" Don't have my code book handy to verify; however, the use of a #10
conductor
on a 5HP/240V/1PH motor even if legal, would at best be considered a bad
installation.

The difference between the I^2*R heat losses of that #10 vs the lower heat
losses of a #8 would soon consume any material savings made during
installation.

After that, the power company would be laughing all the way to the bank.

Personally, I'd use #6 wire /w/ a 2P-40A c'bkr for a 5HP/240V/1PH motor.


Lew,
The plate on my 5 HP motor indicates that it draws 20 Amps at full
load. If it was a motor that operated for extended periods at full
load, or on a long wire, I'd agree with your thinking on the I squared
R loss. But my table saw will rarely load the 5 HP motor, and the run
of #10 wire is less than 20 feet. Since I have a pretty good feel for
the tradeoffs involved, I elected to use the smaller wire, not because
of cost, but because it's easier to pull. Others may certainly choose
another path.

DonkeyHody
Even an old blind hog finds an acorn every now and then.