3 phase transformer secondary sizing

[Jpflex]

I’m trying to figure out secondary conductor sizing for assumed Y generator 300 KVA at 480 volts secondary at 3 phase

300,000 / 480v x 1.732 = 360.85 i total amperes


But is this total amperes to be divided equally between 3 phases AB, bc, ac? So that secondary are to be wired at 120.85 amperes x 1.25 for continuous loads? Does pick look right?

You would not wire each 3 legs at total 360.85 i amperes correct?

 

[synchro]

To draw 300 kVA from the generator, each of the line currents on the conductors for A, B, and C would be 361A as you have calculated. If this current is all coming from balanced L-L loads across A-B, B-C, and A-C, then then each of these L-L loads is drawing 361A / 1.732 = 208A.

 

[don_resqcapt19]

And the conductors between the generator and the first OCPD, must be sized at 115% of the 361 amps requiring a 600 kcmil copper conductor if using a single conductor.

 

[Jpflex]

And the conductors between the generator and the first OCPD, must be sized at 115% of the 361 amps requiring a 600 kcmil copper conductor if using a single conductor.

Why 115% but I thought it was 125% of largest motor plus sum of remaining FLC of motors or continuous loads for feeder?

 

[Jpflex]

To draw 300 kVA from the generator, each of the line currents on the conductors for A, B, and C would be 361A as you have calculated. If this current is all coming from balanced L-L loads across A-B, B-C, and A-C, then then each of these L-L loads is drawing 361A / 1.732 = 208A.

Dam This is so confusing as what you are saying sounds as if total current output is 361A x 3 phases but not 300 KVA / 3 phases

 

[synchro]

"Total current" is not a very meaningful expression unless you are considering the sum of different currents being either supplied to (or drawn from) the same conductor.

It may be easier to think about having all line-to-neutral loads, where each phase would contribute 361A x 277V = 100 kVA of the total kVA. Then the total kVA drawn by all loads will be 3 x 100 kVA = 300 kVA.

But the same line currents of 361A on each of phases A, B, C can result from line-to-line loads of 208A across each pair of the phases (i.e., A-B, B-C, A,C). For example, the current on phase B would be the sum of the load currents on A-B and on B-C. But that sum is not 2x 208A = 416A, which would be the case if the these two L-L load currents were in-phase. Instead, the two L-L load currents sum to 1.732 x 208A = 361A because they are shifted 60 degrees from each other.

 

[don_resqcapt19]

Why 115% but I thought it was 125% of largest motor plus sum of remaining FLC of motors or continuous loads for feeder?

See 445.13

 

[Jpflex]

See 445.13

I will check