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I'm a massive amateur and I haven't done any serious electrical projects before.

I used some two gauge cable to replace the secondary winding in a 220V to 2100V transformer and it caught on fire really quickly. I know my cable had a current of 1000A running through it, the primary winding has 450 turns and the cable had 2 turns. Pretty much every other video I've seen of someone modifying an MOT like this, their transformer lasted for way longer without catching fire. Why did it catch on fire? Here's a video of it catching fire if it'd help.

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    \$\begingroup\$ You might want to clarify what is a MOT. \$\endgroup\$
    – Justme
    Commented Dec 29, 2023 at 12:54
  • \$\begingroup\$ @Justme: Microwave Oven Transformer. Popular target for hobbyists. \$\endgroup\$
    – Dave Tweed
    Commented Dec 29, 2023 at 13:11
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    \$\begingroup\$ "I'm a massive amateur" - please don't play with potentially life-altering equipment if you're not knowledgable on the subject. \$\endgroup\$
    – Attie
    Commented Dec 29, 2023 at 13:20

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Looking at your video, and post-mortem photo, it seems you have cut the core apart, but not reassembled it. This leaves a collosal air-gap in the magnetic circuit, which means the primary will draw many times more than its original magnetising current. This will overheat the primary very quickly, pretty much regardless of what you are doing with the secondary.

The way to rewind a MOT secondary is to leave the core intact. Use a hacksaw to cut off one side of the secondary, then hammer it out with a dowel or punch from the other side. Then carefully thread the few turns of your new secondary through the core.

You might think you can cut the core apart at the welds, and reassemble it. You are unlikely to manage to get them as close together as they were originally, without heavy tools. Any increase in the original air-gap means an increase in the already far-too-high magnetising current that MOTs are designed to run at. Make sure the core faces are undamaged and clean, before reassembling them with as much clamping force as you can bring to bear.

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Every kind of cable/wire has a heat rating for the insulation printed on it unless its some overly cheap wire. With so few turns on the secondary you are playing with a lot of current but at a lower voltage. Current and resistance is the main cause of heat. I couldnt see your video so ill just have to assume something shorted together even if it doesnt look like it and the current greatly exceeded the wires ampacity and such a small piece of wire left nowhere else for the heat to go but out.

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