3 phase supply size

James sparks · Apr 13, 2014

Hi all,

I need to put in a 3 phase supply for a piece of equipment in a commercial unit. They've sent the spec sheet for the item over and all it says is

Current type (Ph/V/Hz): 3/420/50
Power rating (kW): 7.8

I just want to clarify if it's 7.8kW per phase or 7.8kW total over all 3 phases.

Thanks!!

Darkwood · Apr 14, 2014

davesparks said:
Consider a 32A three phase mcb, that's 32A per phase. Same goes for KW rating.

Only fly in the ointment I can think of is from UKPN's paperwork where a 100A 3ph supply is described as being a 69KVA 3ph supply.

Good point Ive come across before very confusing TBH when the rest of the industry works the other way.

Guest111 · Apr 14, 2014

I would have said KW per phase myself,hazarding a guess I'd say it's some kind of pressure washer and you can get some very large ones with a fair old inrush on the motor.

Spoon · Apr 14, 2014

Are you intending to use a DOL starter? If not then what?

Guest111 · Apr 14, 2014

Spoon said:
Are you intending to use a DOL starter? If not then what?

It may well have it's own control gear built in,It would have been nice if the op had actually told us what the machine was.lol

Spoon · Apr 14, 2014

It well could have, but then it should have a label on stating what it requires. Us leccies know everything except when it comes to the opposite sex..

Guest111 · Apr 14, 2014

Spoon said:
It well could have, but then it should have a label on stating what it requires. Us leccies know everything except when it comes to the opposite sex..

Oh yes,and then 99 times out of a 100 you're wrong.

happysteve · Apr 14, 2014

Interesting. Surprised there is such a gulf between opinions.

I am a trainee, we're covering motors at the moment. I happen to have Trevor Linsley's book open at this page:

[ElectriciansForums.net] 3 phase supply size

This suggests there's a "root three" in there somewhere!

I'm not proffering an opinion, just interested to learn. This could well be one of those cases where the text book means diddly squat. Be useful to know, though, if that's the case!

micknew · Apr 14, 2014

The text book is 100% correct Steve.
Some of the opinions expressed above are not.

Darkwood · Apr 14, 2014

Yes the book is 100% correct never argued the calcs ...what the OP asked was is the KW of a 3ph load per phase or not ...

If you use the calcs shown above to work it out you indeed do apply root3 to establish the running current per phase.

If you have a Motor the Kw rating is the power output energy through the motor shaft (also in HP) its not an electrical measurement par se' and using either calcs as above or the plated amps this will show Amps per phase.

I think i see why your arguing my point but think you have approached it a different way... im been careful to word it correctly as telling the OP 7.8Kw is not per phase may give the impression to divide by 3 and hey presto but when reading a power plate its the Kw per phase and yes you use the root3 in the calcs to convert this to amps per phase.

mechelec · Apr 14, 2014

Have to say I have never heard anyone quote KW or KVA per phase in relation to 3 phase. KW is power and usually means total power required by the machine. It would be like quoting total amps for a 3 phase machine when amps per phase is the norm.

Darkwood · Apr 14, 2014

It maybe my teaching is old hat but was taught as you rightly say that the Kw or Kva is the power required but because some machines especially with heating banks can have a KVA or Kw value given as a single Phase value where the machine can sometimes be converted to 3ph you divide the value by 3 first to get your 3Ph Kw rating.

It may not be correct to ask the question if the Kw is across the phases or per phase but instead ask how to calculate the load per phase of a 3phase - 7.8kw machine.

I work on chinese old solder machines and the Kw needs dividing by 3 as they stated the total of each phase and not like we mark them over here common on machinery that can be converted to 1ph by linking together element banks.

Hindsight I should have shown the formula early on... but for the OP -

Hindsight I think showing the equation early on would have cleared the confusion up...

I = P/(voltage * PF * 1.732)

I = 7800 (400 * 0.8 * 1.732)

I = 7800 / 554.24

I = 14.07 amps per phase.

If its indeed a motor then efficiency should be in the calcs as well. Note PF is estimated at 0.8 for a pump motor? - info lacking

James sparks · Apr 14, 2014

This is a link to the item, it's a pressure washer

HDS 10/20-4 M | Kärcher UK

NickD · Apr 14, 2014

Kärcher Centers & Stores | Kärcher UK

Give 'em a call?

"Whether for Industry / Professional or Home & Garden, Kärcher Centers are committed to providing professional advice on selecting the correct cleaning solution for your requirements. The staff at Kärcher Centers are experienced in all products groups and applications, having undergone thorough training at the Kärcher Academy."

Darkwood · Apr 14, 2014

Thanks for the Link .... Would have been useful earlier

...

http://www.karcher.co.uk/document/BTA-5339762-000-02.pdf

Page 41 .... took 30-seconds to find

It requests a fuse size as slow... I would confirm with their tech' support if a type C mcb will be suitable as its vague but does give you a fuse rating for each model and voltage.

Marvo · Apr 14, 2014

darkwood said:
If you get a motor and it says 4kw 400v then its per phase.

If you buy a 4 kW motor it will be 5.5HP regardless of whether it's single phase or 3-phase. I'd respectfully suggest that 4kW indicates the total energy consumption regardless whether it's a single phase or 3-phase motor.

I would expect the 7.8kW stated in the OP to be the total energy consumption so it would be 7.8kW across 3 phases.

As Nick says above, don't take my word for it, give them a call and confirm.