DIY Spot Welder from Re-Purposed Microwave Oven Transformer

[Scotsman Hosie]

...However, you should at least watch the last episode in the series, which shows the latest iteration of the secondary winding which has turned out to be the best.

That is an impressive bit of engineering – culminating in an excellent build. I would suggest the thing that has eluded your understanding, of the failed single coil secondary, is two-fold.

1st, stranded wire will always carry more current, than the same sized solid copper. This is due to the "surface effect" of electrical current. It travels on the surface of the wire. More conductors, many times more 'surface' than a same-sized solid conductor.

The 2nd would seem to be not making a complete loop, through the actual magnetic field of the transformer. But, rather, making a U-shaped – in-and-back-out – with your solid copper secondary [almost a] coil.

Regardless, you've completely redeemed yourself, with the latest iteration and performance of your latest build. For build quality, a tool in a class by itself.

As an addendum, hydroelectric generators are often wound with hollow tubes. the design feature making use of both the inside and outside surfaces of the conductors, for maximum current-handling.