How to calculate power requirements for 3 phase? (89 Views)
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Frequent Advisor
Ryan Clerk
Posts: 44
Registered: ‎05-12-2002
Message 1 of 5 (89 Views)
Accepted Solution

How to calculate power requirements for 3 phase?

Hello experts,

I hope this is the right place to post this. One of our customers is upgrading their site UPS and generator. The question came up about how to figure out the power needs for the 3-phase Liebert AC unit. I have found the voltage and amps ratings. Do I just multiply Volts x Amps x 3 for 3-phase stuff?

Thanks in advance,
Ryan
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Acclaimed Contributor
A. Clay Stephenson
Posts: 17,825
Registered: ‎07-16-1998
Message 2 of 5 (89 Views)

Re: How to calculate power requirements for 3 phase?

Well, you were in the ball park. Normally, your UPS/gen. vendors will be glad to do all this planning for you.

The actual formula is:

P = IV * 3^0.5 * pf

where P = power in watts
I = Current in Amps (look for the FLA - Full Load Amps on the label)
V = Voltage
3^0.5 --> square root of 3
pf = power factor (Cos of phase angle); a reasonably good estimate for loads is about 0.85

If you want apparent power (VA's) drop the pf term.

Regards, Clay


Regards, Clay
If it ain't broke, I can fix that.
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Frequent Advisor
Ryan Clerk
Posts: 44
Registered: ‎05-12-2002
Message 3 of 5 (89 Views)

Re: How to calculate power requirements for 3 phase?

Clay, Thanks very much. Just to check my math.

208 Volts and 85 Amps would be
P = 208 * 85 * 1.732 * .85 = 26028.

Is this correct?

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Acclaimed Contributor
A. Clay Stephenson
Posts: 17,825
Registered: ‎07-16-1998
Message 4 of 5 (89 Views)

Re: How to calculate power requirements for 3 phase?

Ryan:

If you were one of my old students, I would ask 26028 apples? peaches? furlongs per fortnight? watts?

Now if you meant:
P = 208V * 85A * 1.732 * .85 = 26028W
or ~ 26KW then I would agree with you. We who groove on physics tend to be absolute fanatics about units - and don't you forget it.

You see 1 Volt = 1 Joule / Coulomb and 1 Ampere = 1 Coulomb / Second so that when you multiply those terms, you get Joules/Second and any child on the streets of Starkville knows that 1 Joule/s is a watt.

Not that I'm picky, mind you, Clay




If it ain't broke, I can fix that.
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Honored Contributor
Alexander M. Ermes
Posts: 2,122
Registered: ‎06-15-1997
Message 5 of 5 (89 Views)

Re: How to calculate power requirements for 3 phase?

Hi Ryan.
Clay's solution is absolutely right and he has a good sense of humor.
Clay, about these apples / pancakes etc I have heard that sentence so often from my physics teacher i could not help laughing when i saw this one.
I love to be a part of this.
Ryan, no points pls.

Rgds
Alexander M. Ermes
.. and all these memories are going to vanish like tears in the rain! final words from Rutger Hauer in "Blade Runner"
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