Motors, Part 9

Motors, Part 9Different conductor sizing rules apply, depending on motor type and some other factors. The rules are in Article 430, Part II. This seems like an awful lot to wade through, but you can skim rather than wade.

First, start with 430.22 and scan through the subsection headings, looking for your application in A through G. Note that more than one may fit. For example, you may have a multispeed motor [430.22(B)] that is also not continuous duty [430.22(E)]. Make a note of all applicable requirements.

Two other motor types fall outside 430.22:

  1. Wound-rotor. Do you have a wound-rotor secondary or a squirrel cage motor? The wound-rotor motor produces high torque with relatively lower starting current. This makes it great for applications such as cranes. If you have this kind of motor, you’re going to size your conductors based on 125% of the full-load secondary current of the motor [430.23]. That’s different from sizing them based on 125% of the full-load motor current, as you do for a squirrel cage motor.
  2. Constant voltage direct-current motor. For this type of motor, apply 430.29 for the power resistors.

Does this application involve more than one motor? If so, apply 430.24 and/or 430.25 as appropriate. If you have load diversity, you can apply feeder demand factors [430.26].

If your motor has capacitors, apply 430.27. If your motor drive is power factor corrected (and most are), it has capacitors.

Apply 430.28 if you supply your motor with feeder taps.

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