how do I work out kva from kw with 3 phase supply
how do I work out kva from kw with 3 phase supply
(OP)
Hi.
Apologies in advance for a basic question. Although my background is mathematics, I have very little electrical knowledge so please bear with me!
My factory runs on a 3-phase power supply of 415 volts. In addition, if all our machines were to run at once, the maximum load demand is 260kw.
My understanding is that this equates to 330kva (assuming 80% power for constant supply). With a 3-phase system does this mean we need a minimum of a 460amp supply?
[3ø, Amps = (kVA*1000)/Volts/1.732 (SqRoot of 3)]
I know that there is a 600amp cable from the mains supply in the street which at present changes to a 300amp cable into our building. Would it make sense to arrange for an upgrade to the full 600amp supply? If this is the case, then is this equivalent to requesting a service availability of 500kva?
Any help would be appreciated!
Lisa
Apologies in advance for a basic question. Although my background is mathematics, I have very little electrical knowledge so please bear with me!
My factory runs on a 3-phase power supply of 415 volts. In addition, if all our machines were to run at once, the maximum load demand is 260kw.
My understanding is that this equates to 330kva (assuming 80% power for constant supply). With a 3-phase system does this mean we need a minimum of a 460amp supply?
[3ø, Amps = (kVA*1000)/Volts/1.732 (SqRoot of 3)]
I know that there is a 600amp cable from the mains supply in the street which at present changes to a 300amp cable into our building. Would it make sense to arrange for an upgrade to the full 600amp supply? If this is the case, then is this equivalent to requesting a service availability of 500kva?
Any help would be appreciated!
Lisa






RE: how do I work out kva from kw with 3 phase supply
260kW would equate to 330kVA if the power factor were 78.8%.
kW/kVA=PF
You state that 260kW is the "demand". The system is not typically designed to supply 100% demand at all times.
I'm not sure what you mean by "80% power for constant supply", possibly 80% loading, but for this you must first find the load in kVA(and therefore amps), not kW, and then add 25%(equivalent of 80% loading).
Are you having some kind of trouble with the system as it is? I am curious why you are concerned about this issue if you are not an electrician or electrical engineer.
RE: how do I work out kva from kw with 3 phase supply
However, in the new year I am considering replacing some of our machinery with more productive and more energy efficient machinery. This would mean that the demand would decrease. At present, I have asked the electricity company for a quote to ascertain the cost of the power upgrade. (On the one hand the additional power might be good for the future but on the other, if it's too expensive it would make more sense to invest in equipment that uses less power.) However, in order to get their surveyor to come on site to give the quote I have had to answer certain questions from them. These include: "my estimate of the new total After Diversity Power Requirement in kVA/KW for the site". My engineer has supplied me with a list of our equipment and their KW requirements and I assume that this requirement is simply a total of the KW we would need at any one time.
In essence, I am hoping to have a better understanding of the issues involved so that we make the right decision when the surveyor when he visits. However, with no electrical background I appreciate that this may be a little optimistic!
Either way, thank you for having taken the time to reply.
RE: how do I work out kva from kw with 3 phase supply
RE: how do I work out kva from kw with 3 phase supply
RE: how do I work out kva from kw with 3 phase supply
You need to understand the per unit system. You can pick up by reading any power engineering text book.
Vbase = 415 volts -> Voltage = 415 volts or 1.0 pu
KVAbase = 100 kva -> Power = 330 kva or 3.3 pu @80% pf
Ibase = KVAbase/Vbase/1.732 = 0.14 kA
Ipu = KVApu/Vpu = 3.3/1.0 = 3.3 pu
Iactual = Ipu * Ibase = 0.46 kA or 460 Amps
Hope this will help.