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Determine the VA of a transformer

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New Member
I have two large power transformers that I do not know the VA of. I found this site that has some general rules, but I was wondering how accurate it is.

*** t3h GeeK ZonE ***

The transformers are 4in by 3.25in cross section looking at the top of them. According to the site (and some interpolation) it looks like they could be from 2000VA to 2500VA. Could that be right?


New Member
I get the volume and weight vs. VA from catalogs and compare to the subject 'former.
A 1 kW microwave oven 'former weighs about 8# and is about 60 cu. in.


Most Helpful Member
Are you going by the center of the core cross section?
If so I would put them in the 2.5 KVA to 3 KVA range.
Got a picture?
As transformer cores get larger the VA capacity per square inch goes up.
That chart is not so acurate as you get further away from the 500VA capacity.


New Member
Here are a couple pics of one of them. Jewel case and 9v battey for comparison. They are for some 300w audio amp kits that I have, which I haven't put back together. 120v to 53-0-53v. Not really sure what other use they could be atm.



Most Helpful Member
rough guess based on similar transformers I have is its probably a 1 KVA give or take.


New Member
by using the very latest in cad design technology (a rule on the monitor), it appears the lamination is the same width as a cd case, so 140mm, and the stack is approx 90mm

at 60hz, that's going to be about 1.5KVA


Well-Known Member
Most Helpful Member

Another way is to look at the wire thickness that makes up the windings.
You can then compare the wire gauge/thickness to a decent size for
transformer windings, like 600 circular mils per amp, which will give you
the amps. If you look at the primary and it comes out to 10 amps and
the primary voltage is 120, then you know it is a 1.2kW transformer.

Alternately, stand the transformer upright with side facing camera,
with ruler on the table right up against the side at the bottom.
That will give us the stack height nearly exactly.
Then, stand the transformer on its back (on one of the windings) so
that we can see the inside part of the transformer which is the center
part of the stack, with a ruler sitting on top so that we can see the
edges of the center stack AND the ruler. That will give us the stack
This will enable us to give a better estimate too.
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