How much can utilities undersize transformers?

[MattEng]

I know utility companies don't have to follow the same NEC rules as us peasants, but to what degree are they allowed to undersize the utility owned transformer?

I've got 4000amp 480V switchgear for a new service, the transformer we're being proved is a 750kVA which feels awfully small to me even when accounting for not all of the 4000A capacity being used at once, and utility allowed to undersize their equipment compared to if it was a customer owned transformer. I just don't know what that typical rule is, and don't envy trying to argue back with a utility but yeah I have concerns if this will meet the needs of the customer without going to an early grave.

Also not sure if this is the right forum, but it's specifically NOT an NEC question.

 

[charlie b]

I am accustomed to utility company transformers being sized around 40% of the NEC service calculation. Your example is well below that, and I would push back.

This forum covers more than NEC issues, so your question is appropriate.

Is this ComEd? What area of Illinois is your project?

 

[ggunn]

I am accustomed to utility company transformers being sized around 40% of the NEC service calculation.

I once saw a 150kVA 480/277V (180A) transformer on a 400A service, and I called the POCO to ask about it. The guy I talked to wasn't concerned.

 

[charlie b]

I once saw a 150kVA 480/277V (180A) transformer on a 400A service. . . .

That's 45%, and we'll within what I have seen for utility transformers.

 

[MattEng]

I am accustomed to utility company transformers being sized around 40% of the NEC service calculation. Your example is well below that, and I would push back.

This forum covers more than NEC issues, so your question is appropriate.

Is this ComEd? What area of Illinois is your project?

WE Engergies, I do stuff in northern Illinois as well as southern Wisconsin

In between when I asked my question and now- a project manager got back to me. Apparently, the initial application had the large switchgear for future tenants, but the first tenant was only expected to have 2 400A meters for warehouse. And there's been shakeup and now the tenant will be a factory and using the full capacity but the service application never got updated.

So it's a discussion, but at least I see where the issue happened

Thanks for the info that being down to 40% of size is still in the normal range for future reference. I've seen it all over the place but didn't know what the lower floor was just that this raised alarm bells to me.

 

[jim dungar]

As long as they can provide the correct voltage, within their tolerances, they usually don't worry about their transformer.

 

[David Castor]

We typically sized them at 50% of the NEC-calculated load.

 

[electrofelon]

I once saw a 150kVA 480/277V (180A) transformer on a 400A service, and I called the POCO to ask about it. The guy I talked to wasn't concerned.

I inquired about their transformer size once and was told not in the politest tone, "let us worry about sizing our equipment."

 

[hillbilly1]

I was able to get a bigger transformer from my local poco, they wanted to put in a 450 for a 1600 amp 480 volt service, but would put in a 750 for a couple thousand more. The owner was willing to cough up the extra money. They were kinda grumpy about it, because they only had one 750 in the yard, and would have to order another as a spare.

 

[ggunn]

I inquired about their transformer size once and was told not in the politest tone, "let us worry about sizing our equipment."

How politely did you ask about it?

 

[petersonra]

I know utility companies don't have to follow the same NEC rules as us peasants, but to what degree are they allowed to undersize the utility owned transformer?

I've got 4000amp 480V switchgear for a new service, the transformer we're being proved is a 750kVA which feels awfully small to me even when accounting for not all of the 4000A capacity being used at once, and utility allowed to undersize their equipment compared to if it was a customer owned transformer. I just don't know what that typical rule is, and don't envy trying to argue back with a utility but yeah I have concerns if this will meet the needs of the customer without going to an early grave.

Also not sure if this is the right forum, but it's specifically NOT an NEC question.

They don't really care about the rating of your switchgear. It is mostly about your calculated load. From what I can tell they will size their transformer at 30-50% of the calculated load. But my sample size is very small.

They will also whatever transformer size they stock. You are not going to get a special. If they are out of the next size up, or very low in stock on them, they might have given you this size cause it is what they have.

It is doubtful your calculated load is anywhere near 4000 amps. If it was you would probably not have 4000 amp switchgear.

Incidentally, I am not sure the nec has any transformer sizing requirements in it.

 

[MiamiValleyelec]

A guy from utility company told me they’ll run transformers at 300%. I was surprised.

 

[petersonra]

A guy from utility company told me they’ll run transformers at 300%. I was surprised.

Of calculated load. I am not surprised. Nec load calculations are very conservative.