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Discuss working out Kva etc! in the Commercial Electrical Advice area at ElectriciansForums.net

S

sparkyork

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Hi guys n gals,

wondering if anyone knows the best method of working something out:

if ive got a 3 phase supply, rated at 45Kva then how many amps do i have in total and per phase? does square root of 3 come into anywhere?
also be handy to have the above solved and equated into KW's as well :)

the above relates to commercial unit in leeds and this supply is what the shop has got.

cheers
 
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G

Grae79

Total power for balanced star, P = sqrt 3 x VL x IL x PF Is all i have for now, off to giym. :)
 
S

sparkyork

  • Thread Starter Thread Starter
  • #3
mmmm feeling a bit thick today!! is the above formula gonna give me a wattage of
38277.2 or 38.3Kw? i used .85 as a pf but dont know if this is right, if someone could break down the above and give me amps per phase or total available amps and watts likewise that would be great!
 
G

Grae79

In a perfick world:

IL = P/(sqrt 3 x VL x PF)

IL = 45000/(1.73 x 400 x 1)

IL = 65A
 
S

Shakey

mmmm feeling a bit thick today!! is the above formula gonna give me a wattage of
38277.2 or 38.3Kw? i used .85 as a pf but dont know if this is right, if someone could break down the above and give me amps per phase or total available amps and watts likewise that would be great!
well 45KVA is your apparant power, and is simply a product of supply volts by supply current

your watts (or KW) is your true power, and is a product of the supply current and v dropped accross the resistive element of the load

KVAR is your reactive power, and is current times v drop accroos reactive element of the load

of course, all this is theortical, using vectors etc etc

but what it means is your KW is smaller than your KVA

in fact true power / apparant power is p.f

so your p.f of o.85 x 45kVA = 38.25 KW

but of course, your loads may (or may not) be at 0.85 p.f

you should have your pf as high as possible, (close to unity), which means you are 'using' more of the KVA supplied

I have been in large installations running many inductive loads (motors etc) where they p.f correction capacitor banks fiited

anyway, lets say you have 38.25Kw

divide by three = 12.75Kw per phase divided by 230V (because its single phase) = 55.4 amps per phase

note this a very rough calc, and assumes perefctly balanced phases and loads, a fixed p.f of 0.85 etc etc

of course, your PF is not fixed

so using your 45KVA, divide by three = 15KVA per phase divide by 230V = 65A

this is 'available supply'

reality is, without a constantly chjanging p.f, so would your answer


oh, and i just made all that up because i am not an 'electrican' according to some on here:p
 
S

sparkyork

  • Thread Starter Thread Starter
  • #7
cheers grae, so thats gonna be total amps available then. seems pretty poor really for a commercial premises!
 
S

sparkyork

  • Thread Starter Thread Starter
  • #9
didnt see your post up there as well! thanks shakey.

yeah im getting simlar figures as well, so potentially if ive got a nicely balanced loading, with some evenly matched and as pure as poss on the pf, im looking at about 195A across all three phases.

motor wise, after talking to customer today he's getting some energy efficient single phase air con units, so hopefully they will have good pf as well as 10 refrigerators.

cheers for your response guys.

oh, and i just made all that up because i am not an 'electrican' according to some on here:p

lol! :) i hope it all sorts it self out n everybody becomes friends again!
 
S

sparkyork

  • Thread Starter Thread Starter
  • #11
just doing a load survey for me leeds job, just realised i was working it out wy wrong to begin with! i was doing 45000/400 and coming out with 112 or summat!! bit different to actually having 65a ish per phase!! lol
 
S

Shakey

didnt see your post up there as well! thanks shakey.

yeah im getting simlar figures as well, so potentially if ive got a nicely balanced loading, with some evenly matched and as pure as poss on the pf, im looking at about 195A across all three phases.

motor wise, after talking to customer today he's getting some energy efficient single phase air con units, so hopefully they will have good pf as well as 10 refrigerators.

cheers for your response guys.

oh, and i just made all that up because i am not an 'electrican' according to some on here:p

lol! :) i hope it all sorts it self out n everybody becomes friends again!

hey no problems:p
 
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