Transformer Size for 3000A Service

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jonny1982 said:
In general, what size transformer is used for a 3000 amp commercial service. It is 277/480.

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Just because you have 3000 amps of service conductor/overcurrent protection doesn't mean your actual load ever reaches 3000 amps. Therefore POCO's typically look at the actual demand or at least an estimated demand when installing new. They sometimes even look at peak demand, for how long, and what kind of cooling off period may be normal and take that into consideration, and are willing to push a smaller transformer to certain limits than to use a larger than needed transformer.

I installed a 1200 amp service that only has about 600 amps max of load on it. POCO used a 225kVA transformer (208 volts) which is only about 625 amps when at full rating. My 1200 amp service has more room for additions. POCO may need to upgrade transformer if load goes up someday.
 
There are exceptions of course, but the vast majority of the time I see utilities install transformers with half to 40% of the NEC service size. On the other end of the spectrum, I have seen them size to meet the LRC of a large motor. Right now I have a job with 3x167 units (1390 secondary amps) on a 120/208v 1000 amp service. This was existing so I don't know why such large units were selected.
 
jonny1982 said:
In general, what size transformer is used for a 3000 amp commercial service. It is 277/480.

Thanks
Click to expand...

It would depend on the nature of the load. That’s a fairly big service at 480Y/277. After looking at the plans, and seeing what load history we might have for similar customers, I might have called for one as small as 1000 kva. Or 1500 kva. There’s a big difference between service size (NEC) and actual load (POCO) in the vast majority of cases; others have noted this as well. In my experience we often sized our stuff as small as 20% or 25% of the service size. We had a saying: “If you don’t burn one up every once in a (great) while, you’re way oversizing things.”
 
My clients are industrial. Most own their own transformers. The philosophy generally is:
If we can line the system up to where we can pull 3000A, that is what we want.
No we don't want to burn one up every now and then. Down time is way too costly.
Keep the Vd down to where the HIDs don't shut off when large motors start.
We want to run flat out 24/7
We will pay some extra to get the MTBF up​

Given these constraints, for a 480V, 3000A secondary main, I would expect the transformer to be 2MVA. FLA ~ 2400A. 2400A X 1.25 = 3000A

Which didn't answer your question, you asked about "commercial". And I don't know.
 
iceworm said:
My clients are industrial. Most own their own transformers. The philosophy generally is:
If we can line the system up to where we can pull 3000A, that is what we want.
No we don't want to burn one up every now and then. Down time is way too costly.
Keep the Vd down to where the HIDs don't shut off when large motors start.
We want to run flat out 24/7
We will pay some extra to get the MTBF up​

Given these constraints, for a 480V, 3000A secondary main, I would expect the transformer to be 2MVA. FLA ~ 2400A. 2400A X 1.25 = 3000A

Which didn't answer your question, you asked about "commercial". And I don't know.
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OP was inquiring about transformer for a service, which generally means POCO's transformer.

Customer can still desire more transformer than POCO intends to supply, usually for a cost.
 
Sometimes, POCO's use whatever they have in stock. If it's oversize or somewhat undersized, they save money using stock equipment instead of special order. Most large utilities buy in quantity, based on expected customers and past history.
 
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