Article 430, Part X provides the NEC requirements for adjustable-speed drive systems. Notice the word “systems,” which means this covers more than the drive itself.
Size the branch or feeder circuit conductors supplying the power conversion equipment of adjustable-speed drive system to an ampacity at least 125% of the rated input power of the conversion equipment [430.122(A)]. Informational Note No. 1 states this equipment can have multiple power ratings and corresponding input currents. Informational Note No. 2 is new with the 2020 NEC, and it addresses the issue of conductor breakdown susceptibility. It lists some contributing factors; be smart about this and consider these as part of a mitigation strategy.
Size the output conductors (those between the power conversion equipment and the motor) at an ampacity at least 125% of the motor full-load current as determined by 430.6(A) or (B) [430.122(B)]. But you don’t have to follow this rule if power conversion equipment is listed and marked as “Suitable for Output Motor Conductor Protection.” The rule you follow instead is one of the two enumerated in the Exception Note.
If your system has a bypass device, ensure the conductor ampacity for the bypass is at least that determined by 430.6 [430.122(C)]. And if you use a bypass, the circuit conductors supplying the power conversion equipment must be sized at 125% of either the rated input current to the power conversion equipment or the motor full-load current rating as determined by 430.6.
If you have other loads than one motor, ensure the ampacity of the supply conductors conforms to 430.24 [430.122(D)].
If motor overload protection is provided by the adjustable drive system, you don’t need to supply additional motor overloads [430.124(A)]. In fact, it’s better if you don’t. Overtemperature protection of the motor must be provided by the adjustable drive system, if the motor is not rated to operate at the nameplate rated current over the speed range required by the application [430.126]. One reason for this requirement is a fan-cooled motor that is turning more slowly is also cooling more slowly. You’ll find detailed requirements for overtemperature protection in 430.126.
You can relocate the motor disconnecting means to the incoming line to the adjustable drive system [430.128], but only if it has a rating of at least 115% of the rated input current of the conversion unit.
The branch-circuit short-circuit and ground-fault protection requirements for single motor circuits are in 430.130. In essence, you’re going to use the normal sizing process outlined in 430.52(C)(1), (3), (5), or (6). But there’s an exception, and following it are two Informational Notes. And if the manufacturer has protection requirements listed on the equipment, you cannot exceed those values.
The branch-circuit short-circuit and ground-fault protection requirements for circuits including more than a single motor are in 430.131. A key takeaway here is for purposes of sizing the branch-circuit short-circuit and ground-fault protection devices, you consider the conversion equipment to be a motor controller.
