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Sizing 3 Phase Service

Sizing 3 Phase Service

Sizing 3 Phase Service

I am having issues with available amperage for three phase. Just wanted to confirm this:

For a 400A switch gear (320A usable), 277/480V pole top installation, the individual transformers would need to be 100 kVA each?

Assuming the full 320A is used, 320A*480V*1.732 = 266kVA 3 phase. So divide that by 3 to get the three individual transformers, 88kVA, rounded up to the next available transformer size would be three 100 kVA transformers, correct? (you could probably round down and use three 75 kVA transformers too). [b]Available amperage is 300kVA/(480*1.732)=360A 3 phase, and that is the amperage allowed on each individual phase (100 kVA/277V = 360A).

The main point here is that you cannot add up the currents on each phase to get the 3 phase amps you are looking for, correct? The other side of the discussion is trying to explain that three 37.5kVA transformers will be needed. (37.5kVA/277=135A, multiplied by 3 would provide 406Amps three phase capacity).

Please help me understand if I am wrong, thank you!

RE: Sizing 3 Phase Service

The numbers look correct. A 3 x 100 kVA bank will have a full load capacity of 361 A at 480 V.

But your concept of "usable amps" being 80% of the rating of the switchboard isn't quite correct - at least in NEC land. If the main breaker is rated for 400 A, it will have a maximum continuous rating of 80% of 400 A or 320 A. Continuous is defined as three hours or more. So non-continuous loads could exceed the 320 A. If this is a utility-owned transformer bank, the NEC doesn't apply and a smaller bank could be installed, based on the fact that the actual load will be less than the calculated NEC load. It's common for utilities to install smaller transformer banks than the NEC would require based on their experience and the type of facility being served.

RE: Sizing 3 Phase Service

Also, the sizing of transformers used by utilities has little relationship to what the rest of us would reasonably use. They own it, they set their own rules on how much stress they want to put them under. years ago I had a building with a 200A 3 Phase service drop to it, but the transformers looked too small to me and one day while on the roof for something else, I walked over and could see they were clearly marked 25kVA.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Sizing 3 Phase Service

Your answer is correct if you are doing an installation to meet NEC. If you are a utility, your answer will be you need to determine the load class of the customer and use historical data to size the transformer. Many utilities use watts per square foot of the customer property based on data from similar properties and then calculate the load. For industrial customers or customers with large single loads (such as water pumping stations), those will play a bigger factor than relying on historical load data. But most of the time, the historical load data with some multipliers are used. Overhead oil-filled transformers have capability of short term overloads as long as they are not sustained and there is a period after the overload where they are loaded less than full load. As such, a 400A service very frequently will see transformers much smaller, possibly 75kVA or 150kVA isn't uncommon. But you need to have the load data to back that up.

RE: Sizing 3 Phase Service

For comparison of one instance only.
A small sawmill estimate of transformer size:
The tariff depended on the ownership of the transform.
If the transformer was owned by the customer and metered on the primary, then it must be sized to NEC standards.
By NEC standards the transformers would be approximately 1 KVA per HP.
If the transformer was owned by the utility and secondary metering was used, the transformer would be sized at 1/2 KVA per HP.

Ohm's law
Not just a good idea;
It's the LAW!

RE: Sizing 3 Phase Service

You have to look at loads, not just the service size. It may be that your 200 A service is twice a large as it needs to be rather that the transformer being half the size that it needs to be.

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