One method I've used with success is as follows:
If belt-driven, remove the belt from the compressor.
Using a clamp-on amp meter, measure the running current with no load on each phase.
If the meter has a peak-hold feature (my Fluke 36 does) also measure the peak draw on startup with no load.
Repeat the same steps with the belt on and the compressor running. Record those readings. If there is no unloader valve (releases pressure in the cylinder(s) when the motor stops, you'll hear the hiss at stop if it has one), then it would be good to repeat the measurements with the tank full.
Set your overloads to 125% of the highest running (not peak!) readings. If you have nuisance trips, try 135% of highest running amps. (The setting should roughly equal the peak current +10%/-30%, not a concrete rule but seems to work out that way.)
Also look carefully at the overload trip curves, some have longer trip times to allow for hard starting motors to ramp up without tripping.
I used this method to set the overloads on a pipe organ starter I designed and so far it has worked out perfectly. The only trips have been due to phase loss and one case of dry bearings.
Larry: Horsepower ratings are usually of little or no use in compressor applications, especially with modern units. Check out Sears' "5HP" compressor which will plug into a std. 15amp circuit.
For other motors, you've got the right idea as most HP ratings seem to fall within a standard current range. :thumbsup:
A bit OT, but in my pipe organ application, when does the blower draw more current? When the organ is idle (lots of backpressure) or when playing "full stops" (basically with little or no back pressure)? The answer may surprise you.