30kva ups 400voltage 3phase ====amps?

Question

30kva ups 400voltage 3phase ====amps?

Answer 1

30kva/400 = 75a. divided between 3 phases, that is 25 amps per phase.

Answer 2

Actually the 3 phase LINE current is 43.3A.

You forgot the square root of 3.

You can prove this a few ways.

We know that there are two 3 phase connections that are used most frequently. Delta and Wye.

Lets analyze both.

Suppose the 3 phase KVA is 30 and we have a wye system.

With a wye system the line voltage is 400V as given, the phase voltage will thus be 400/1.732 = 230.9V

In a wye system, by inspection, line current = phase current.

So you can say there is 10kva per phase. So 10kva per phase divided by the phase voltage = 10000/230.9 = 43.3A

Since this is a wye system phase current = line current and the line current you will observe is 43.3A.

You can simplify this by coming straight out and saying 30000/(400x1.732) = 43.3A.

But lets now use this example with a delta connection.

With a delta connection, the line voltage = phase voltage.

Line current = phase current * 1.732. Therefore phase current = line current/1.732.

So doing like we did before you can divide the kva between phases with 10kva per phase.

Letting phase voltage be 400 and the kva of that phase be 10, then we can find the phase current of the delta connection.

Phase current (delta) = 10000/400 = 25A

But this is phase current. The line current is phase current x 1.732 = 25 x 1.732 = 43.3A.

Again you can simplify the calculation and just say line current = 30000/(400 x 1.732) = 43.3A.

If its 3 phase, do not forget to put in the square root of 3.

If its wye or delta and if you're looking for phase current or phase voltage you have to know the laws that govern the phase current and voltage of each particular system.

Most times in the power world we do not concern ourselves with actual phase currents or phase voltages and worry about line current exclusively (this is how we determine the wire size and how the conductors/equipment will be protected).

Straight up to calculate line current in a 3 phase application the calculation is (KVA x 1000) / (Line voltage * 1.732).