# Thread: Transformer sizing

Join Date
Jul 2004
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where can I find info on or how can I size transformers and determine fuse protection

2. Good question. I'd like to hear other responses because I have not found a sizing chart that was simple for low voltage controls. I've found all sorts for larger loads in the NEC.
Here is one, but a chart/table that listed various total VA and wire sizes and lengths would be a lot nicer.

http://www.kele.com/archive/2002/Tr1...ltSelGuide.pdf

We've used fire alarm wiring length voltage drop guidelines along with adding up max used VA of each device on the circuit to figure out our transformer and wiring sizes. I then oversize the transformer by at least 25%. What do you mean by fuse protection? Are you trying to protect the transformer or the controller? For a transformer I've always took the VA rating of the transformer divided by the secondary volts.
For example a 24 volt secondary rated at 75 VA.
Amps x Volts = VA
So we know the VA and the volts so
Amps = VA / Volts
Amps = 75VA / 24V
which equals 3.125 Amps.
The transformer is rated to continously handle 3.125 Amps. I would throw in a 3 amp fuse here - if I didn't have a transformer with a manual reset circuit breaker on it. I have put in higher amperage rated fuses in certain applications as due to the inrush amps of a contactor pulling in a fast acting fuse will pop but you may be right at 3.2 Amps or even 3.5 when the contactor pulls in. A slow blow would have been better but when it's a service call late at night you use what you have (5 amp fast acting). This did not bother me in these applications as I know the only reason the amps got that high was due to the contactor momentarily pulling in. Also if something got shorted the 5 amp fast acting would still protect the transformer.
To protect the controller you would put in a fuse rated at/just below the max amp rating of the control.

[Edited by crab master on 09-10-2005 at 12:23 AM]

3. the smallest transformer you can get with manual reset is a 75va. that's for 3.2 amps as mentioned before.

let me reface that, the smallest transformer i've seen in supplyhouses with a cb is 75va.

4. r2 - saved by the "let me reface that, the smallest transformer i've seen in supplyhouses with a cb is 75va."

I have one setting below my PC a 50 VA RIB with a CB on it
Functional Devices Model # TR50VA004.
;o)

5. crabmaster

i have always been taught to build in a safety facator when protecting a transformer...by your example, I would use a 2 amp fuse..

6. Key - Yes, but check out the rating most circuit breakers on transformers are. They are all right at continuous load amps when you do the math. That tells me there is a bit of a factor built in above the continuous load rating.

7. Professional Member
Join Date
Apr 2005
Location
Pennsylvania
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Well this is simple ohm's law.

VA + VA = Total VA

VA / 24V (or whatever your voltage is) = amperage

Remember VA is nothing but wattage. If you have a 120 wall light bulb in your home @ 120V how many amps does it draw?

On the 50VA XFMR question, I believe you can get a CB on a 50VA XFMR it is multi-tap, but that is only from my experience.

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