multi motor service size vs feeder OCPD

[mjmike]

Having a debate with myself. When calculating a service disconnect with multiple motors and other loads, the calc is based on the largest motor FLC x 1.25% + remining motor FLC + other loads (1.25% if continuous). However, this can result in a smaller main than a feeder OCPD with multiple motors, so which is correct?

Here is an example: Enclosed breaker or fused service disconnect feeds an ATS which feeds a panel with main breaker and individual branch breakers for 3 motors at 20HP 208V 3-phase each and 10A 3 phase other non-continuous load.

Service calc: (62.1FLC x 1.25%) + 62.1 + 62.1 + 10 = 211.8A so 225A service disconnect breaker or fuse size.
Feeder OCPD calc: 62.1 FLC x 2.5 = 155.25A = 175A breaker + 62.1 + 62.1 + 10A= 309.2A = 350A.

So should the service be at 225A and the panel main breaker be at 350A per code? The reason a I suspect a motor feeder OCPD is large is due to the motor inrush to insure the OCPD does not trip when motors start. However, this would carry through to the service disconnect as well so why should it not be sized in the same manner? Shouldn't the entire system and service be rated for 350A? What if the service went right to the panel and the panel main was the service disconnect? It only needs to be 225A correct?

 

[wwhitney]

First, the language in 430.63 of "not less than" is a bit weird, as it would appear to allow arbitrarily large OCPD without regard to conductor ampacity. So I'm not 100% clear that 350A is the correct answer for the feeder OCPD calc. But that's a separate issue from your question, so let's assume it is correct.

Then 230.90(A) Exception 1 would permit the service OCPD to be 350A as well. While the service and feeder conductors would only need to have the 212A ampacity you calculated

Cheers, Wayne

 

[mjmike]

First, the language in 430.63 of "not less than" is a bit weird, as it would appear to allow arbitrarily large OCPD without regard to conductor ampacity. So I'm not 100% clear that 350A is the correct answer for the feeder OCPD calc. But that's a separate issue from your question, so let's assume it is correct.

Then 230.90(A) Exception 1 would permit the service OCPD to be 350A as well. While the service and feeder conductors would only need to have the 212A ampacity you calculated

Cheers, Wayne

Thanks for the info. The conductors would be rated for the OCPD so I'm not concerned with the service conductors or the conductors from the disc. to the sub panel. I'm concerned with having the correct minimum OCPD at the service and the sub-panel (can always go larger). So would the sub-panel main breaker minimum rating need to be sized per 430.63 as I calculated above? For the service, would the ampacity I noted above be accurate, but it does not make sense if the sub-panel main breaker needs to be larger.

 

[wwhitney]

The conductors would be rated for the OCPD

Doing that on a motor circuit would either lead to possible nuisance tripping (if you size the OCPD to match the minimum conductor size) or to unnecessarily large conductors (if you size the conductors to match the allowed OCPD).

The conventional solution for your arrangement is to use 212A service and feeder conductors and 350A service and feeder OCPD. [Where again I find the wording of 430.63 a bit odd and would want to consider again whether that should or could be 300A rather than 350A.]

Regardless, as to your basic question, the service OCPD is the OCPD for the downstream feeder, so the service OCPD always has to be at least the minimum size feeder OCPD given by 430.63.

Cheers, Wayne