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#137473 - 07/09/03 11:23 AM 3 phase kVA to amps per phase query
Steve|Uk Offline
Member

Registered: 07/09/03
Posts: 18
Loc: Horley, Surrey, UK
Hi all. Call me noobie Been reading for a few days, and like it.

Long story short, I'd like a refresher on how to ascertain the amps per phase if given a kVA / 3 phase rating.

For instance, an appliance is 75 kVA three phase. What is the current per phase ?

I've seen various (differing) answers on websites, which in turn give different results; Only one can be correct.

Would anyone be so kind as to refresh my memory - I'm due to start a job in industrial electrics; I've spent the last 15 years house bashing :-)

TIA

Steve|Uk

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#137474 - 07/09/03 12:11 PM Re: 3 phase kVA to amps per phase query
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Welcome to ECN! I like your use of the | divider.

The kVA is the easy one:

current = VA/(phase-to-phase voltage x square root 3 )

or in another form

current = VA/(phase-to-neutral voltage x 3)

75kVA on a 230/400V system is

75000/(230 x 3)= 109 Amps


[This message has been edited by C-H (edited 07-09-2003).]

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#137475 - 07/09/03 12:14 PM Re: 3 phase kVA to amps per phase query
Steve|Uk Offline
Member

Registered: 07/09/03
Posts: 18
Loc: Horley, Surrey, UK
Hi. Thanks for the welcome.

Yeah I did think it was that, but was getting all sorts of confuzzled input.

Just as you replied, I found this helpful page right here at electrical-contractor.net
http://electrical-contractor.net/ubb/Forum15/HTML/000043.html

I can see me liking this place

Bye for now.

Steve

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#137476 - 07/09/03 12:38 PM Re: 3 phase kVA to amps per phase query
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Hi Steve, and welcome to ECN.

I see you've already gotten a really quick answer to your question so there's nothing I can add there.

I reckon the industrial side will be quite different to what you've been doing. I've not been inivolved in that side of the job either; I do mostly residential with just a little very light commercial from time to time. That being the case, even where 3ph supplies are involved it's mostly single-phase loads, so it's so easy to forget those forumlas if you're not using them regularly.

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#137477 - 07/09/03 02:24 PM Re: 3 phase kVA to amps per phase query
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
For 400V 3ø, a rule of thumb is 1.4(4) amperes/kVA.

600V >> 1A/kVA
480V >> 1.2A/kVA
240V >> 2.4A/kVA
208V >> 2.8A/kVA

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#137478 - 07/09/03 04:30 PM Re: 3 phase kVA to amps per phase query
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Gidday there Steve,
Welcome along to ECN, mate!.
The other guys have just stolen all the thunder out of this one, no worries though.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#137479 - 07/09/03 10:45 PM Re: 3 phase kVA to amps per phase query
old Appy Offline
Member

Registered: 03/18/01
Posts: 120
Loc: Auckland, New Zealand
Very easy to forget if you aint using it!! even when you have just learnt it all again.
CW

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#137480 - 07/10/03 02:53 AM Re: 3 phase kVA to amps per phase query
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Appy,
You've got that right!.
When I last went to do a Refresher Lines Course at the Power Board, we were ALL handed an exam paper, made up by one of the Inspectors, in things like 3P theory, Neutral Currents in Star systems, etc.
I had an OK time with it all, but some of the old Lineys, really kicked up a stink about it and they also failed miserably.
They thought that they were just there to wear some gloves and do a few Pole Top Rescues.
Was good to give the Grey matter a good work out though!
[Steve, sorry about the threadjack!]

[This message has been edited by Trumpy (edited 07-10-2003).]
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#137481 - 07/10/03 03:24 AM Re: 3 phase kVA to amps per phase query
Steve|Uk Offline
Member

Registered: 07/09/03
Posts: 18
Loc: Horley, Surrey, UK
Jack away!!!!

I've said it once, and I'll say it again. I think I'm going to like it here

Steve

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#137482 - 07/10/03 06:58 AM Re: 3 phase kVA to amps per phase query
lyledunn Offline
Member

Registered: 06/30/02
Posts: 159
Loc: N.Ireland
Remember, it is usual to mark plant such as generators and transformers in kva and plant which "consumes" energy in kw. Thus a piece of piece of equipment requiring 30 kw at a power factor of 0.5 would need to be fed from a source that could deal with the current associated with 60kva.
_________________________
regards

lyle dunn

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