# magnetic wire guage ratio between primary and seconday windings

Discussion in 'General Electronics Chat' started by adeel, Jun 28, 2008.

Jun 28, 2008
16
0
hi ,
Friends i need ur help i want to build a transformer to step down voltage i have 1.5*2 (inch) core and i want to make 25 amp 12 volts transformer but i am surprised i have no guage ratios i have turn ratio formula but not the guage ratio. Description is here
220 volts for primary 50/60 hz
12 volts for seconday
if u know guage ratio for primary and secondary (like 22 or 12) please give me the formula thanks .

Apr 5, 2008
19,914
4,141
3. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,724
496
A good working rule of thumb is to make the current density in each winding the same.

In other words, the current divided by the cross sectional area of the winding should be the same for both the primary and the secondary. You will have to select primary and secondary wire gauges such that the winding window is as full as possible while maintaining equal current density in the two windings.

4. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
Transformer voltage ratios go by turns in primary/turns in secondary. The wire gauges are important, but the core structure is the determining factor in the amount of power the transformer can handle. Winding the primary and secondary with heavier gauge wire will not increase power handling.

5. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Sorry to dissapoint you, but with that size of transformer core at 50/60Hz, you will be lucky to get over 2 Amperes of current at 12v. You need a much larger core, more like 5" * 6" or thereabouts (this is a "ballpark" guess.)

The wire size is determined by the amount of current expected in the circuit.
Here is a website that has a table of wire gauge vs current:
http://www.powerstream.com/Wire_Size.htm
You can use "chassis current" for your transformer. The smallest size you could use for a 25A secondary carrying 25 Amperes would then be AWG 15.
Your primary side will likely have around 1.5-1.6A of current. The smallest size you could use would then be AWG 27.