I found a "horsepower-amps-volts" clarification posted to this
newsgroup by John T. Horner. There's probably other good ones, too. I
thought it would be constructive to repeat it here, especially for
those ignorant of the horsepower-amp-volts thing, like me. John said
(with some minor edits by me)
First, Sears isn't the only company to use the misleading Developed
Power labeling .... but may be the most egregious about it.
"Developed" horsepower is much like "Peak RMS Power" was in the world
of amplifiers. In the US the advertising of "Peak Power" was outlawed
in the 1970s as misleading advertising.
"Developed" horsepower is calculated by jamming the motor in a stall
(not rotating) and turning on the power. The power drawn by the motor
under those conditions (amps x volts = watts) is measured. The
measurement is then converted to horsepower though the conversion 1
horsepower = 746 watts. A 3 "Developed Horsepower" motor is thus one
which draws 2,238 watts at a dead stall. Using standard 115 volt house
wiring we have 19.5 amps being drawn at a stall. A plain 6.12 ohm
resistor of sufficient power dissipation capacity (that is one very
physically large resistor!) thus also "Develops" 3 horsepower. Of
course this is all pure Horsepucky.
Electric motors are not 100% efficient at converting electrical power
into rotational mechanical power. Real "rated" horsepower is measured
as the mechanical power output of the motor at speed and under load.
The www.baldor.com site has good information on the specifications of
their ac motors for background reading if anyone is interested. An
excellent (big bucks!) single phase motor might have a full load
efficiency of 80%. The more common ones are in the range of 55% to
Now let us do some simple math:
A true 3 horsepower motor of 65% efficiency running at full load needs
(3HP X 746 watts/HP)/0.65 eff = 3,443 watts input power. At 115 volts
this implies 29.94 amps, obviously a serious overload for a standard 15
amp wall socket. At 230 volts the current draw drops to 14.97 amps,
within the capabilities of a 230 volt, 20 amp feed.
Doing the same calculations for a 1.5 HP motor you need about 15 amps
at 115 volts. Thus a power tool with a 1.5 HP motor running on 115
volts should be connected to a 20 amp circuit, not a 15 amp circuit.
This is why the rated horsepower of woodworking machines generally
doesn't go above 1.5 HP for 115 volt equipment.
There are issues of starting current, line losses, temperature margin
and such which have been left out of this quick primer.
I wish the government would put an end to this nonsense of developed
horsepower in the same manner they squashed the analogous situations
which once prevailed in the worlds of audio equipment and automobile