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Motors, Part 5

The requirements for motor rating determination and for sizing the conductors that supply motor-related equipment are in subsection 430.6. This subsection starts off by essentially telling you to select your conductors the same way you would for any other circuit.

What it doesn’t tell you is that getting the ampacity right is the tricky part. Article 430 provides detailed instructions, later.

Once you know the correct ampacity for a given motor circuit conductor, then you can turn to the ampacity tables in 310.15(B). As with other conductors, you also have the option of calculating the ampacity per 310.15(C).

To determine the motor rating, you first need to decide what type of application this is. You have four choices:

  1. General motor application.
  2. Torque motor.
  3. AC adjustable motor.
  4. Valve actuator motor assembly.

If it’s not one of the last three, it automatically defaults to the first one. Let’s look at those requirements.

Base motor overload protection on the motor nameplate current rating [430.7(A)(2)]. For all motors except low-speed, high-torque, or multi-speed, don’t use the actual current rating on the motor to determine the ampacity of conductors. This same restriction applies to determining the ampere ratings of switches and overcurrent protection devices. Instead, use the NEC’s Table 430.247, 430.248, 430.249, or 230.50 as applicable.


  • Torque motors, use the locked-rotor current.
  • AC adjustable motors, use the maximum operating current on the motor nameplate.
  • Valve actuator, use the name-plate full load current.

« Part 4Part 6 » | Source: Mark Lamendola | Mindconnection