Sizing a panelboard after a 75KVA transformer

[elec_eng]

I had a debate with other engineer on how to size the panelboard on the secondary side of the 75KVA transformer.
We have a 75KVA (480V to 120/208V) transformer to feed a 120/208V. Our load at the panel will be less than 140A. He insists that he needs a 400A panel with 300A main to meet the code. Here is what he did.

FLA of secondary is 208A. Secondary OCPD is 208x1.25=260A. Next size up is 300A (per 450.3). His secondary conductor will be 350KCM per 240/21(c).

I would say that is totally a waste of money and that is not the code requirement.

What I would do is, first calculate the load and decide what size transformer I would need. With 140A load, I would need a 75 kVA transformer (140x208x1.732=50.4kVA). For a secondary panelboard, I would provide a 225A panelboard with 225A main since my load is only 140A and the FLA of 75kVA transformer is only 208A. My transformer conductor will be #4/0. I think this is perfectly in compliance with the code.

What would you guys normally size the secondary panelboard for 75kVA transformer?

 

[iwire]

225 with 4/0 copper

 

[TimK]

A little confused?

A little confused?

I had a debate with other engineer on how to size the panelboard on the secondary side of the 75KVA transformer.
We have a 75KVA (480V to 120/208V) transformer to feed a 120/208V. Our load at the panel will be less than 140A. He insists that he needs a 400A panel with 300A main to meet the code. Here is what he did.

FLA of secondary is 208A. Secondary OCPD is 208x1.25=260A. Next size up is 300A (per 450.3). His secondary conductor will be 350KCM per 240/21(c).

I would say that is totally a waste of money and that is not the code requirement.

What I would do is, first calculate the load and decide what size transformer I would need. With 140A load, I would need a 75 kVA transformer (140x208x1.732=50.4kVA). For a secondary panelboard, I would provide a 225A panelboard with 225A main since my load is only 140A and the FLA of 75kVA transformer is only 208A. My transformer conductor will be #4/0. I think this is perfectly in compliance with the code.

What would you guys normally size the secondary panelboard for 75kVA transformer?

It sounds like you are trying to size your panelboard off of the transformer? Size your panelboard from you load and possible future need, I don't get sizing per transformer ahead of panelboard? I would think a 200A would be suffcient.:?

 

[jim dungar]

The sizing of the secondary panel has absolutely nothing to do with the size of the transformer.

For decades we have fed 200A main breaker panels with 75kVA (at 208Y/120), but now that 225A panels are not as much of a special order so they are becoming more common.

 

[petersonra]

I had a debate with other engineer on how to size the panelboard on the secondary side of the 75KVA transformer.
We have a 75KVA (480V to 120/208V) transformer to feed a 120/208V. Our load at the panel will be less than 140A. He insists that he needs a 400A panel with 300A main to meet the code. Here is what he did.

FLA of secondary is 208A. Secondary OCPD is 208x1.25=260A. Next size up is 300A (per 450.3). His secondary conductor will be 350KCM per 240/21(c).

I would say that is totally a waste of money and that is not the code requirement.

What I would do is, first calculate the load and decide what size transformer I would need. With 140A load, I would need a 75 kVA transformer (140x208x1.732=50.4kVA). For a secondary panelboard, I would provide a 225A panelboard with 225A main since my load is only 140A and the FLA of 75kVA transformer is only 208A. My transformer conductor will be #4/0. I think this is perfectly in compliance with the code.

What would you guys normally size the secondary panelboard for 75kVA transformer?

His numbers make sense if you want to get the absolute most out of the xfmr.

Based on the load, it is overkill.

Based on giving the end user the maximum flexibility down the road is something else.

IMO, this is a design decision, and not a code issue.

You could use 1/0 and save even more with a 150A CB, or maybe 2/0 with 175A CB for more cushion.

 

[elec_eng]

Thank you for the responses. That was what I thought. Just wanted to make sure I didn't miss anything.

 

[ron]

It is a future flexibility issue IMHO. I always provide the full sized output panelboard. My clients don't even know what they are having for dinner that night, never mind know the future load.