When to use 746W/HP

[Nickarus]

Hi everyone!

I'm studying up for my AE PE in April, and something's come up that's always bugged me in my cross-discipline practice: When discussing electrical motor loads with ME's and often in the context of energy modeling, I've often been referred to the "746W/HP" conversion to determine electrical requirements from mechanical equipment schedules. This straight conversion seems at odds with the procedure in electrical systems design that starts with the voltage/hp and determining the FLA using the NEC motor FLA tables at the end of Article 430.

I'm trying to understand where to draw the line between using the straight 746W/hp conversion and when to work from the NEC FLA's, as the AE PE exam apparently contains solutions that require the use of both approaches for different questions... a

Is it correct to say the FLA tables in NEC Art.430 are only to be used when sizing conductors/OCP/services and that in any other case concerning a motor's load given in horsepower, the 746W/HP straight conversion should be used?

 

[Kahrlo]

Hi everyone!
I'm studying up for my AE PE in April, and something's come up that's always bugged me in my cross-discipline practice: When discussing electrical motor loads with ME's and often in the context of energy modeling, I've often been referred to the "746W/HP" conversion to determine electrical requirements from mechanical equipment schedules. This straight conversion seems at odds with the procedure in electrical systems design that starts with the voltage/hp and determining the FLA using the NEC motor FLA tables at the end of Article 430.

I'm trying to understand where to draw the line between using the straight 746W/hp conversion and when to work from the NEC FLA's, as the AE PE exam apparently contains solutions that require the use of both approaches for different questions... a

Is it correct to say the FLA tables in NEC Art.430 are only to be used when sizing conductors/OCP/services and that in any other case concerning a motor's load given in horsepower, the 746W/HP straight conversion should be used?

That's correct. NEC is used by EEs as a guide to ensure safety in electrical installation. If you look at article NEC 430.6, to size the motor conductors, short circuit and GF protection, you need to use the tables given in the code book to determine the FLA rather than using the nameplate rating of the motor. But if you're looking for the actual current draw/ active power drawn by the motor, 746W/hp should be used.

 

[cableguy]

As for "when to use it during the exam", I took the approach that if the exam mentioned "NEC" in the problem, I approached it from the tables. If it was a generic (non-NEC) problem, I would have used 746 w/hp.

 

[Nickarus]

Thanks so much fellas!