Verifying total amp draw on a 3 phase panel?

[OneStepBehind]

When we were testing the total amp draw on a 3 phase service panel the electrician told me that the highest reading of the 3 legs was the total draw on the panel. Is this correct? We had a 200a 208v 3 phase panel and the highest draw was 81 amps with the other two being around 72 and 75 as best I remember.
Thanks.

 

[MAS2000]

I don't know what are you testing?
However, from readings given above 81A is the highest among the three phase currents; so we can say that the highest 3-phase current was 81A at time when you performed measurements and corresponding 3-phase kVA is (assuming balanced load) 1.732 x 208/1000 x 81 = 29.2kVA.


M.A.Sh.
Elect. Engr.

 

[Skogsgurra]

If your thinking is that the sum of the three phases is the total load, then think again. Three-phase loads are usually given exactly the way your electrician told you.

If, on the other hand, you had three outgoing groups from a single-phase system, then the total load would be the sum of the three measurements.

Sounds funny? Perhaps, but that's the way it is done. One of the secrets of the trade, so to say.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[OneStepBehind]

Thanks for the reply. It was what I was looking for.
I will assume also that on a single phase panel the same thing applies? You would read both legs on the service entrance side of the panel and the highest leg will be what the panel is drawing?
Thanks again.

 

[waross]

Single phase or three phase- The highest leg is not what the panel is drawing. It is drawing what is measured (with possible corrections for power factor and possibly phase angle).
But for the purpose of sizing feeders, supply transformers and protection, you generally can't protect the conductors independently at different amperages and you can't buy three or four conductor cable with different wire sizes in the three hot conductors and you can't buy transformers with different ratings for the three phases, so you must use the reading from the highest leg to size these components.

Bill
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"Why not the best?"
Jimmy Carter