Single phase circuit on 3 phase current calc

Hi,

I have a question regarding calculating current in a 3 phase supply. I have my head around how to calculate the current in a 3 phase circuit. For example if we have a 22kw load assuming a power factor of 0.8 would be 22000/(1.732x415x0.8) = 38.25A

However if I’m installing a 7.2kw single phase circuit on a 3-phase supply I.e using L1 (or L2 or L3) and neutral. Is the calc 7200/(1.732x230x0.8) = 18.06 amps

It’s not the usual 7200/230 (simplified calc) where the current would be 31.30A we would use for a single phase supply?. Even though it is a single phase circuit?.

Thanks,

A.
asked 3 months ago
spark83192
spark8319

1 Answer

Looks like in your first example you are using 415 V, and 230 in your second.  The common convention is 415/240 or 400/230 V  [line-line/line-neutral].

This is how I like to calculate:

First example: 22000/0.8 = 27500 VA.  I =  (27500/3) / 240 = 38.2.  Same as your answer, just a slightly different way of calculating (and using 240 V, whereas 415/1.732 = 239.6 more accurately).  

Second example: 7200/0.8 = 90000VA = I = 9000/230 = 39.1 A.  

In your formulae using 1.732 x voltage assumes the voltage is line-line.  In the second example, the voltage is line-neutral. 

Hope this helps.

answered 3 months ago Steven McFadyen 246
Steven McFadyen

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