# Finding the primary VA when winding your own power transformer?

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

• 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? – Solar Mike Jul 16 '18 at 20:48
• 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://… – Nano Gonzalez Jul 17 '18 at 0:17