3 phase calcs
3 phase calcs
(OP)
I would like someone to verify an engineers calculations:
I have a delta / Y-grounded 45 KVA-3phase 480-208/120V transformer.
I calculate the rated line current at I primary @ 54.12A and I secondary @ 125A. The individual phase current should be Ip = 54.12/sqrt(3) = 31.25A and Is = 125A. The individual feeders are sized according to the phase current correct? Is this the obvious conclusion, I want to be sure? The secondary feeders (Al) are going to be like 500 feet long! (outside in winter though)
Thanks for the help.
I have a delta / Y-grounded 45 KVA-3phase 480-208/120V transformer.
I calculate the rated line current at I primary @ 54.12A and I secondary @ 125A. The individual phase current should be Ip = 54.12/sqrt(3) = 31.25A and Is = 125A. The individual feeders are sized according to the phase current correct? Is this the obvious conclusion, I want to be sure? The secondary feeders (Al) are going to be like 500 feet long! (outside in winter though)
Thanks for the help.






RE: 3 phase calcs
For the secondary side - 500 feet at 208 is a long run, regardless of the weather. You need to check the voltage drop and perhaps increase the conductor size.
You also have to take into account the NEC requirements for transformer protection and the impact of this on the feeder sizes.
RE: 3 phase calcs
RE: 3 phase calcs
RE: 3 phase calcs
The voltage drop will be determined by the load as follows:
Vdrop[%]=((VA[3 phase]*(Line Impedance/distance)*Distance)/(root(3)*Voltage[line-line]))/Voltage[line-line,nominal]
I have a spreadsheet that does all this automatically - If you want it I can email it to you.
RE: 3 phase calcs
RE: 3 phase calcs
utilization; and
-not exceed 3% in a feeder or branch circuit.