current calculation for 3 phase

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dataca

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Hi all. I've always been confused about current calculations for 3 phase power. I'm sure someone will be able to clear this up for me. I see this equation everywhere: I= volt amps/(voltage* power factor * 1.73)

Suppose I have 120/ 208Y, 3 phase, 4 wire source. I like to put a 800VA load between L1 & N (which is 120v).
My question is, is the current:

I= 800VA /(120V* power factor * 1.73)
or
I= 800VA/(208V* power factor * 1.73)

I guess I don't understand which voltage to use.
Thanks for your time.
 
dataca said:
Hi all. I've always been confused about current calculations for 3 phase power. I'm sure someone will be able to clear this up for me. I see this equation everywhere: I= volt amps/(voltage* power factor * 1.73)

Suppose I have 120/ 208Y, 3 phase, 4 wire source. I like to put a 800VA load between L1 & N (which is 120v).
My question is, is the current:

I= 800VA /(120V* power factor * 1.73)
or
I= 800VA/(208V* power factor * 1.73)

I guess I don't understand which voltage to use.
Thanks for your time.
Click to expand...
Line to neutral load, as you note, is single phase. No sqrt(3) factoring.

Also VA is the wrong unit measure: P = E?I?pf ...where P unit is watt.

I = 800W ? 120V ? pf
 
dataca said:
I like to put a 800VA load between L1 & N (which is 120v).
Click to expand...
That is a single phase circuit. The factor of 1.732 (i.e., the square root of 3) never comes into play for a single phase circuit. For a three phase circuit, that factor is going to show up somewhere along the line. The thing that you need to learn is where to put that factor within any given equation.
dataca said:
My question is, is the current:
I= 800VA /(120V* power factor * 1.73)
or
I= 800VA/(208V* power factor * 1.73)
Click to expand...
Both are wrong, for two separate reasons. The first is wrong because, as I said before, the 1.732 does not show up in a single phase circuit. Both are wrong because when going from a value of VA to a value of current, you do not bring the power factor into the equation.

  • For a single phase, 120 volt load, I= 800VA /(120V) = 6.7 amps.
  • For a single phase, 208 volt load, I= 800VA /(208V) = 3.8 amps.
  • For a three phase load, I= 800VA/(208V* 1.73) = 2.2 amps.
 
Smart $ said:
Line to neutral load, as you note, is single phase. No sqrt(3) factoring.

Also VA is the wrong unit measure: P = E?I?pf ...where P unit is watt.

I = 800W ? 120V ? pf
Click to expand...

Actually, if the OP really does mean VA the calculation is just 800/120.
 
Besoeker said:
Actually, if the OP really does mean VA the calculation is just 800/120.
Click to expand...
Good of you to reply.
It's not altogether unusual to have someone pose a question as their first and only post then disappear back into the woodwork.

I see it here and elsewhere.
It's rude.
 
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