I have scoured through my engineering books for an answer to a simple question, and can't seem to find it.

When you tie three single phase transformers together for a 3-phase service, is the kVA rating of the 3-phase system that of the rating of the single phase transformers or is it additive?

i.e.: I have 3 single phase 37.5kVA step down transformers tied for 3 phase 208Volts. Do I run my calcs as 3phase 37.5kvA OR 3phase 112.5kVA?

Any supporting calcs would be awesome. My personnal experience is that they are additive and I would have 112.5kVA in my example, but I can not seem to find a reference to support it.

Thanks for the help!

Hopefully the impedances are the same (or very close). Yes you add them so 37.5 x 3 is 112.5 kVA.

Thanks Jim, that was my feeling. However I talked to a couple of engineers and an Sr electrician and no one had a concrete answer.
Is there a formula that proves the point? Or is it as simple as P = VI (sqrt 3) x 3 (with similiar Z)

Yes, it is that simple. kVA (3 phase) = kVA (1 phase) x 3 Assuming all three phases are equal.

And to continue the thought... an accompanying question is typically... What would the impedance of the 3 transformers connected as a single 3 ph transformer be? They are simply not added together.

The maximum current through each transformer stays the same whether it is connected single phase or three phase. Therefore, for a Wye connection the kVA is E X I X Sqrt 3. --> 37.5 X 1.73 = 65 kVA. In Wye, the phase current = the line current. If you were making a Delta connection the total kVA would be 37.5 X 3 since the line current = the phase current * sqrt 3. In delta, kva = E X I X sqrt 3 X sqrt 3 = E X I X 3. Hope this helps.

If the three single-phase transformers are identical, then their per unit impedances will also be identical. The per unit impedance of the assembly will be the same as its components.

I don't think so, or perhaps both yes AND no.
The KVA RATING of each transformer stays the same, regardless of the configuration. [And in the above I assume you are talking primary side configuration]

To do an apples to apples comparision, you still have to be applying 1PU voltage across the winding.

In your Wye case, your Line voltage also needs to be multiplied by Sqrt3 to get you back to 1PU on the transformer, getting you back to an overall KVA rating of E*I*3.

Or if you want to look at it the other way, you need to recalculate the NEW transformer KVA 1PU base at the "new" 1PU voltage, and the new total KVA will still be 3 * the new KVA base. [Yes, the new total KVA would be 65]