Tranformer selection, sizing, and calculations

[katwalatapan]

Hello,

I would like to receive opinions on my transformer selection and calculation process to rectify any problems so I could be on the right track.

To feed a facility's electrical load via 3 phase, 800 A, 120/208V electrical service, I performed the following calculation to help me select a transformer. The utility line voltage is 25 KVA.

Minimum transformer size that I may require is 288 KVA, 3 phase. This may change based on how each phase is loaded to balance the system.

Usually utility may provide 288 KVA, 3 phase via either configuring three, 1 phase transformers of appropriate rating OR by supplying pad-mount transformer. Is it normal practice by the utility to provide 3 phase 288KVA via pole mount configuration?

Am I missing any other type of calculation that may be required in selecting and sizing a transformer correctly to send the information to the utility?

Thank you.

 

[davidbeach]

Generally you tell the utility what your projected loading is and the do sizing per their practices and standards. They certainly won't come up with 288 as that isn't an available size. Unless you can show that your load will be at its maximum continuously they won't even exceed your number. My guess is that they'll either put in a 225 or even a 150. Different rule apply on the utility side of the meter than apply on the customer side.

 

[shamsdebout]

As was mentioned when you submit your building load information, the utility will perform calculations to ensure they provide a transformer adequately size for your needs. They will apply quite a bit of diversity and plan for the future what size the transformer needs to be.

They like it when you can give them a breakdown of the type of loads you will have. Some are satisfied with a panel schedule that lists the type of loads and I have worked with some that wanted me to provide the breakdown to the nth degree.

Typically a breakdown like the follow is probably much more than they will need:

Lighting
Appliance (receptacle, etc.)
Plumbing (sanitation, etc.)
Mechanical (ventilating)
Mechanical (heating)
Mechanical (cooling)
Chiller
Transportation (elevators)
Kitchen


I did this breakdown for a utility on a smaller project and had to do more detailed breakdown on a larger project. Normally at most the transformer they provide will be 65% of what you calculated so I agree that a 150 kVA or 225 kVA would not be likely for them to provide.