Transformer sizing for PV

[electro7]

Hi,

I am getting a little confused on my transformer sizing and was wondering if I could get some input.

I am looking at 3 inverters output current total equals 240A at 480V.
240 X 1.25 = 300A at 480V
Do I need to size the transformer with taking into account the 1.25 multiplier for continuous load? If so I think I need a 250kva or next size up would be 300kva for 3 phase. If not, and I take the 240A, I think I would need a 200kva or next size up 225kva.
According to my calculations a 225kva has a current rating of 270A on the 480V side.
(225,000/(480x1.732)=270.6A
If my max output total is 240A why wouldn't I be able to use a 225kva?

Next is primary side (utility) OCPD.
Would I use this calculation (ohms law):
300A X 480V = 144,000W
144,000W / 208V = 692A
USE 700A OCPD on primary side?

I feel like I am getting a little lost in my numbers. Appreciate the help!

 

[BandGap1.1eV]

No need to complicate it. Base the transformer KVA on the cumulative inverter kW value. 200 kW worth of inverters = 200 kVA transformer. One caveat is that some commercial inverter manufacturers recommend 5-10% of headroom.

Using your stated current values it looks like your using 65 kW three phase inverters. I suggest spec'ing a 200 kVA unit and moving on from there.

 

[BandGap1.1eV]

For the OCPD's use the traditional 1.25 multiplier on the nominal current at each voltage.

200,000/480/sqrt(3) * 1.25 = 300A OCPD
200,000/208/sqrt(3) * 1.25 = 700A OCPD

Obligatory caveats apply for continuous duty ratings of fuses and breakers, and series ratings of devices, etc.

 

[Carultch]

For the OCPD's use the traditional 1.25 multiplier on the nominal current at each voltage.

200,000/480/sqrt(3) * 1.25 = 300A OCPD
200,000/208/sqrt(3) * 1.25 = 700A OCPD

Obligatory caveats apply for continuous duty ratings of fuses and breakers, and series ratings of devices, etc.

Keep in mind that inverter current does not necessarily scale directly with nominal power. Some inverters may have a larger current limit, to account for non-unity power factor or lower-than-nominal voltage. In any case, use inverter current directly from datasheet(s), and add them up accordingly. Scale by the voltage ratio, assuming the transformer is 100% efficient, to determine the maximum continuous current and corresponding circuit/ocpd sizes on the utility-connected side.