Around the same time our central air conditioning system faded and died. We live in southern California and do not have many really hot days, so we decided to not repair it again and just let it be.
The 220V 50A circuit for the AC happened to terminate on one wall of the shop. So, taking advantage of the situation, I decided to set up a subpanel using the former AC circuit. I am not an electrician, but have done other electrical work around the house, including wiring attic fans, adding new outdoor circuits, etc.
I added the subpanel and ran a 220/20A line (10-2G NM-B cable) to the tablesaw and bandsaw and two 120/20 A lines (12-2G NM-B cable) for new receptacles. The saws are each rated at 220V/13A and are never run concurrently, One of the 120V receptacles was dedicated to a dust collector rated at 120V/18A or 220V 9A.
I have been running this setup for about two years with no apparent problems. I recently removed the cover on the subpanel to check something and found that the neutral wire for the line feeding the dust collector, and connected to the neutral bar was charred for about two inches from the bus. Most of the insulation had been burned off along those two inches. Also, the black plastic around the neutral bus shows signs of having melted around the perimeter of the neutral bus bar.
The circuit breaker on the dust collector line was still engaged. I also tested the cb and found that it does shut off power to the circuit.
I checked the screw which held the charred wire to the neutral bus bar and it was tight I also checked the screw holding the neutral wire for the other 120V line, and it was also tight.
The 50A line from the main box to the workshop subpanel is Aluminum (house is mid 70's vintage) and the Murray subpanel in the shop is rated for both Al and Cu wire.
Does anyone have any ideas as to what the problem could be?