# how to determine the current capability of a transformer

Hello I have this transformer from ebay its from a north court 50 amplifier. There is not much information online. It has 400 0 400 volt ht i think it ran a pair of KT66 valves with gz34 rectifier

dimensions are about L x B x h 4" 3" 4" if anyone can help me out on this one it would be great

• Weighing it will give you approx. power rating. – Marko Buršič Jan 20 '18 at 14:31
• Nominal power is approx.300VA. – Marko Buršič Jan 20 '18 at 14:40
• Why on earth did you buy a transformer from ebay (or anywhere else) when you don't know what it does!? This just boggles the mind. – Olin Lathrop Jan 20 '18 at 14:40
• well i have a rough idea i want to build stereo amp – user134721 Jan 20 '18 at 14:43
• @MarkoBuršič To clarify, you are saying VA = .6 * Mass * 100? You should make that an answer. Issue in this instance is the secondary looks like it has multiple windings. Mass equation won't help figure out how much amps can be drawn form each. – Trevor_G Jan 20 '18 at 16:03

The main determiner of transformer power rating is the material and cross-sectional area of the core. This sets the power level at which the core will saturate, and manufacturers don't want to use any more steel than they absolutely have to.

As a rough estimate, since these transformers generally use the same kind of steel laminations and the same relative geometry, it is sufficient to make a judgement based on the volume/mass of the core. Since the core is the majority of the total mass of the transformer (the windings are a tiny fraction), simply weighing it is enough, which is why Marko made his comment.

• ok thank you but how do i calculate it? – user134721 Jan 20 '18 at 16:20
• If you want a precise calculation, find a reference book on transformer construction. But for rough estimates, the cube-square relationship between mass and area tells us that the power handling capacity should vary proportionally to the 2/3 power of the weight. For example, given @marko's data point, a 10- kg transformer would be expected to handle about 475 VA. – Dave Tweed Jan 20 '18 at 16:31

0.3mm^2 wire can withstand 1ampere. Thus measure the diameter of secondary winding wire.You will get the approximate idea.