Transformer Capacity and Calculated load on Panelboards

[aelec84]

Hello Forum,

I have a 45kva (480V-120/208V,3Ph,4w) transformer and it feeds a panelboard section#1 and 2 (MLO). It has a 150A, 3P MCB in the panelboard section#1 and section#2 is feed thru lugs. Both sections are rated for 225A. Overall calculated loads fed by transformer via (2) panelboards equate to 75kva but individual panelboard loads are under the bus and MCB rating of the panelboard. Would this be considered a code violation? Thank you.

 

[jim dungar]

A little confusing.
Does you circuit go:
Transformer secondary
Panel 1 150A main breaker
Panel 1 loads
Thru feed lugs
Panel 2 main lugs
Panel 2 loads.

 

[wwhitney]

I have a 45kva (480V-120/208V,3Ph,4w) transformer and it feeds a panelboard section#1 and 2 (MLO). It has a 150A, 3P MCB in the panelboard section#1 and section#2 is feed thru lugs. Both sections are rated for 225A. Overall calculated loads fed by transformer via (2) panelboards equate to 75kva

With an NEC calculated load of 75 kVA, if we assume it is balanced (best case), that's a secondary current of 208A and a primary current of 90A. That means that per 215.3 your primary side OCPD must be at least 90A, while the sum of your secondary OCPDs must be at least 208A.

However, the rated currents for the 45 kVA transformer are 125A on the secondary, and 54A on the primary. Table 450.3(B) requires that you either provide primary protection of at most 125% * 54A = 68A or provide secondary protection of at most 125% * 125A = 156A.

These 450.3 maximum OCPD options are both lower than the minimum OCPD imposed by 215.3. Thus it is impossible to comply with both 215.3 and 450.3 for a 45 kVA transformer with 75 kVA of load.

Cheers, Wayne

 

[aelec84]

A little confusing.
Does you circuit go:
Transformer secondary
Panel 1 150A main breaker
Panel 1 loads
Thru feed lugs
Panel 2 main lugs
Panel 2 loads.

Correct

 

[augie47]

Agree with Wayne, Art 215 would require feeders sized to carry the load and your 45kva transformer would be undersized for the feeders and load.

 

[wwhitney]

Agree with Wayne, Art 215 would require feeders sized to carry the load and your 45kva transformer would be undersized for the feeders and load.

I'm not aware of anything in the NEC that requires you to size the transformer to the feeder or the load.

What the NEC does have is that given the minimum OCPD size in 215.3, the transformer has to be large enough to be properly protected by the OCPD used per 450.3. The net result, if the primary or secondary transformer rated current happens to match a standard OCPD size, is that the transformer could be rated at 80% of the calculated load, but no smaller.

Cheers, Wayne