kVA - Amps Formula for Three Phase

[Paultrim123321]

Hi gents

Just wondering if anyone can advise me on my working out.
Have been asked to advise on supply conductor csa for a sub distribution supply. The power requirements are 10kVA 400V supply.

I'm used to working with kW power just not sure on kVA - Amps formula which I have used below: -

I = 1000 x kVA / (1.732 x VL-L(V)
Therefore 10000 / (1.732 x 210V)
Therefore 10000 / 363.72
Amps = 27.49?

I'm a little confused as if I transpose kVA - KW :-
kVA x PF
10 x 0.95 = 9,500kW
So if I use 9.5kW,
9500 / 1.732 x 400 x PF (0.95)
my design current drops to 14.43Amps?

Any advice would be great thanks all

 

[steve50]

have a look here kVA to amps (A) conversion calculator

 

[Paultrim123321]

Cheers Steve, cracking link. My calculation works

 

[Silly Sausage]

Your 1st calc was almost right, but where did you get 210V from?
Your 2nd calc was almost right as well, notice that the power factors cancel out, so you don't need it!

 

[Paultrim123321]

Your 1st calc was almost right, but where did you get 210V from?
Your 2nd calc was almost right as well, notice that the power factors cancel out, so you don't need it!

Cheers Archy, I took 210V to be Line voltage - line voltage.
I see what you mean about PF! Doh
Have I miscalculated tho somewhere as if I work out kW from kVA my result is different, substantially when I transfer kW to amps? It as if I haven't root3 correctly?

 

[Paultrim123321]

Doh I meant 400V line - line voltage.....

 

[Lucien Nunes]

kVA is exactly what it looks like: kilovoltamps, so when you divide VA by the voltage, by definition you get the current. Hence it being called the apparent power, i.e. what the power appears to be from multiplying the voltage and current, without any regard to the power factor.

Real power in kW is the apparent power in kVA multiplied by the power factor, so if your power factor is less than unity the load won't be able to do as much useful work but the circuit current must still be calculated from the apparent power in kVA, without involving the power factor.

Note that for 3-phase power calcs the quick way in your head, it's not necessary to put sqrt(3) in anywhere. Just divide the rating in VA by the line-neutral voltage to give the sum of the line currents, then divide by 3 to get the line current. In your case (10k/230)/3 = 14.5A. It's useful to remember that a balanced 3-phase load on 400V takes 1.45A line current per kVA.