Not saying I agree with it, but NM-B is derated to 60C so #8 should not be used for 60A.
Technically #8 is ok for 40A, #6 55A and #4 70A. If you used 75C which the datasheet specs for temp, then #6 could handle 65A.
Would it be a problem? Hard to say but seems unlikely.
Nobody is saying that #8 NM-b can handle 60A. Code says that the wire only has to have 35A ampacity, which #8 NM-b DOES have. You can still put it on a 60A breaker in this case, in fact, code says you could even use a 70A breaker. This is fine because:
1. The motor is never going to pull more than roughly 30A, so the wire can handle it no problem.
2. This is a dedicated circuit for this motor only. No other loads will be plugged into this circuit, potentially overloading it and causing more than 30A to go through the wire (unlike a regular receptacle branch circuit, where anyone can plug any combination of devices into receptacles, pulling who knows how much current)
3. If something went wrong with the motor where it's pulling more current than normal (For example, a bearing going bad that makes the motor more difficult to spin), the motor has a built-in thermal overload that will trip before anything bad happens.
4. If something happened to the wiring between the breaker panel and the motor and created a short circuit, even a 70A breaker would trip immediately (a dead short will create several hundred amps of current, much more than 70A).
The whole "#14 wire = 15A breaker, #12 wire = 20A breaker, #10 wire = 30A breaker" thing goes out the window when dealing with hardwired motor circuits. The reason you may want to use a larger breaker is to avoid nuisance tripping from the large in-rush of current when the motor starts. You don't HAVE to use a 70A breaker; that's just the maximum. If you don't get any nuisance tripping on a 40A or 50A breaker, then use that. I have a 40A breaker on my 5 HP compressor.
