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Anyone care to help confirm my thinki...

# Anyone care to help confirm my thinking on the 3PH to Single Phase??? Please!

DownTownFool
#1 Dec 7, 2012
Ok, here are the facts:

I have 3PH 480 coming in from the power company (kVA is irrelevant at this point).

I want to install the following:

Three (3) single phase 480 to 240/120 25kVA transformers
One (1) single phase 480 to 240/120 10kVA transformer
One (1) 15HP 480V 3PH motor

According to Ugly's Book which references NEC, a single phase 25kVA has a FLA of 52.1, a single phase 10kVA transformer has a FLA of 20.8, and a 3PH 15HP motor has an FLA of 24.15 (FLA's obtained from Ugly's Book).

They will be wired as follows:

1phase 1phase 1phase 1phase 3phase
25kVA 25kVA 10kVA 25kVA 15HP
Phase - A X X X X

Phase - B X X X

Phase - C X X X X

Does this mean EACH phase will see the following amperage?

Phase - A - 52.1 + 20.8 + 52.1 + 24.15 = 149.15A

Phase - B - 52.1 + 52.1 + 24.15 = 149.15A = 128.35A

Phase - C - 52.1 + 52.1 + 20.8 +24.15 = 149.15A

For a given 480V to 240/120 single phase transformer...will BOTH primary 480V legs ( say A and B for the first 25kVA listed above) see the same current (52.1 amps assuming the transformer is 100% loaded)?

OR

will BOTH primary 480V legs see 26.05A (half of the 52.1 amps assuming the transformer is 100% loaded)?

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