What causes the connected wire nuts in a buck booster to melt?

Answer 1

Answer for USA, Canada and countries running a 60 Hz supply service.

Assuming the splices were made correctly, meaning tight, and assuming the wire nuts did not otherwise fail, there is more heat than the wire nuts were designed to withstand. Such splices in a transformer should be limited and eliminated if at all possible. Such splices in a transformer should not be made in close proximity to the coils. If there is not space or a compartment to isolate the splices from the heat, you need to install a different method of splicing that can withstand the heat.

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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.