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#181312 - 10/02/08 08:46 PM Help with transformer calculations
Merlin Offline
Member

Registered: 01/27/04
Posts: 170
Loc: 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 - 10/02/08 10:08 PM Re: Help with transformer calculations [Re: Merlin]
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
Merlin,
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?
Joe

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#181318 - 10/03/08 03:41 AM Re: Help with transformer calculations [Re: JoeTestingEngr]
Merlin Offline
Member

Registered: 01/27/04
Posts: 170
Loc: 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?

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#181321 - 10/03/08 12:03 PM Re: Help with transformer calculations [Re: Merlin]
SJT Offline
Member

Registered: 07/24/02
Posts: 242
Loc: PATCHOGUE, N.Y.
Hello,
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

Safe Day

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

Member

Registered: 04/03/02
Posts: 6786
Loc: Brick, NJ USA
Merlin:
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.
_________________________
John

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#181326 - 10/03/08 05:00 PM Re: Help with transformer calculations [Re: HotLine1]
maintenanceguy Offline
Member

Registered: 12/02/01
Posts: 303
Loc: Southern NJ, USA
Merlin,

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.

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#181327 - 10/03/08 05:58 PM Re: Help with transformer calculations [Re: maintenanceguy]
Merlin Offline
Member

Registered: 01/27/04
Posts: 170
Loc: 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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