Rules Of
Thumb (Approximation)
At 1800 rpm, a
motor develops a 3 lb.ft. per hp
At 1200 rpm, a motor develops a 4.5 lb.ft. per hp
At 575 volts, a 3phase motor draws 1 amp per hp
At 460 volts, a 3phase motor draws 1.25 amp per hp
At 230 volts a 3phase motor draws 2.5 amp per hp
At 230 volts, a singlephase motor draws 5 amp per hp
At 115 volts, a singlephase motor draws 10 amp per hp
Mechanical Formulas
Torque in
lb.ft. =

HP x 5250
rpm 
HP =

Torque x rpm
5250 
rpm =

120 x Frequency
No. of Poles 
Temperature Conversion
Deg C = (Deg F
 32) x 5/9
Deg F = (Deg C x 9/5) + 32
Deg F = (Deg C x 9/5) + 32