Discuss 3 phase supply size in the Electrical Forum area at ElectriciansForums.net

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James sparks

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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!!
 
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J

James sparks

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[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)
1330x750x1060
Detergent tank (l)
20
 

micknew

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

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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..
 

micknew

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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

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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.
 
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davesparks

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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

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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.
 
G

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

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

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

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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..
 
G

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

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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:

uploadfromtaptalk1397480029966.jpg

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! :)
 

darkwood

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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

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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

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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
 
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NickD

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Kärcher Centers & Stores | Kärcher UK

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Marvo

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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

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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
 
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darkwood

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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.
 
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happysteve

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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.
 

happysteve

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That he does. :)

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