Per whatever standard you're referencing, does the service factor applying to HP (not current), only hold true when voltage is equal to nameplate value?
Never mind, I answered my own question. You must be referencing this standard?
The National Electrical Manufacturers Association (NEMA) defines service factor in section MG1 – 1.43 of their manual as: “The service factor of an alternating current (AC) motor is a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor.” The conditions under which service factor may be applied are described in NEMA MG1 – 14.36 as: “When the voltage and frequency are maintained at the value specified on the motor’s nameplate, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate.”
So in order for service factor to apply, V and F have to match nameplate. They're constants.
P=I×E.
If E is a constant, then there is only one variable which can affect P, and that is I. Current.
So your argument is nothing but semantics. Service Factor is based on Current (at nameplate voltage), and therefore HP (at nameplate voltage). And the way you broadly stated it to be based on HP alone (no mention of nameplate voltage) makes the argument less than true.
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