Power Consumption formula 1

[warmulli]

Hi,

Can anyone help clear the air, I am trying to calculate the total KVA demand from a set of reading i took from a 3-phase supply. I used a power quality meter to record the max current reading from each phase. 120A 140A 143A, i am trying to convert the reading to a total KVA demand but having trouble with my figures. Assuming that power factor is around 85%. Please could some help with the formula.
Thanks

 

[dpc]

"Demand" is generally referring to values averaged over a period of time (15 min to 60 min). Peak demand would be the maximum of these demand values.

For three-phase circuits, kVA = kV x Current x 1.732 (where kV is line-to-line voltage). You would use the average of the three currents. Or you can calculate on a per-phase basis, using the line to neutral voltage times the current and simply add the three kVA values together.

If you are interested in kVA, the power factor is irrelevant. If you are interested in kW, you need to do a little basic review of electrical power concepts.

None of this is going to tell you the "demand" as generally defined in the electrical power community.

David Castor

http://www.cvoes.com

 

[warmulli]

Thanks for you reply, A bit confused on the two methods. If you use the first method KVA = kv x current x 1.732 this gives a much higher KVA then the second method. Is there a reason for this?

 

[dpc]

The reason is that you are doing one or both of the calculation incorrectly.

Make sure you use kV if you want kVA - or use Volts and multiply by 1000. If you are doing per phase, the 1.732 is not needed.

Really, this is very basic for a EE. Refer to one of your first-year textbooks, or an EE Handbook.

David Castor

http://www.cvoes.com

 

[waross]

Assume 480V
120 A = 33.26 KVA
140 A = 38.8 KVA
143 A = 39.63 KVA
Total = 111.69 KVA

(120 A + 140 A + 143 A)/3 = 134.33 A ave.
134.33 x 480 x 1.73 = 111.68 KVA
Seems pretty close.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[warmulli]

Thanks Bill thats great.