in me designing it? I simply don't understand how these are entirely separate issues, it seems like they go hand in hand. Again, this is all very new to me and I may be totally wrong, just trying to explain my perspective so you understand where I'm coming from.

If you put a nameplate on the control panel, the EC will look at that and not do the NEC calculations.

 

[wwhitney]

The ampacity is 30 A - 10 AWG copper wire (if I'm not doing this wrong, this would always be bigger than (a) unless there are heater loads)

Two comments: If UL 508A matches the NEC, then the #10 AWG may well have an ampacity above 30A (e.g. if the insulation and terminations are rated at least 75C, the ampacity is at least 35A), but 240.4(D) further limits the OCPD size to 30A. That 240.4(D) limit is not an ampacity limit, though, which distinction occasionally matters.

Secondly, can't you resolve this conflict simply by changing your design to use #8 AWG? Then the maximum OCPD is at least 40A, per UL 508A 32.3.1.(b).

Cheers, Wayne

 

[jim dungar]

The NEC uses some very conservative ampacities because of the vast number of assumptions which may be made incorrectly, by the designer.

UL 508A has more control over the assumptions therefore its ampacities can be tighter.

 

[tortuga]

That was just an example of a possible set of values, but I can do a real one demonstrating the same thing.

Let's say I have the feeder CB-1 going to 9 branch CBs, CB-2 through CB-10 (my actual panel). 24V DC power. Each component consumes 75 W (not my actual panel, but demonstrates my point), all continuous loads. To size the CBs for these (NEC 210.20):
(75 W)/(24 V) * 125% = 3.9 A
CB-2 through CB-10 = 4 A

Following UL 508A 32.3:

For (a):
Rating of largest branch circuit + load of all other branch CBs = 4 A + 8*(75 W)/(24 V) = 29 A
Note that nothing is multiplied by 125% because there are no heater loads.

For (b):
The ampacity is 30 A - 10 AWG copper wire (if I'm not doing this wrong, this would always be bigger than (a) unless there are heater loads)

The larger of (a) and (b) is 30 A.

Following NEC 215.3:

Continuous Load * 125% = 9*(75 W)/(24 V) * 125% = 35.2 A

Thus the contradiction, cannot go lower than 35.2 A according to NEC, cannot go above 30 A according to UL 508A.


I'm aware my design is governed by UL 508A instead of the NEC. But what do you even mean, practically? Let's go with this example where the UL 508A result indicates you need a 30 A circuit breaker and the NEC result indicates you need 40 A. If I design according to UL 508A knowing it conflicts with the NEC, then when whoever it is goes to install it, are they supposed to go against my design and use a 40 A CB?

Electricians are used to seeing labels that say maximum overcurrent protection (MOP) and Minimum Circuit Ampacity (MCA).
So if you want a 30A MOP for whatever reason you could perhaps just specify a MOP / MCA on your label, just make sure your terminations will accept a larger wire as it may get upsized for any number or reasons, de-rating, voltage drop etc.. Keep in mind we will need to follow any instructions you include in the listing per 110.3(B)