Proper installation of adjustable speed drives (ASD’s) hinges in part on compliance with the requirements of National Fire Protection Association (NFPA) 70 – the National Electrical Code (NEC). Beginning with the 2005 Code, requirements for ASD’s were consolidated in a single section, Part X, under Article 430 (Motors, Motor Circuits, and Controllers). This consolidation was made with the explicit intent that other relevant parts of Article 430 would also apply unless specifically addressed in Part X. A brief overview of these requirements will assist in understanding correct installation, and increase the likelihood of a trouble-free drive application.

Branch circuit and feeder conductors supplying ASD’s are to be sized for 125% of the drive’s rated input current, unless a bypass is part of the drive system (more on that shortly). Note that the rated input current is based on the drive electronics’ ability to handle amperage without over-heating and as such factors in over-load capacity. For a given application, the input current also varies when a drive is de-rated for single-phase supply; manufacturers typically require a 50% de-rating, which means that, for a given load, a single-phase drive  will draw approximately twice the input current of a three-phase unit. So when designing the feeder ratings for a drive system, it is critical that input amp ratings for a properly sized drive are used.

If the drive system incorporates a bypass, the bypass conductors are required to be sized for the greater of 125% of the drive’s input rating OR 125% of the motor full load amperage as defined in NEC section 430-6. This is to ensure rated conductor temperatures in the bypass circuit are not exceeded.

Additional motor overload protection is not required if the ASD includes overload protection and is marked as such – if the drive is not marked, then the Authority Having Jurisdiction must assume that the ASD does not include such protection. Fortunately, most drives include overload protection and proper labeling. However, additional overload protection is needed in the following cases:

  • If the drive is not equipped with motor overload protection, then external protection must be installed, sized per Article 430, Part III.
  • If the drive system contains a bypass. In that case, overloads must be provided in the bypass circuit to protect the motor when bypassing the drive. Again, the overloads are to be sized per Part III.
  • If a single drive supplies multiple motors, properly sized overload (and in most cases over-current) protection must be provided in each set of motor leads on the load side of the drive.

Motors supplied by ASD’s require proper overtemperature protection in some cases. Such cases include those in which the motor “is not rated to operate at the nameplate rated current over the speed range required by the application” (Sec. 430.126(A), NFPA 70, 2008). In these cases, the protection is to take the form of one or more of the following:

  • a motor thermal protector per sec. 430.32
  • An ASD system with speed- and load-sensitive overload protection with thermal retentive memory. (But thermal memory retention is not required for continuous duty loads.)
  • An overtemperature relay with thermal sensors in the motor
  • An ASD system accepting input(s) from motor thermal sensors

This is a mouthful, but in essence the intent is to ensure that self-cooled motors controlled by drives are protected from over-heating in cases where motor speed is insufficient to move the amount of air needed to keep the motor at or below allowable temperature rise. And since a rise of 10 degrees C above rated can cut motor insulation system life in half, this is an important consideration. Keep in mind, though, that motors capable of running at slow speeds without over-heating, such as totally enclosed non-ventilated (TENV) types, are not required to have this protection. But care should be taken in the case of motors using supplemental cooling, such as water or auxiliary blower, to ensure that failure of cooling systems is recognized quickly and the motor protected. This can take the form of interlocks which shut down ASD control if the cooling systems fail.

Finally, the disconnecting means supply the ASD system can be located in the supply line to the drive and must be rated for a minimum of 115% of the drive’s rated input current.

I hope this overview has provided some insight into some of the factors needed for a quality drive installation. Please feel to share your thoughts and experiences with other readers in our Comments section. And should you need help or information, please contact us at or 815-725-9696. Thanks for reading!

Jay Baima - Author


Jay Baima
Joliet Technologies