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#181312 - 10/02/08 11:46 PM Help with transformer calculations  
Merlin  Offline
Joined: Jan 2004
Posts: 170
NW Indiana
Hello to all. I need to size a transformer for a small project. Well to be honest, I'm not 100% percent sure that my calculations are correct. I need to install a transformer in a remote location to power up a "high water level" security system. (simply to notify me that i have dewatering pump problems) I currently only have 480 VAC 3 phase at the site. I need approx. 5-6 amps at 120 VAC. I would like a little bit of extra, maybe 10 amps.

Can someone please show me the proper calculations to verify my thinking? Thank you

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#181313 - 10/03/08 01:08 AM Re: Help with transformer calculations [Re: Merlin]  
JoeTestingEngr  Offline
Joined: Nov 2005
Posts: 785
Chicago, Il.
We need more info than you have provided. 5 or 6 amps is overkill for phase loss, high water, low oil, and other typical status monitoring. Are you powering radio transmitters, battery chargers, or other devices that would take additional power?

#181318 - 10/03/08 06:41 AM Re: Help with transformer calculations [Re: JoeTestingEngr]  
Merlin  Offline
Joined: Jan 2004
Posts: 170
NW Indiana
Yes. It will be powering a cellular transmitter, a 12V backup power supply, and a Hoffman 200 watt cabinet heater. The heater i know is 1.7 amps at 120VAC.The rest of the equipment i am unsure. The engineer only was able to tell me that he only needed about 2 amps. That is why i am trying to slightly oversize it. Does this info help?

#181321 - 10/03/08 03:03 PM Re: Help with transformer calculations [Re: Merlin]  
SJT  Offline
Joined: Jul 2002
Posts: 241
A 3KVA transformer would give you approx. 7.2 Amps @240V, and the primary is approx. 3.6 Amps @ 480V.
Next size up is a 4.5KVA, this would give approx. 10.8 Amps @240V, and the primary approx. 5.4 Amps @480V

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#181324 - 10/03/08 07:04 PM Re: Help with transformer calculations [Re: SJT]  
HotLine1  Offline

Joined: Apr 2002
Posts: 6,857
Brick, NJ USA
You may want to consider a SQ D "Mini Power Zone' factory assembled unit Primary CB/Sec CB/ & 120-240 volt branch breakers all in one 3R setup, right out of the box.

MPZ5S40F is a 5 kva; 10 single pole spaces for QO CB's

May be considered pricey....but ya get what ya pay for, and it's all ready to go.


#181326 - 10/03/08 08:00 PM Re: Help with transformer calculations [Re: HotLine1]  
maintenanceguy  Offline
Joined: Dec 2001
Posts: 300
Southern NJ, USA

for what you're doing, the calculation is simple. You can do it yourself.

Voltage x amperage = Watts. transformers step voltage up or down but watts in always equals watts out. Watts are also known as VA (or volt amps).

(somebody's going to correct me and say that VA is different than watts and they're right but for something this simple, you don't need to worry about that)

So you want 120 volts and 10 amps. 120*10 =1200 Watts or 1.2 KVA.

Just for fun, if we need to know the input amperage at 480V we still use V x A = W. or 480 x ? = 1200. We get 2.5A.

So 2.5A at 480 volts input give us 10A at 120V output.

#181327 - 10/03/08 08:58 PM Re: Help with transformer calculations [Re: maintenanceguy]  
Merlin  Offline
Joined: Jan 2004
Posts: 170
NW Indiana
Thank you much to all. I have since figured it out. It was actually too easy. I thought that was the way it was figured but was unsure. I double checked will a local engineer today and he verified my calculations. Once again thank you all.

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