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For EC&M Magazine
By Mike Holt, NEC® Consultant
Here's the follow-up to yesterday's newsletter.
This includes the answers to the questions sent, so you can see how you did.
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Figure 01
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Figure 01
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For EC&M Magazine
By Mike Holt, NEC® Consultant
Here's the follow-up to yesterday's newsletter.
This includes the answers to the questions sent, so you can see how you did.
Note: The answers to these questions are based on the 2020 NEC.
Q1. What is the Code definition of a motor controller?
A1. Controller. Any switch or device used to start and stop a motor by making and breaking the motor circuit current [430.2 Definitions].
Author's Comment
►A controller can be a horsepower-rated switch, snap switch, or circuit breaker. A pushbutton that operates an electromechanical relay is not a controller because it does not meet the controller rating requirements of 430.83. Devices such as start-stop stations and pressure switches are control devices, not motor controllers.
Q2. What is the Code definition of Electronically Protected motors?
A2. Electronically Protected (as applied to motors). A motor that is provided with electronic control that is an integral part of the motor and protects the motor against dangerous overheating due to failure of the electronic control, overload, and failure to start [430.2 Definitions].
Author's Comment
►This type of motor will be identified by the words “electronically protected” or “E.P.” on the nameplate, just like the motors with internal thermal protection have the words “thermally protected” on the nameplate.
►A reference to this type of protection was added to 430.32(A)(2) that previously only covered thermal protection.
Q3. What is the Code definition of Thermally Protected motors?
A3. Thermally Protected (as applied to motors). A motor or motor-compressor that is provided with a thermal protector [430.2 Definitions].
Author's Comment
►The definition for “Thermally Protected” appears in Article 100 but is included here for the reader’s convenience.
Q4. What is the Code rule for sizing of conductors that feed an adjustable-speed drive system?
A4. Branch/Feeder Circuit Conductors. Circuit conductors for an adjustable-speed drive system must have an ampacity of not less than 125 percent of the rated input current to the power conversion equipment [430.122(A)]. Figure 01
Motor Branch-Circuit for Adjustable Speed Drive Example [430.122]
Question: What size branch‑circuit conductors are required for an adjustable‑speed drive system with a rated input of 25A and terminals rated 75ºC?
(a) 14 AWG (b) 12 AWG (c) 10 AWG (d) 8 AWG
Answer: (c) 10 AWG
Solution:
Rated Input from Adjustable-Speed Drive = 25A
Branch Circuit Conductor = 25A × 125%
Branch Circuit Conductor = 31.25A
Use 10 AWG rated 35A at 75ºC [Table 310.16 and 110.14)(C)(1)(a)(3)].
Note 1: Adjustable-speed drive systems can have multiple power ratings and corresponding input currents.
Note 2: Circuit conductors on the output of an adjustable-speed drive system are susceptible to breakdown under certain conditions due to the characteristics of the output waveform of the drive. Factors affecting the conductors include but are not limited to the output voltage, frequency, and current, the length of the conductors, the spacing between the conductors, and the dielectric strength of the conductor insulation. Methods to mitigate breakdown include consideration of one or more of these factors.
Q5. What is the Code rule for sizing branch-circuit short-circuit and ground-fault protection for circuits containing an adjustable-speed drive system?
A5. Circuits Containing an Adjustable-Speed Drive System. Circuits containing an adjustable-speed drive system must be protected by a branch-circuit short-circuit and ground-fault protective device in accordance with all of the following [430.130(A)]:
(1) The rating and type of protection must be determined by 430.52(C)(1), (C)(3), (C)(5), or (C)(6), using the motor’s full‑load current (FLC) as listed in Tables 430.248 and 430.250.
Ex: The rating and type of protection is determined by Table 430.52 using the adjustable-speed drive system rated input current where the adjustable-speed drive system is listed and marked “Suitable for Output Motor Conductor Protection.”
Note 1: Motor conductor branch-circuit short-circuit and ground-fault protection from the adjustable-speed drive system to the motor is provided by an adjustable-speed drive system that is listed and marked “Suitable for Output Motor Conductor Protection.”
Note 2: A motor branch circuit using an adjustable-speed drive system, including equipment listed and marked “Suitable for Output Motor Conductor Protection,” includes the input circuit to the power conversion equipment.
(2) The maximum branch-circuit short-circuit and ground-fault protective ratings must be in accordance with the manufacturer’s instructions.
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