3 phase supply size

[James sparks]

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!!

 

[uksparks]

I would imagine it's in total not per phase.

 

[Darkwood]

usually if you use a kw rating in the same spec as 3ph its per phase ...

 

[uksparks]

I suppose the best thing would be to get more data from the device

 

[micknew]

I would imagine it's in total not per phase.

I agree with you.
If not it should have said 'per phase.

 

[James sparks]

[h=3]Technical data[/h]
Current type Ph/V/Hz		3 / 420 / 50
Flow rate (l/h)		500 - 1000
Pressure (bar/MPa)		30 - 200 / 3 - 20
Max. temperature (°C)		155 - 80
Power rating (kW)		7.8
Consumption fuel oil (kg/h)		6.4
Fuel tank (l)		25
Fuel tank (cc³)		-
Weight (kg)		171
Dimensions (L x W x H) (mm)		1330 x 750 x 1060
Detergent tank (l)		20

 

[micknew]

That is telling you that the total load of the machine is 7.8KW.

 

[uksparks]

So it's 7.8kW split across phases, so not much then, what is it you say? A pump?

 

[Darkwood]

its per phase 7.8kw per phase otherwise it would have stated single

 

[micknew]

its per phase 7.8kw per phase otherwise it would have stated single

No it wouldn't.

 

[Darkwood]

If you have a spec asking fot 3ph and a Kw rating its Kw per phase .. . no disrespect to other posters but your guessing and not experienced in 3ph so express this in your post or you may mislead the OP ... I assume a 3ph device is part of the setup like a motor; motors are specd per phase not devided by 3..

 

[NickD]

No it wouldn't.

I would ask the question of the supplier/manufacturer/whoever. Assumption is the mother of all cock ups.

 

[micknew]

If you have a spec asking fot 3ph and a Kw rating its Kw per phase .. . no disrespect to other posters but your guessing and not experienced in 3ph so express this in your post or you may mislead the OP ... I assume a 3ph device is part of the setup like a motor; motors are specd per phase not devided by 3..

You are wrong and most of my work is industrial 3 phase.

 

[Darkwood]

You are wrong and most of my work is industrial 3 phase.

If you request a supply from DNO in Kw then its per phase.
If you get a motor and it says 4kw 400v then its per phase.
If you have a machine that asks for a 3ph supply 10kw then thats per phase.


25yrs of industrial intalls design and machine panel building plus several requests to DNO for supplies or upgrades im sure im not wrong....

Only exception is the occasional machine usually from china and 2nd hand that does go over 3ph and checking the wiring plan or load characteristics can usually identify this but other than that ill say your wrong.

Its always good to cross check these values Ive seen architects mess up if a reading is in KVA on a machine or supply then its likely to be a total demand across the phases as dave points out below but the OP spec'd a Kw value.

 

[davesparks]

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.

 

[Darkwood]

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]

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]

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

 

[Guest111]

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]

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]

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]

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:



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]

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

 

[Darkwood]

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]

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]

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]

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

HDS 10/20-4 M | Kärcher UK

 

[NickD]

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]

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]

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.

 

[sparks1234]

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.

Sorry Darkwood, you are wrong on this one

Total load, split across the phases

 

[Darkwood]

Yes Marvo im getting the impression my old school terminology differs but just to confirm a 4KW motor means it output 4KW of energy down the shaft it is not what it consumes, my thinking really here as i mentioned is its wrong to ask if the KW is across the phases or per phase as the KW has no electrical meaning its a modern convertion of horse power ratings.

You wouldn't ask if the horse power is per phase or across them you would ask if the plated ampage is if anything... this is why I have approached it the opposite way and yes confused matters but the OP's back and getting relevant info'

Ill hold my hands up ad say my advice was incorrect in the way it was given and i should have explained it better by giving the KW to I equation expressing the KW is a energy available at shaft and requires the calculation to turn it into something more meaningful.

 

[happysteve]

Allan, my course tutor, has just told me that (in his opinion) if the plate on a 3 phase motor says (eg) 10kW, that's the total consumption.

 

[micknew]

Allan, my course tutor, has just told me that (in his opinion) if the plate on a 3 phase motor says (eg) 10kW, that's the total consumption.

Your tutor knows his stuff.

 

[happysteve]

That he does.

