3 phase calcs 1

[dinkelja]

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.

 

[dpc]

You started out pretty well, but kept going too long. The phase current will be 54 amps on the primary side. The transformer winding connection (delta or wye) has no impact on the line currents (for equal kVA ratings). The only difference is in the internal winding current inside the transformer and you don't generally care about this when sizing feeders.

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.

 

[jghrist]

The individual feeders are sized according to the load of the feeders. This could be a lot less than the full load rating of the transformer.

 

[tinfoil]

If possible, I'd look at relocating the xmfr closer to the load, as 125A @ 120/208V is going to experience a lot of voltage drop over 500 feet.

 

[whycliffrussell]

You want to keep the voltage drop in the feeder to less than 3%:

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.

 

[dinkelja]

I am now informed that the xfmr is by the construction trailers (loads) and the 480 is run out to it. I thought that feeders under 600V were 5% VD, either way, they will have to make it work. Are branch circuits 3% or 5% VD?

 

[whycliffrussell]

-not exceed 5% from the supply side of the consumer’s service (or equivalent) to the point of
utilization; and
-not exceed 3% in a feeder or branch circuit.