Sizing switchboard for 2000kVA 480/277V service transformer.

[EngineerFrank]

We are installing a new service transformer for some new equipment an industrial client is adding to their plant. We originally had some negotiations with the utility to oversize the transformer for some possible future loads and some possible load migrations. While we were in negotiations, schedule dictated that we needed to purchase the service switchboard, so we purchased a 3000A switchboard we thought would be large enough to feed anything they would supply. We originally thought 1500kVA would be the largest they would agree to supply. Turns out they had a 2000kVA they previously ordered and decided to use it for my application.

My question is it within NEC code to service a 2000kVA, 480/277V transformer with a 3000A switchboard with a 3000A MB. My concern is that 2000kVA/(480V * sqrt(3)) * 1000 * 1.25 = 3007A. Which is technically more than the bus rating. If we make sure never to overload the transformer is it within code to use the switchboard we already have purchased? The switchboard will feed various MCCs and the current loading will be around 650A (without diversity) and with future loading possibly going too 1200A total pending approval of future projects. I don't ever see this thing getting anywhere near fully loaded. I have seen example calcs online where they round of the 2000kVA fla to 2400 which makes the 1.25 multiplier come to an easy 3000A. Is my switchboard undersized for my transformer?

 

[charlie b]

What matters is the overcurrent protection that you provide for the switchboard. It has a rating of 3000 amps. You plan to install a 3000 amp breaker. You are done.

I will take it one absurd step further. You can start with the 2000 KVA transformer, install a 1000 amp switchboard, protect it with a 1000 amp main breaker, and you will be within code requirements. You would have wasted some capacity, and someone's money. But those are not code considerations.

Welcome to the forum.

 

[don_resqcapt19]

Even if you have multiple service OCPDs, as long as the calculated load is less than 3,000 amps you are good to go, even if the sum of the ratings of the OCPDs exceeds 3,000 amps.

 

[GoldDigger]

Just make sure that the available fault current from the larger transformer does not exceed the interrupting rating of the the breakers (and enclosure) you have specified.

 

[topgone]

Just make sure that the available fault current from the larger transformer does not exceed the interrupting rating of the the breakers (and enclosure) you have specified.

This! Then, you're good to go.

 

[electrofelon]

I think it has been implied but not explicitly stated, that the NEC has no control over the utility transformer sizing. Very typical to see transformer size for large services 50-40% of the NEC service size.