Sizing service to 500 kVA utility transformer

[Hv&Lv]

Was there enough room in the terminal cabinet of the 150kVA transformer for all the conductors (presuming you have at least 4000 amps of conductors to terminate there)? Or was this more of an open transformer inside some kind of enclosure/barrier or even up on pole/pole supported platform?

You just stack 3 on each side over a 6-8 hole spade. They all will go.
It’s crowded in there.. barely had room for the CTs

 

[Russell39]

From a utility guy perspective.
Put in what the customer wants..
we will get a bigger transformer.
More kWh = more revenue..

We have a mill that shut down. They have a couple of 2000 amp panels. The usage is so low there we put in a 150kVA.
If they get cranked up again we will make it bigger.

No load losses add up pretty quic

From a utility guy perspective.
Put in what the customer wants..
we will get a bigger transformer.
More kWh = more revenue..

We have a mill that shut down. They have a couple of 2000 amp panels. The usage is so low there we put in a 150kVA.
If they get cranked up again we will make it bigger.

No load losses add up pretty quick

How often is the customers demand compared to the utility transformer rating? I never really thought much about it, but I would think it would be automated with software and monitored at each location. The POCO rep on this project indicated they would know if it needed to be upgraded in the future.

 

[Hv&Lv]

How often is the customers demand compared to the utility transformer rating? I never really thought much about it, but I would think it would be automated with software and monitored at each location. The POCO rep on this project indicated they would know if it needed to be upgraded in the future.

Your assumption is spot on.

We keep a database with all transformers on system with all loading. From the smallest 5kVA for a sign to our 2.5MaVA XF.
We get a report every month with transformer loading. I can change a parameter to look at all transformers that are say, 80% loading.. set it and see results..
it’s set now to automatically run monthly and send the results via email for all over 90% loaded. That way we stay on top of it.

 

[magno78]

You're right to be cautious. With a 500 kVA 480Y/277V transformer, you're maxing around 601A (line-to-line). Installing a 1200A panel with an 800A main could be misleading and risky, even if load calcs are under. Utility may allow overload, but from your side (liability-wise), I’d suggest sticking with a 600A main—match capacity to transformer.


Think of it like house wiring: brown (live), blue (neutral), green/yellow (earth) — clear roles, no overpromise. Keep it safe and spec-aligned.

 

[kwired]

How often is the customers demand compared to the utility transformer rating? I never really thought much about it, but I would think it would be automated with software and monitored at each location. The POCO rep on this project indicated they would know if it needed to be upgraded in the future.

If billing contract involved demand charges it is constantly monitored and demand penalties are administered if you go over your allowed demand. POCO may not automatically start look into whether the transformer is sufficient for the load, but with the penalties the customer may start to watch their load pattern or possibly want to renegotiate new demand terms in their contract to save on those penalties which possibly might be a sign to the POCO to check out whether their equipment is sufficient to supply this new agreement.

Many POCO's here such demand arrangements often begins when they have about 100 kVA or larger transformer involved and applies to both single or three phase supply.

 

[Knightryder12]

You're right to be cautious. With a 500 kVA 480Y/277V transformer, you're maxing around 601A (line-to-line). Installing a 1200A panel with an 800A main could be misleading and risky, even if load calcs are under. Utility may allow overload, but from your side (liability-wise), I’d suggest sticking with a 600A main—match capacity to transformer.


Think of it like house wiring: brown (live), blue (neutral), green/yellow (earth) — clear roles, no overpromise. Keep it safe and spec-aligned.

All utility transformers, and I mean all, are undersized here in Florida. I cannot tell you how many times I have designed a 4000 amp, 480/277v service and the utility sets a 1500kva transformer.

 

[CoolWill]

All utility transformers, and I mean all, are undersized here in Florida. I cannot tell you how many times I have designed a 4000 amp, 480/277v service and the utility sets a 1500kva transformer.

Yep. The utility has tons of real-world data to back up their transformer selection. They've seen it all. Rarely do we connect some novel load that no one has ever seen before.

 

[kwired]

All utility transformers, and I mean all, are undersized here in Florida. I cannot tell you how many times I have designed a 4000 amp, 480/277v service and the utility sets a 1500kva transformer.

You ever looked at the actual demand after the fact to see if it was ever pushed too hard for too long.

They will allow a certain amount of overloading as long as there is sufficient cool down time in between peak demand periods. They have methods of determining what is acceptable for this on their equipment. We can't normally do that from NEC perspective though.

 

[Knightryder12]

You ever looked at the actual demand after the fact to see if it was ever pushed too hard for too long.

They will allow a certain amount of overloading as long as there is sufficient cool down time in between peak demand periods. They have methods of determining what is acceptable for this on their equipment. We can't normally do that from NEC perspective though.

I have. i have gone back to the same facility 2 years later to do another addition and the worst case I have ever seen is about 55-60% loaded, and that was on a manufacturing facility. The NEC makes us oversize our services to an extreme to say the least.