Transformer Primary Current

[Backyard Bob]

Hi,

I'm running into a mental block regarding primary current for a 3kVA 480/240V control transformer. The primary is fed from 2 legs of a 480V/3PH circuit. I know that when dealing with 3-phase, I = P/1.732*E. So in the case of a 3kVA/480V transformer primary for 3PH, I = 3,000/1.732*480, which comes out to 3.6A. But here's where I'm stuck.. If I'm only using 2 of the 3 legs, is current draw now just going to be as though it were single-phase, in which case current is simply I=P/E?

The secondary is split-phase 120/240V with a grounded center tap. So the secondary is obviously a single-phase circuit, and L1-L2 FL current will just be calculated as I=3000/240, which is 12.5A. But I'm thoroughly confused now about whether or not I'm correct in using a 3PH calculation for the FL primary current? Is the 3PH calculation only correct when dealing with a balanced 3PH load utilizing all three phases like in the case of a 3PH motor?

Any help on this would be thoroughly appreciated.

-Bob

 

[MD Automation]

...If I'm only using 2 of the 3 legs, is current draw now just going to be as though it were single-phase, in which case current is simply I=P/E?

Yes, don't get bogged down here in 3Ø calculations - the primary of your single-phase control transformer is fed from a single-phase source. In your case, 480 VAC.

The fact that a little farther "upstream" that 480 single phase comes from a 3Ø source is irrelevant. Your control transformer just sees the single phase voltage that come from 2 of those legs.

So, assuming full load utilization, your primary current is simply 3kVA / 480V = 6.25 A.

 

[Backyard Bob]

Thanks for the reply MD. That makes a lot more sense now.

 

[LarryFine]

The fact that a little farther "upstream" that 480 single phase comes from a 3Ø source is irrelevant. Your control transformer just sees the single phase voltage that come from 2 of those legs.

Exactly. A two-wire source is always one phase.