# current capacity of wire in smps transformer

Discussion in 'General Electronics Discussion' started by eem2am, Sep 30, 2010.

1. ### eem2am

422
0
Aug 3, 2009
Hello,

The following datasheet for enamelled copper wire (ECW) states that ECW of diameter 0.56mm (SWG24) has a current carrying capacity of 0.381A.

Enammelled copper wire datasheet:
http://www.farnell.com/datasheets/24248.pdf

However, the following web page states that wire of diameter 0.574mm (AWG23) has a transmission current carrying capacity of 0.729A.

American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies

So, whn I am winding my SMPS transformer primary winding, in which the RMS current is 0.58A, will I be OK with ECW of diameter 0.574mm?

…according to the second reference, I will be ok, according to the first, I will not be OK………..who should I believe?

2. ### Militoy

180
0
Aug 24, 2010
The guidelines for current density limits in magnet wire are just that – guidelines. They’re offered as a starting point to work with, and as a way to compare different wire sizes. In an SMPS transformer, winding turn numbers are typically small, and winding lengths and thermal path distances are short. As opposed to a fixed limit of current for a given copper wire size, a designer will usually design for a maximum desired resistive loss in each winding – considering skin effect and proximity losses. Allowing for operating frequency - it would not be unusual to use 23 gauge magnet wire at over 2 Amps current.

3. ### trobbins

83
0
Jun 15, 2010
eem, have you determined skin depth for the fundamental frequency? Remember that if you are using square waveforms, then the harmonics will have smaller skin depths.

Have you determined a layer/interleaving build and determined proximity effect influences.

You may want to convert to A/mm2 or /mil2, as many people design to certain acceptable levels of current density for a given skin depth percentages - which usually involves going to bi-tri..filar or a power litz.

Ciao, Tim