3 phase current

[stunlawless]

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...

 

[GaryMo]

I = (kVA x1000) / (V x root3)

In your case that equates to 8.66A per phase.

 

[stunlawless]

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

 

[GaryMo]

I thought so - pretty darn close!

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

 

[stunlawless]

Hallo Garymo,

Yes, got there in the end !

Thanks for your help, David

 

[Pensdown]

Afternoon everyone,

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

Motor rated at 6kVA.

What's the Power Factor?

 

[GaryMo]

Would you not only need the PF if the power was quoted in kW?

 

[ColJack]

Handy online calculator..

 

[Pensdown]

Would you not only need the PF if the power was quoted in kW?

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?

 

[LiamPope]

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.

 

[ColJack]

230 x (sqrt 3) = 398.37

close enough to spit on...

and it's

VA / V / Sqrt 3... not VA / ( V x Sqrt 3 )

 

[GaryMo]

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.

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

 

[GaryMo]

Would you not only need the PF if the power was quoted in kW?

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?

I see where you're coming from now.

 

[ColJack]

80000 / 400 / sqrt3 = 115.47A

80000 / 230 / 3 = 115.94A

 

[GaryMo]

I missed his /3 bit