I have been researching how to build power transformers but I am stuck on finding the proper wire gauge for winding the primary. In order to find the correct wire size I have to know the current in the primary coil. I have two different formulas from different sources for this that say different things.

METHOD 1 The first is an article written back in 1947. Which says "the primary circuit VA is 1.4 times the total secondary VA.

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  • METHOD 2 the other formula is from a transformer calculator which is Vp x Ip = Vs x Is. Then divide primary VA / Vp (115)

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My example problem is this

  • Vp (primary voltage) = 115
  • Ip (primary current) = ???
  • Vs (secondary voltage) = 19.2 volts
  • Is (secondary current) = 1.5 amps

When using Method 1 we get Ip = 0.35 amps. This seems to makes sense because the primary should be more current than the secondary to make up for any losses.

When using Method 2 we get Ip = 0.25 amps. This has the primary and secondary VA being equal. Which contradicts the first method which says primary VA is 1.4 times the secondary VA.

So which one is correct?

  • \$\begingroup\$ That first article, did the 1.4 come from the turns ratio calculated earlier and on part of the page that you did not copy? \$\endgroup\$
    – Solar Mike
    Jul 16, 2018 at 20:48
  • \$\begingroup\$ No, it seems to be an approximation that is very close to all the calculators I have used. Here is a link to the article google.com/url?sa=t&source=web&rct=j&url=http://… \$\endgroup\$ Jul 17, 2018 at 0:17

1 Answer 1


There appears to be a contradiction but you are lacking a vital piece of information; the primary will not have infinite inductance - it will have finite inductance and although this doesn't consume power (unless the core saturates) there will be a VA associated with it in addition to load current VA transferred from secondary to primary.

So the primary consists of load VA and magnetization VA. Typically for a 230 V power transformer (circa 100 VA on secondary), the primary inductance will be upwards of 5 henry and this will have an impedance of 1570 ohms at 50 Hz and will take 230/1570 amps = 150 mA.

So the magnetization VA is 33.7 VA and total primary VA will be about 134 VA. Not exactly 0.35/0.25 but near enough given that there will be copper losses and leakage inductance losses that I have not accounted for.


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