Single phase circuit on 3 phase current calc


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?.



asked 5 months ago

2 Answers

Hi Steven,

I have a related question.  

We often utilize 240V 3PH to supply mixed 3PH loads and 1PH 240V loads on any two phases.

I want to confirm the calculation of current in the single phase loads at the line-to-line voltage in the following simplified example where the current for a single phase load of 1kVA = (1000VA / 2) / 240V = 2.08A (as opposed to 1000VA/240V = 4.17A).  The resulting system would then have a balanced 3PH load of approximately 9A.




answered 3 weeks ago tnhokie27 2
edited 3 weeks ago

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 5 months ago Steven McFadyen 246
Steven McFadyen

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