Agreed and I have always sized the conductors to the breaker because that is how I was taught (by the dinosaurs, as Mike would say). Or usually ask the HVAC guys what size wire to run because at that time the AC unit is not in site. I know that I could have possibly used a smaller conductor on a lot of AC units in the last 30 years.
AC motor 208 vs 240 volts
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Agreed, but maybe I should have rephrased my question. The calculation part was not about what size wire or breaker but rather wattage as it relates to a load calculation of a dwelling. I guess I was wondering if the manufacturer's number was based on the wattage of 25 amps with a 240 volt system or the wattage of 25 amps of a 208 volt system and it appears its the 208 volts system as this seems to be the worst case scenario.
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Different issue, you are conflating amps with watts. Watts are watts are watts, regardless of the voltage. The AMPS will change with the voltage, the watts do not. Load is watts.
Even if MCA matched anything, watts is Volts x Amps x K x power factor. K is 1 for single phase and the square root of 3 (1.732) for 3 phase. Or put another way in a motor you have two currents, flux and torque. The torque current does work, that is watts, power factor = 1.0. The flux creates a rotating magnetic field that turns the rotor. It does NO work. It is pure reactive 90 degree lagging current. Watts = 0. Power factor is zero. On the NEC motor charts by the way they assume power factor is 0.8. It is normally 10-15% of the current. At no load power factor is almost zero. It gets up to usually over 0.85 only at full load. Motors with lots of poles are particularly bad.
So you can’t measure current and get watts. You can however measure bolts, amps, and either power factor or phase angle to get it.
This is where I'm getting confused. This is not the mini-split I was referring to, but this is what I was looking for as far as wattage goes with different voltage systems. Why does this show different wattage if it stays the same regardless of voltage? I'm looking for the number to put in for my load calculation
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Ah, information not provided... Your title was about MOTORS.
That's because with RESISTANCE HEATING, the watts do change with voltage, because the resistance stays the same (Ohm's law). So if a resistance heater has 100ohms of resistance, Ohm's Law says that P (watts) = V2/R. So at 240V it is 576W, but at 208V it is 432W.
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