Multiple Transformer EI predictions

Thread Starter


Joined Jan 8, 2013
Hi All,

I am trying to do the age old estimation for a scrap transformer.

Looking around online i have found multiple rules of thumb, however some arent clear if they are meant for 50Hz or 60Hz operating freq and i am getting quite varied results.

The Transformer specs i have:
Only Markings on the transformer are one line stamped into the cover "121-481388-7DR", It was stripped from an old UK Appliance, so expect typical 230-240 rating.

RPrim = 294 Ohms ,RSec = 0.850 Ohms

This means based on 240V Primary, im expecting around a 12V Secondary. (this ties up with the first 2 digits of the markings)

It is an EI transformer, (I believe a EI-48 size based on catalog EI dims also -48 in the part marking) the cross section of the middle of the E is approx 30mm*16mm = 480mm^2 or approx 0.744 Square inches.
Transformer + approx 100mm of connection wires is approx 450 grams (0.99 pound)

So one calc is

A = (VA)^2 / 5.58 (no idea if this is for a 50Hz or 60Hz)
A being Centre CSA of the EI in square inches, VA being the approx VA rating. (From a post by Studiot from 2007 on this forum)
So that would be (0.744 * 5.58 )^2 = 17.24VA

but then according to Wikipedia, for a 12KiloGauss estimate,
a = 0.206*SQRT(VA)
this now gives, (0.744 / 0.206)^2 = 13.04VA

based on the estimate that it is 20W per Pound, at 450grams, 0.99 pounds, so 19.8VA

So i have varying results from 13.04VA, 17.24VA to 19.8VA, my gut would say i am probably looking around 18VA.

But then, looking at RS, UK they sell 12VA Chassis mount transformers which weigh approx 400gram and then 20VA at 550gram. which would then lead me to believe that the transformer is more likely a 12VA rating as its closer to 12VA/400g end of that scale.

Does anyone else have any suggestions? I do have access to a Variac at work (and LCR meter if needed) but its locked away and the guy who has the key works a different shift (I work nights)

Thanks :)


Joined Nov 6, 2012
There is no getting away from WEIGHT,
unless You go with a Toroidal-Core,
and even then the materials of construction,
and the construction techniques,
will vary substantially.

The easiest "rule-of-thumb" is,
when You think you've nailed-down a value,
cut it in half and you'll probably be happy with the results.

Then there are "specialty" Transformers, such as MOTs, ( Microwave-Oven-Transformers ),
which are extremely "lossy",
and they scrimp on the Copper until it squeals.
So for a project centered around an MOT,
if you need a reasonably efficient Transformer,
is to use 2 identical units with the Primary-Windings in series,
and the newly installed Secondary-Windings split between the 2 Cores.
You can usually get a reliable, continuous, ~1000VA out of this type of setup,
and get more space for heavy Secondary-Windings.


Joined Aug 21, 2008
In short, make a test primary winding and see what you get. If necessary, make a secondary winding and see how its output is affected by load.

Careful! Keep your hands away while power is applied and be aware that you might have a whole lot of magnetizing current in the primary.