What the heck does that mean? De-rating of motors is done primarily to allow for increased duty cycles, or increased ambient temperature operation. But it's usually accomplished via just buying a larger motor than what the mechanical specs call for, not actually changing the nameplate of the motor. "HP" is a shorthand expression of torque at a given speed. So if the motor was built as a 250HP, but they put a name plate on it as 200HP, I fail to see how they could have LOWERED the torque capability of the motor.
Originally Posted by MrJLH
Here's what I think. I think it sounds as though you may have stumbled across a motor importer that has taken a standard IEC 315kW 380V 50Hz motor and is telling you to use it at 460V 60Hz. 315kW = 235HP, but IEC motor specs have no concept of "Service Factor", so if you take a requirement for 200HP with a 1.15SF, that's 230HP; close enough in their mind, that extra 30HP is covered in the converted HP of that kW rated motor.
Why then are they calling is "250HP de-rated?" Because if you look at the TORQUE of a 315kW 50Hz motor and a 250HP 60Hz motor, they are closer to being the same. Torque = HP x 5250/RPM. Assuming (although not important) that this is a 4 pole motor, the 50Hz torque would be 235 x 5250/1455 = 847 ft-lbs, and a 250HP 60Hz motor would be 250 x 5250/1725 = 760 ft-lbs. The 315kW 50Hz motor is actually going to supply more torque than a 250HP 60Hz, but not enough to be called 300HP, so we jump from 250 to 300, they are calling it 250HP.
What's important to you? In this case actual motor nameplate FLA. If you look at the specs for a NEMA size 5 starter it is rated for 270A, which is actually a little higher than the NEC chart for 200HP (240A). This is because the NEMA rating has to cover lower pole counts (slower base speed) motors. That may work for you here in not requiring you to upgrade to a Size 6 starter. As to conductor sizing, I would interpret the NEC as saying you need to go with the 250HP aspect.
Many people are shocked when they discover I am not a good electrician...