3 phase current

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stunlawless

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Afternoon everyone,

Trying to remember how to specify cable for a three phase motor.

Motor rated at 6kVA.

6000 VA /230 V = 26.1 A

26.1 A/ 3 = 8.7A.

Am I right in thinking that each phase will carry 8.7A, and so the (3+E) cable must be spec'd to carry 8.7 A ?

Seems too simple to be true...
 
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I = (kVA x1000) / (V x root3)

In your case that equates to 8.66A per phase.
 
Cheers Garymo,

Apart from using the wrong voltage & forgetting to squareroot the 3, I was correct.

Memory is not what it used to be. Coincidence that my (incorrect) method gives very nearly the right answer - two wrongs almost making a right...

Thanks, David
 
I thought so - pretty darn close!

Obviously you worked out that V in this case is 400.
 
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Hallo Garymo,

Yes, got there in the end !

Thanks for your help, David
 
GaryMo said:
Would you not only need the PF if the power was quoted in kW?
Click to expand...
I was only asking because motors are normally rated in watts/kW or in old money horse power. I was just wondering how he arrived at 6kVA.

4.8kW @ 0.8PF?
 
Stunlawless got the right answer because there's nothing wrong with his calculation! Think of his method as considering the load being shared equally by three 230V single-phase supplies, which is basically what a 3-phase supply is. :)
 
230 x (sqrt 3) = 398.37

close enough to spit on...

and it's

VA / V / Sqrt 3... not VA / ( V x Sqrt 3 )
 
LiamPope said:
Stunlawless got the right answer because there's nothing wrong with his calculation! Think of his method as considering the load being shared equally by three 230V single-phase supplies, which is basically what a 3-phase supply is. :)
Click to expand...

OK...try the same with 80kVA and see what you get!
 
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