Transformer current / power rating.

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Lord Loh.

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I have a 12-0-12 transformer which is rated 5A.

So the power I shall get is 60W.

Suppose I try to get 24v from this transformer, should I derate the maximum current to 2.5A ? to keep the power constant at 60W ? There is no power specification given for the transformer.

I belive that I can still draw 5A as the thickness of the secondary coil wire is still the same...and is capable of handeling of 5A.
 

12-0-12 means you can get TWO 12VAC outputs frm the transformer. If you said it's rated at 5A, then each output can deliver 5A. So, each output power is 12 x 5 = 60VA (not watt as it depends on the power factor of the load). So, the total power can be drawn frm the outputs is 120VA. If you connect it as 24V output, you still can draw 5A, as the total power is still 120VA.

Thickness of the secondary wire is similar to that of primary doesn't mean it can handle the similar current rating that can be handled by primary. This is because the number of wire turn for secondary winding might be more and therefore the winding resistance is higher.
 

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    Lord Loh.

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You can estimate current rating of secondary winding by measuring wire diameter of it. Formula for 4A/mm² current density is I=2*d². Measured in mm. If your transformer is EI type you may allso measure iron core area and use formula P=S², S is measured in cm². This is just to give you an orientation. At thoroid cores power rating estimation is different but formula for wire diameter is the same.
 

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