va per phase vs. amps per phase

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collier

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i have a piece of equipment that is 20kw 3 phase 208 volts. the panelboard schedule says the following:

phase A va = 6665
phase B va = 6665
phase C va = 6665

phase A amps = 18.5
phase B amps = 18.5
phase C amps = 18.5

i thought each phase should pull 55.5, not 18.5?

very puzzled. thanks for any help
 
Re: va per phase vs. amps per phase

It does make sense! :D

It's a common error, a common misunderstanding. I say "it does make sense," in the sense that I can see where it comes from. You take the 20,000 VA, divide by 208 volts, and divide again by the square root of 3. You get the answer of 55.5 amps that you have already mentioned. Then some people see that as some type of "total amps." They think, incorrectly, that the amps per phase is one third of this "total amps." So they divide 55.3 by 3, and get 18.5.

It is all nonsense. I agree that the following statement is counter-intuitive, but it is true nonetheless: Phase A amps (55.5) plus Phase B amps (55.5) plus Phase C amps (55.5) equals a "total" (if you will allow me the use of this imprecise term) of 0.0 amps. That is because the three Phase currents are out of phase from each other by 120 degrees of arc, and that causes the sum to equal zero.
 
Re: va per phase vs. amps per phase

If all we are given is the
piece of equipment that is 20kw 3 phase 208 volts.
Click to expand...
then who's to say it's connected, internally in the equipment, wye or delta?

The math simplifies, and I, personally, can easily hold in my mind's eye, three equal parts of this load (6.666 kVA) connected in a wye, each with 120 volts across. . .then 6666 VA ? 120 V = 55.5 A.

The reality is that the load will possibly not be evenly distributed, but, for this simplistic approach, the summing of vector currents slips back to straight arithmetic.

And, regardless of how the load internal to the equipment is actually configured, the phase leg current is going to look like 55.5 A for a 20 kVA 208 V 3? load.
 
Re: va per phase vs. amps per phase

Never mind ,They did the same thing I did at first glance they used 2kw instead of 20kw.

[ July 15, 2005, 07:40 PM: Message edited by: ronaldrc ]
 
Re: va per phase vs. amps per phase

Hey Ronald,

Nice to see you here. :cool:

I like the tribute you have for Bennie at your site.

He had a delightful way to stoke the smudge pots and occasionally fan the flames. . .
 
Re: va per phase vs. amps per phase

Thanks Al

He did indeed.

On this post I divided his 6665 by 208 and then 1.73 and didn't pay attention to the total Kw.
I believe he was using 2Kw.

Ronald
 
Re: va per phase vs. amps per phase

ronaldrc,

Neat graphics. Do you mind if we post them on the forum?
 
Re: va per phase vs. amps per phase

Hi Sam

No I don't mind use them any way you want.Most of them are not that good.Just don't make fun of them. :)
 
Re: va per phase vs. amps per phase

Cool, :) I'm sure they'll come in handy for an explaination here and there. I think I've alredy got your site bookmarked somewhere but it's been so long since I've seen it I forgot it's there.
 
Re: va per phase vs. amps per phase

Al, I am with you. If we look at the problem as a wye source and wye load, it is dog simple to compute the line currents. Doesn't matter what the actual configuration is.
 
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