This post reminds me of an installation I did years ago as an apprentice.
A pottery person had two kilns on an old service, and kept blowing fuses.
She had a 100A service, with two 30A kilns on fused disconnects. She knew she could only run one kiln at a time, but would blow fuses when the elements shorted during an explosion in a kiln.
She wanted breakers instead of fuses. They didn't make them anymore for her panel. She couldn't afford a main panel replacement. We couldn't find a 60A breaker for a sub-panel. We did find a 60A main breaker that would fit her panel. It was a screw-in as opposed to push-in.
We talked to the local inspector, called the national onsite approval people, drilled and tapped her existing bus bars, and installed the 60A to feed a sub-panel.
All authorities agreed and passed it because they estimated her 'old' bus bars capable of 300A.
