Chicago said:
Now I'll wait for the lambasting.
No lambasting from me. You're a professional, and that's one of the choices you make. I'm not comfortable exceeding manufacturers horsepower ratings as labeled on their switches. There are two reasons for this.
One, the UL has not tested the switches for greater than the indicated horsepower. This will put some liability on me if something should happen down the road.
UL General Guide said:
SNAP SWITCHES (WJQR)
While many of these snap switches will operate successfully
on circuits that have some reactance in general, an inductive load should
not exceed one-half the amp rating of the switch at the voltage involved.
However, some of these snap switches are marked with additional horsepower
ratings at one or more voltages which indicate that a switch which
is so marked has been tested for the control of a motor of the horsepower
and voltage rating indicated.
Since the UL General Guide is not a permissive document as the NEC is, you cannot exceed the horsepower rating of a switch that is so rated.
Secondly, the NEC has a few rules.
nec said:
430.110 Ampere Rating and Interrupting Capacity.
(A) General. The disconnecting means for motor circuits
rated 600 volts, nominal, or less shall have an ampere rating
not less than 115 percent of the full-load current rating
of the motor. [ROP 11–79]
Exception: A listed nonfused motor-circuit switch having
a horsepower rating not less than the motor horsepower
shall be permitted to have an ampere rating less than
115 percent of the full-load current rating of the motor.
Notice that the original switch Mike selected did not have a straight ampere rating. It had ratings for various load types. You can't apply the 115% rule to this switch, because it has no pain ampere rating. You could apply that 115% rule to an ordinary general use snap switch that did not otherwise have a horsepower marking.
Yes, if you installed the 1/4hp switch on a 1/3 horsepower motor, it would probably be just fine for a great while. I am resolved, however, that it would be a violation to do so and it's life (as compared with a higher horsepower rated switch in the same installation) would be shortened. Would I install it? Probably so, but I couldn't pretend that it was permissible or proper.