# Adding single phase transformer kva together

I have been working with quit a few 3 phase transformer banks consisting of 3 single phase transformers tied together lately. Particularly either pole mounted units, or 3 “cans” at ground level, and the question arose, if each transformer “can” , is listed at 50 kva, is the xfmr bank going to be a 150kva bank? Methodology being: simply add the 3 values together, OR will the resulting 3 phase kva be 86.6kva? Methodology: taking 1 units kva value and multiplying it by the square root of 3?
At first glance I supposed you would add the banks together, but then I realized the power will be out of phase on each leg, leading me to want to use the square root of 3.
All of this is in an effort to understands the xfmrs fla.

I thought I new my xfmr equations but then got tripped up with these 3 single phase units!!! Help appreciated!

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The total capacity of the bank is 150kVA.
KVA 1ph = Vph * Iph
KVA 3ph = 3 Vph * Iph = SQR3 * VL * IL

WYE system: VL = SQR3 * Vph and IL=Iph
Delta system: VL=Vph and IL=SQR3 * Iph

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150kVA and if one of the units fails it will still operate in open delta at 57% capacity. (150 x 0.577 = 86.55KVA)

The transformer has a capacity of 150 kVA and can be configured in either a star or delta connection. If you choose the delta configuration, it will continue to operate even if one of the transformers is removed from the circuit. In this case, it will act as an open delta connection with a rating of 86.55 kVA, which is calculated by multiplying 0.577 with the original rating of 150 kVA.

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