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Posted By: Steve|Uk 3 phase kVA to amps per phase query - 07/09/03 06:23 PM
Hi all. Call me noobie [Linked Image] 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
Posted By: C-H Re: 3 phase kVA to amps per phase query - 07/09/03 07:11 PM
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).]
Posted By: Steve|Uk Re: 3 phase kVA to amps per phase query - 07/09/03 07:14 PM
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
https://www.electrical-contractor.net/ubb/Forum15/HTML/000043.html

I can see me liking this place [Linked Image]

Bye for now.

Steve
Posted By: pauluk Re: 3 phase kVA to amps per phase query - 07/09/03 07:38 PM
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.
Posted By: Bjarney Re: 3 phase kVA to amps per phase query - 07/09/03 09:24 PM
For 400V 3ΓΈ, 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
Posted By: Trumpy Re: 3 phase kVA to amps per phase query - 07/09/03 11:30 PM
Gidday there Steve,
Welcome along to ECN, mate!. [Linked Image]
The other guys have just stolen all the thunder out of this one, no worries though.
Posted By: old Appy Re: 3 phase kVA to amps per phase query - 07/10/03 05:45 AM
Very easy to forget if you aint using it!! even when you have just learnt it all again.
CW
Posted By: Trumpy Re: 3 phase kVA to amps per phase query - 07/10/03 09:53 AM
Appy,
You've got that right!. [Linked Image]
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! [Linked Image]
[Steve, sorry about the threadjack!]

[This message has been edited by Trumpy (edited 07-10-2003).]
Posted By: Steve|Uk Re: 3 phase kVA to amps per phase query - 07/10/03 10:24 AM
Jack away!!!!

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

Steve
Posted By: lyledunn Re: 3 phase kVA to amps per phase query - 07/10/03 01:58 PM
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.
Posted By: Steve|Uk Re: 3 phase kVA to amps per phase query - 07/10/03 03:52 PM
Funnily enough, I had remembered that kW needs power factor calculations, I had just gone completely blank on the kVA - but hey ho, if I hadn't got stuck, I wouldn't have stumbled upon this place.

Cheers

Steve
Posted By: Trumpy Re: 3 phase kVA to amps per phase query - 07/10/03 11:23 PM
Just a note about power factor.
In a reactive circuit (one that has elements of inductive or capacitive reactance),the reactive power (kVAr) still has to be allowed for, even though it does no real work.
True power(kW), is the only component that does real work, this is dissipated in the resistive elements of the circuit.
Apparent power(kVA) is the product of RMS voltage and RMS Current, this is made up of both True and Reactive power.
Reduction of the reactive and Apparent power components, improves over-all power factor.
Just a note on the kVA rating of transformers, they are rated this way because the load that they supply, may have a variable power factor, depending up on what is connected to them. [Linked Image]
Posted By: j a harrison Re: 3 phase kVA to amps per phase query - 07/11/03 07:14 PM
Steve.

Again welcome to ECN, i do mainly commercial and industrial installation works, (up to 33kv if needed.

If you need any pointers, just post it on the site and we will endeavour to help you out.

Bye for now,

John H
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