Three phrase power correction calcs

[amateur_24]

Hi
I dont get how to work out power dissipated for a three phase load. I understand for a single phase it's
P=I*V*pf. What is the formula for three phase? IS it the squareroot(3) x V x I x P.f. ?
Also where the heck does the square root of 3 come from?


The question am trying to answer is
A 400v, 50 hz supply has a line current of 50A at 0.8 lagging p.f. The power dissipated will be ?
According to my answer sheet it's 27.712KW.

 

[JimB]

What is the formula for three phase? IS it the squareroot(3) x V x I x P.f. ?

Yes, provided that the three phase load is balanced (ie the same on all three phases).

Also where the heck does the square root of 3 come from?

It is from the phasor sum of the line voltages (or line currents).
I can only suggest that you read up on three phase circuits.

The question am trying to answer is
A 400v, 50 hz supply has a line current of 50A at 0.8 lagging p.f. The power dissipated will be ?
According to my answer sheet it's 27.712KW.

I agree with the answer sheet.

JimB