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KVA calculation

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New Dexter
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KVA calculation

Post by New Dexter »

Earlier this year I tried to find an electrician who could assess my workshop's KVA requirement and that proved impossible. It wasn't that I wanted it done for nothing, I was quite prepared to pay but no-one could bother to be here.
Now I was hoping that someone on this forum might be able to calculate the requirement. I have four machines that run from a 16 amp 3 phase supply. One a compressor, a cut-off saw, a Colchester Student lathe and a 320amp welder. I don't use any of them at the same time, that would take multi-tasking a bit too far but occasionally the compressor does blow the trip when it starts.
It was suggested that 12 KVA would suffice but does anyone on this forum know better please?

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watkindj
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Re: KVA calculation

Post by watkindj »

whats the load amperage of the compressor and the lathe ? and whats your current KVA supply ?

as a 12 kva supply should be what you have now at 16 amp
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Sydsmith
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Re: KVA calculation

Post by Sydsmith »

Are you using three phase 440 volt equipment or is some or all of it 240 volt?

Much will depend on the answer to that question as 440 volt equipment uses more or less half the current consumed by 240 volt equipment.

It is much better to have too big a supply than a limited supply even if you do not intend to use more than one piece of equipment at a time.

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Re: KVA calculation

Post by KV8 »

Sydsmith wrote: Thu May 24, 2018 12:15 pm Are you using three phase 440 volt equipment or is some or all of it 240 volt?

Much will depend on the answer to that question as 440 volt equipment uses more or less half the current consumed by 240 volt equipment.

It is much better to have too big a supply than a limited supply even if you do not intend to use more than one piece of equipment at a time.
HI
The UK nominal single phase voltage is actually 230V not 240V - result of EU normalisation in 2004. Thus all single phase equipment carrying a CE mark is rated at 230V. The three phase voltage available in domestic and light industrial premises is thus 230 x sqrt3 =398V (nominally 400V). Actually nothing changed as the nominal 240V/415V UK supply was actually distributed at about 235V/407V which was well within the tolerance of the new 230V/400V rating.

Many years ago I have encountered 440V 3 phase in heavy industrial plants that have their own substations however the derived 13A wall sockets were thus rated at 254V and gave problems with some ancillary equipment.

Point is if the OP has 3 phase gear it should be rated at 400V (CE)or 415V (if pre 2004) as he will not be able to get a 440V supply easily.

H

Ian Slade
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Re: KVA calculation

Post by Ian Slade »

Tripping and KVA are not the same thing, KVA is a simple arithmetic calculation multiply the volts by the total addition of all the rated amps drawn for each machine. Motor driven machines can use up to 4 times the rated current at start up and the correct breaker must be installed, motor breakers or trips have a delay allowing over current for a limited period of time, it may be possible you have a non inductive breaker installed. Your 320A welder will not draw 320A from the mains the amps are at the torch, not the welder input so could be as little as 16A if a single phase machine or less than 5A per phase if 3phase.
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Sydsmith
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Re: KVA calculation

Post by Sydsmith »

Newdexter is clearly not familiar with supply requirements hence his question. All the theory in the world won't help him and will just go over his head. All he wants to know is what rated supply do I need?

We can't answer that question properly unless we know the current rating and voltage of the equipment he intends to use.

If you ask any man in the street what voltage we use in the UK they will probably say 240 volts, the change in 2004 went almost unnoticed. At the time I raised the matter with the powers that be, because the supply companies were and still are cheating the consumer. A drop in voltage of 10 volts will increase the current drawn by equipment and put up the bill.

My point to Newdexter was if he is using everything on three phase and therefore a higher voltage, the current drawn is halved and he can cope with a lower rated supply.

He has not responded so we can't help him until he does.

Phillmore
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Re: KVA calculation

Post by Phillmore »

I don't think the voltage ever actually changed, rather the tolerance changed. Quoting 230v was within the new harmonised tolerance (to satisfy the EU I believe). Correct me if I'm wrong?
Andy

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New Dexter
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Re: KVA calculation

Post by New Dexter »

Gentlemen I am most grateful for all the replies to my original post. My reason for not responding so far is that I have to do as Ian Slade and others have said and work out the current requirement for each of my three phase machines.
As I said, they all run on a 16amp plug/circuit and as Ian has said a motor, overcoming the back EMF will draw more current on start up than when it's running. However, when the power was re-connected the power company were concerned to know what my welder would require as when the arc is struck the machines needs more power, if only momentarily.
I have noticed that there are occasions when the compressor fires up that it trips the breaker, not always but if the power to the other machines has disappeared mysteriously it will be the compressor that has started up and been the cause of the lack of power.
The Colchester lathe runs fine as does the 320 amp welder and the cut-off saw. I am not really sure why I was provided with so much power.
Once again my grateful thanks and please bear with me until i can provide details of the machines.
Just as a post script. What's a star delta starter.....

KV8
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Re: KVA calculation

Post by KV8 »

Phillmore wrote: Thu May 31, 2018 5:49 pm I don't think the voltage ever actually changed, rather the tolerance changed. Quoting 230v was within the new harmonised tolerance (to satisfy the EU I believe). Correct me if I'm wrong?
You are right. As I said in my previous post the so called 240 line voltage as measured by myself over 50 years using top grade meters (AVO then Fluke) was usually 235V . The highest voltage I read ( apart from 440V 3 phase derived single phase) was 239V. So nothing has changed as you say as these readings still fall within the 2004 CE 230V +/- permitted variance.

Just measured my 13A sockets at 235V. Same reading I usually got nearly 60 years ago!

H
Last edited by KV8 on Thu May 31, 2018 8:41 pm, edited 2 times in total.

KV8
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Re: KVA calculation

Post by KV8 »

Sydsmith wrote: Thu May 31, 2018 1:44 pm
If you ask any man in the street what voltage we use in the UK they will probably say 240 volts, the change in 2004 went almost unnoticed. At the time I raised the matter with the powers that be, because the supply companies were and still are cheating the consumer. A drop in voltage of 10 volts will increase the current drawn by equipment and put up the bill.
As said nothing changed as the power companies just continued at the 235V they appear to have been using since before 1960 (well within old 240V spec.) and well within EU 2004 230V CE spec.

Just trying to avoid confusion as your quoted 440V 3 phase is not relevant to the OP's circumstances.

H

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