# Does the place of the primary matter in a transformer?

Does it matter whether I wind the primary or the secondary first? I imagine with the primary inside the secondary you'll have less leakage. Is that right? Other reasons to put the primary or secondary first?

The main consideration when winding cores is safety and thermals. You want to wind in such a way that you maximize insulation. Also, you want to put the highest heat dissipating windings on the outside. This both usually leads to primary on the inside and secondary on the outside, because the secondary will have n less windings and consequently n times as high current (in an ideal world), so as dissipation scales with $n^2$, your dissipation in the lower turns count will be n times as high - of course when using the same gauge wire. Even though nobody in their right mind uses the same gauge, for most practical applications it's not possible to use sufficiently thicker (let alone sufficiently fine multi-strand wire) to completely negate the dissipation scaling.