Apologies to the OP for diverting this thread. Mods please feel free to delete my posts.

 

[sparks1234]

Yes Marvo im getting the impression my old school terminology differs but just to confirm a 4KW motor means it output 4KW of energy down the shaft it is not what it consumes, my thinking really here as i mentioned is its wrong to ask if the KW is across the phases or per phase as the KW has no electrical meaning its a modern convertion of horse power ratings.

You wouldn't ask if the horse power is per phase or across them you would ask if the plated ampage is if anything... this is why I have approached it the opposite way and yes confused matters but the OP's back and getting relevant info'

Ill hold my hands up ad say my advice was incorrect in the way it was given and i should have explained it better by giving the KW to I equation expressing the KW is a energy available at shaft and requires the calculation to turn it into something more meaningful.

Good onya Darkwood!!!!

 

[Darkwood]

Your tutor knows his stuff.

As we had the general banter here Ill offer an apology expressing you were wrong but hope through my latter posts you see the angle I approached from and a consequence of the overlapping Engineering terminology and general Electrical Terminology I was taught.

The opportunity was missed by all to simplify what the OP wanted at the end of the day and it was ampage per phase so he could design the supply.

He has now been given this and a Link to a PDF for the fuse info'

 

[Richard Burns]

As DW says p41 does give it away somewhat!

 

[Darkwood]

As DW says p41 does give it away somewhat!
View attachment 24293

I sourced this off a different website - the OP didn't have this info - Karcher could be more informative on their original spec ...we can only guess the motor might have some form off soft start hence I would still ring Tech' on this matter.

 

[Richard Burns]

I sourced this off a different website - the OP didn't have this info - Karcher could be more informative on their original spec ...we can only guess the motor might have some form off soft start hence I would still ring Tech' on this matter.

I think clicking on the link "product manuals" under more information from the OP link may have given a clue!

 

[netblindpaul]

There are also still some glaring errors in posts on this thread ignoring the so called "misunderstanding" of kW per phase or across 3 phases that are sufficient to kill people.

 

[Darkwood]

Allan, my course tutor, has just told me that (in his opinion) if the plate on a 3 phase motor says (eg) 10kW, that's the total consumption.

Do us a cheeky favour tell your tutor the KW rating on a motor is the energy output down the shaft and a replacement value for horsepower and not to be confused with what the motor consumes thus the relevant equation has to be done to get the current before you can say whether its across the phases or per phase - Im interested ho you are taught today and what his answer would be!

 

[Marvo]

There are also still some glaring errors in posts on this thread ignoring the so called "misunderstanding" of kW per phase or across 3 phases that are sufficient to kill people.

Yes there are but it's a discussion, not a tutorial and if anything should be taken from the thread by a person looking for advice it should be that they ought to confirm with the agents or supplier before making any assumptions.

 

[trev]

Deleted following a PM conversation

 

[happysteve]

Do us a cheeky favour tell your tutor the KW rating on a motor is the energy output down the shaft and a replacement value for horsepower and not to be confused with what the motor consumes thus the relevant equation has to be done to get the current before you can say whether its across the phases or per phase - Im interested ho you are taught today and what his answer would be!

To be fair to Allan, he did say rather a lot more than the sentence I typed quickly on my phone during the brew break! I realise now that in my rush, I probably misrepresented what he explained. Apologies.

Over 3-4 minuts he talked about how (as you say) it's related to the output power of the motor in terms of the work it can do. He mentioned horsepower, and when he put it in this perspective it started to make sense. He did mention efficiency too; he didn't actually say, "Your total electrical power consumption will be greater than the rated power due to the efficiency always being less than one (100%)," but we covered the equations in an earlier lesson so I guess he left this for us to work out.

I like Allan. He's alright.

 

[Silly Sausage]

@NickD

I'll apologise now for taking this off topic (as I know it's now frowned upon)

Nick do you find Asperger's is much of a hindrance to how you deal with your life on a day to day basis?
My great nephew has it and it's a daily struggle for him.

I reckon he had his Bell End pulled and he's replaced it with this lame attempt.

 

[trev]

Deleted following a PM conversation

 

[Silly Sausage]

I wonder what he'll think when he gets access to Arms!

 

[GLENNSPARK]

I wonder what he'll think when he gets access to Arms!

eh?...

 

[GLENNSPARK]

it`s what others will think....