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# Transformer - primary side - fuse rating

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#### lee321987

##### Member level 5
How do you figure out the fuse rating to use on the primary side of a transformer?

Also, I just bought two transformers from Radio Shack, and neither one of them has a VA rating -- they just say "2 Amp" and "3 Amp". Could someone with a little knowledge guess (or tell me how to guess) the VA rating of these?
Radio Shack doesn't seem to have any datasheets for them, so all I can tell you are the voltage and amps (they are both center-tap transformers).
TRANSFORMER 1:
input: 120VAC
output: 6.3 - 0 - 6.3 VAC @ 3 amps
TRANSFORMER 2:
input 120VAC
output: 12.6 - 0 - 12.6 VAC @ 2 amps

Here's the Radio Shack pages for them:

Fuse rating: 2 x current

6V-0-6V @ 3A => 12V * 3A => 36VA
12V-0-12V @ 2A => 24V * 2A => 48VA

:wink:
IanP

lee321987

### lee321987

Points: 2
Thank you.
I don't have a device to measure the current on the primary side.
Is there an equation I can use?

Roughly, if transformer’s power rating is 60VA and voltage is 120Vac, the primary current for the nominal load is somewhere around 120/60=> 0.5A ..
Add 10% for transformer’s efficiency and multiply by 2 ..

Use "slow-blow" fuses ..

:wink:
IanP

lee321987

### lee321987

Points: 2
Great help. Thanks a lot!

Hello lee,

Calculating primary current is simple.
Just check the total output power of the transformer. The total o/p power = I/p power + loss
Consider loss as negligible we get output power i= input power. Now use the formula P=VI at input side ( You have power and Voltage so get the current)
add 100mA as loss and 2* current = fuse rating

TC

Regards
Nikhil

lee321987

Points: 2