About 2 hours into using an outlet to run a 15 amp tile saw, the 20 amp circuit died. Nothing else was running on the same circuit. The saw ran when I changed outlets, and the original outlet checked out OK. How can we find what caused the problem? Thanks.

A 15 amp saw under a heavy load could pop a 20 amp circuit or your breaker could just be going bad. Do you know if the 15amp rating is for load or no load? Try another circuit and see if the problem follows the saw location.

Replace the circuit breaker. A breaker that says 20amp will trip at *OR BELOW* 20 amps. They are not precise, and can wear out over years. I've had this exact problem in my tool room, and replacing the breaker did the trick.

More importantly when you do this, check the wiring in the circuit panel to make sure it is suitable to carry 20 amps. I don't know the minimum gauge for 20amp. I do know that homeowners often oversize breakers, which can be a dangerous thing to do if the wiring can't support it.

This is worth opening and saving. It answers several questions here. Wire sizes, gauges, for breakers (http://www.greatinspector.com/faq-elec-wiresize.html)