# Help sizing buck/boost transformer?

• posted

Load is 130A FLA at 220V = 28.6KVA. Supply is 3-phase, "Y", 208V.

Using a 120-240/12-24 buck-boost transformer, what KVA should I look for? One source says that this transformer increases voltage by 10 percent, so KVS should be rated at 10 percent also. This means I should look for 3KVA transformer?

Also, if my understanding is correct, because of the nature of buck boosts, under light loading the output voltage will be higher than under heavy load. The equipment should be OK with this voltage swing, and if not, a full isolation transformer should be used. What might this swing be? (Note my use of "might"...)

Is my thinking sound?

Thanks, Sparky

• posted

130 * 12 = 1560 130 * 24 = 3120

That's the VA you need for each case, buck or boost. I assume the transformer is NOT a three phase model - you will use three of them? The VA will be for each, not the total.

Slightly but no more than if you had the correct voltage.

• posted

BTW, I'm considering single-phase b-b transformers. Two of them apparently will be needed for three-phase voltage conversion.

Thanks, Sparky

• posted

Sparky -

I looked at my buck/boost recommendation chart and your amperage falls between using two (2) 3 KVA units and (2) 5 KVA units. The closest voltages listed in the boosting section was 208 - 229. With two (2) 3 KVA units, your available amperage is 125A. With two (2) 5 KVA units is jumps to 208A.

The voltage swings will affect different types of equipment differently. Simple motors should work without any issues. Electronic and computer-based equipment may be negatively affected or not work at all if they have voltage sensors that shut the equipment down if the voltage gets out of range.

You are right. Voltage will be affected by loading, but not a lot. If you need calculated numbers I can get those for you on Monday. That brings up a question. Will the equipment actually draw 130A? If so, you should definitely go with the 5 KVA units. You should never consistently run a transformer at more than 80% of its rated KVA unless you only need it for a short time because its lifespan will be shortened dramatically.

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