how do I work out kva from kw with 3 phase supply

[lisah]

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

 

[DanDel]

It's not only your cable but your overcurrent protection and possibly switchgear that must be correctly rated for the load served.
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.

 

[lisah]

Good question! Unfortunately, running a small business means you get concerned/involved with a bunch of stuff that you'd happily leave alone under normal circumstances! Basically, we started to have some problems with the supply last week. (Bitumen had started to leak out of the main joint box.) When we called in the electricity company they said that it was probably due to a loose connection within the unit. They also commented that they thought that on current usage it might be sensible to upgrade the 300amp cable to a 600amp cable (including any other necessary alterations).

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.

 

[jghrist]

Do you pay a demand charge to the utility? If so, your "After Diversity Power Requirement" would be the maximum of the last year's demand. As you say, it will be lower if you increase energy efficiency. Depending on the form of your bill, you might have to estimate the pf to get kVA.

 

[DanDel]

lisah, you could monitor the incoming circuit for current over a week or so to determine the actual amperage. You could also use an Infrared camera to determine if there is a loose connection at the point you describe. In any case, it sounds like your next move might require calling in someone who is more familiar with this type of problem and has the proper experience and equipment to analyze it and make recommendations.

 

[peguru]

lisah,

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.