# Electrical Control SP



## Tech (Oct 16, 2012)

Am I correct by using the formula KVA=V X A / 1000. So in ordering a Isolation transformer for a 200a service at 480vac. This comes out to be 96KVA? Also what kind of lead way do I need to leave? I was thinking about 20% of 96KVA which gives me 115.2 KVA do's this seem correct to you or am I missing something?


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## MDShunk (Jan 7, 2007)

Tech said:


> Am I correct by using the formula KVA=V X A / 1000. So in ordering a Isolation transformer for a 200a service at 480vac. This comes out to be 96KVA? Also what kind of lead way do I need to leave? I was thinking about 20% of 96KVA which gives me 115.2 KVA do's this seem correct to you or am I missing something?


For a quick and dirty, yeah. It would be nice to work the power factor in there, though. (this single phase, huh?). If this is three phase, you need to work in 1.732 and you'll get something around 160-ish kva.


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## Tech (Oct 16, 2012)

3 phase with power factor of 1


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## MDShunk (Jan 7, 2007)

Tech said:


> 3 phase with power factor of 1


Interesting that you're using an isolation transformer on a straight resistive load. What's the idea behind that?


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## MDShunk (Jan 7, 2007)

Looks like you're buying a 225 kva, by the way.


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## Tech (Oct 16, 2012)

Really just picked up project. the guys can not give me the power factor at there facility due to lack of knowledge. So all I know for sure of is that the service is 480vac and the dissconnect is fused at 200a. I really dont understand why we are looking at a 118KVA 480vac delta primary too 480vac Wye secondary.


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## MDShunk (Jan 7, 2007)

If you pulled the unity power factor out of the sky, you asked about leeway, and power factor and multimotor starting are among the main things that may require a little headroom. You can overload a transformer for brief periods of time, but not all day without measures to combat heat rise.


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## MDShunk (Jan 7, 2007)

Tech said:


> Really just picked up project. the guys can not give me the power factor at there facility due to lack of knowledge. So all I know for sure of is that the service is 480vac and the dissconnect is fused at 200a. I really dont understand why we are looking at a 118KVA 480vac delta primary too 480vac Wye secondary.


Was this 118kva selected under engineer supervision, maybe? Intention to add fan kits perhaps? I have a funny feeling they did a single phase calc and not a three phase calc. If this is existing, the utility might have historical pf data, depending on how it's metered. You could also hook up a PQA for a day if the load's pretty much the same every day.


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## Tech (Oct 16, 2012)

This is a new installation, would not consider the guy an engineer. A fan application is not on the parts list. Actually I am now going over the paper work and found my self in a little bind on this portion.


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## MDShunk (Jan 7, 2007)

Tech said:


> This is a new installation, would not consider the guy an engineer. A fan application is not on the parts list. Actually I am now going over the paper work and found my self in a little bind on this portion.


The only thing I can think of is someone did the calc wrong OR this xfmr is only serving certain loads. Is "the guy" still around to ask him how he came up with that size.


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## Tech (Oct 16, 2012)

No he has left the company. The only thing I see in his notes is that formula. Would that be incorrect for three phase?


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## MDShunk (Jan 7, 2007)

Tech said:


> No he has left the company. The only thing I see in his notes is that formula. Would that be incorrect for three phase?


Yessir. ((volts x amps) x 1.732) x PF

If he had the formula in your opening post in his notes, he effed up. Hopefully you can get return credit for that xfmr.


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