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Protection of Transformer Primary

Protection of Transformer Primary

Protection of Transformer Primary

(OP)
I'm sorry if this comes across as too basic to ask but here goes.

Need to calculate the Full Load Amps on the primary side of a 480V-120/240V, single phase transformer being supplied by only two ungrounded conductors from a three phase system. I'm told that the FLA is the kVA/480 because the load is single phase. I thought the FLA should be kVA/(1.732x480) because the system that "sees" the load is still a three phase system with the phases 120 degrees apart? How do you calculate a "two" phase load on a three phase system?

RE: Protection of Transformer Primary

You were told right. With only two wires, kVA / V = A, regardless of the phase configuration of the source. (You can't discern relative phase angles between two wires.)

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: Protection of Transformer Primary

(OP)
Thanks. That thread on the 3 phase power monitor help me understand it. I'm a very visual person and I was able to see that solution.

RE: Protection of Transformer Primary

I agree except divide by kV, not V to get A.
A = kVA/kV
 

RE: Protection of Transformer Primary

Good catch. Thanks!

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

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