Feeder Conductor Sizing for Motors

[Cory@MAHC]

Working through the Electrical Exam Prep book and on Unit 7 Motor and Air-Conditioning Calculations. Questions 4-6 in the Challenge Quiz offers information regarding a motor that is not continuous duty. The answer key states 'Duty cycle usage of a motor has no impact on feeder conductor sizing, only branch circuit conductor sizing.' The answers to the questions correspond to the application of 430.24 with not regard to duty cycle.

Article 430.24 Exception No. 1 reads:

Where one or more of the motors of the group are used for short-time, intermittent, periodic, or varying duty, the ampere rating of such motors to be used in the summation shall be determined in accordance with 430.22(E). For the highest rated motor, the greater of either the ampere rating from 430.22(E) or the largest continuous duty motor full-load current multiplied by 1.25 shall be used in the summation.

The way I understand this is that I would apply 430.22(E) to my non-continuous duty motor and use that number for my calculations if it was the largest load motor.

Am I missing something? Doesn't the fact that I have a non-continuous duty motor have a POTENTIAL impact on sizing Feeder Conductors?

Realistically I'll probably never run into this issue in the real world but would not be surprised to see something like this come up on a test.

 

[kwired]

Working through the Electrical Exam Prep book and on Unit 7 Motor and Air-Conditioning Calculations. Questions 4-6 in the Challenge Quiz offers information regarding a motor that is not continuous duty. The answer key states 'Duty cycle usage of a motor has no impact on feeder conductor sizing, only branch circuit conductor sizing.' The answers to the questions correspond to the application of 430.24 with not regard to duty cycle.

Article 430.24 Exception No. 1 reads:

Where one or more of the motors of the group are used for short-time, intermittent, periodic, or varying duty, the ampere rating of such motors to be used in the summation shall be determined in accordance with 430.22(E). For the highest rated motor, the greater of either the ampere rating from 430.22(E) or the largest continuous duty motor full-load current multiplied by 1.25 shall be used in the summation.

The way I understand this is that I would apply 430.22(E) to my non-continuous duty motor and use that number for my calculations if it was the largest load motor.

Am I missing something? Doesn't the fact that I have a non-continuous duty motor have a POTENTIAL impact on sizing Feeder Conductors?

Realistically I'll probably never run into this issue in the real world but would not be surprised to see something like this come up on a test.

I haven't looked into code, but I do agree there is potential impact on feeder sizing. That said many non continuous duty motors I have encountered can have smaller conductors run to them than if they were continuous duty motors. Limited duty cycle means there is time to cool off before it can be run and heated up again.

I wouldn't say you will never run into non continuous duty motors, depending on what your job is. One I commonly see is the hydraulic pump motor on automotive lifts, maybe some hoist/winch motors, or linear actuator type motors.