Article 430 is definitely one of the National Electrical Code’s largest, most confusing, and intimidating articles because it contains complex terms and technical requirements utilized by seasoned electricians. Typically, the electricians who work within the guidelines of Art. 430 have an intermediate to mastery level of using the NEC. They are also adept in the following sectors of the electrical industry:
- Industrial (repair, construction, and maintenance)
- Commercial (repair, construction, and maintenance)
- Light commercial (repair, construction, and maintenance)
This article provides guidance on sizing the various components of a motor circuit.
Sizing conductors
Section 430.6(A) of the 2023 NEC instructs electricians to size motor connection conductors based on the allowable ampacity tables in Table 310.16. Electricians constructing 3-phase motor circuits “in the field” use full-load current values listed in Table 430.250; they do not use the motor nameplate current. This table is used for motors running at “usual” speeds for belted motors and motors with “normal” torque characteristics. The system voltage range(s) used in this table are 110V to 120V, 220V to 240V, 440V to 480V, and 550V to 600V listed rated motor voltages.
Field electricians constructing circuits for single-phase motors use the current values listed in Table 430.248, which is for 115V-, 200V-, 208V-, and 230V-rated motors. If the conductors are supplying a single motor used in a continuous duty application, Sec. 430.22 instructs the electrician to select a wire sized not less than 125% of the motor’s full-load current rating. If the conductor is supplying multiple motors, the conductor should have an ampacity not less than the sum of each of the following:
- 125% of the full-load current rating of the highest-rated motor, as determined by Sec. 430.6(A).
- The sum of the full-load current ratings of all the other motors in the group, as determined by Sec. 430.6(A).
- 100% of the noncontinuous non-motor load
- 125% of the continuous non-motor load.
Terminal temperature rating
Another essential factor for field electricians to consider is Sec. 110.14(C). When determining the branch circuit conductor size for a commercial motor, the temperature rating terminals of the equipment and devices matter. This Section of the Code explains that the conductor selected must be coordinated to not exceed the lowest temperature rating of any connected termination, connected conductor, or connected device; the allowable conductor ampacity must be chosen from a column that does not exceed the temperature rating of the terminations. Several steps should be considered in building motor circuits — it’s not cut and dry.
Circuit breakers and fuses
Next is selecting, rating, and sizing protective devices to provide short circuit and ground fault protection for feeders and branch circuits. Motors must be protected from the following: overcurrent; low voltage; and fault currents (short circuit and ground faults).
Commercial single-phase and 3-phase motor circuits should include protection from unexpected abnormal situations. Basic protection from these occurrences is provided by installing circuit breakers or fuses in the controller or starter at a convenient location for the operator or maintenance staff. These devices are usually selected or based on the starting current or characteristics of the motor when it’s under load.
Motor starters
In many cases, the motor manufacturer may include a recommended type and size of protection. Most motor installations provide protection through a “motor starter,” which controls or regulates the electrical power for starting a commercial motor; these electrical devices are also used for stopping, reversing, and protecting electric motors. Motor overheating can cause the motor to burn out and renders the motor practically useless. To protect the motor from potential danger, overload relays can prevent these events.
Section 430.31 covers motor and branch-circuit overload protection for motor control circuits, motor controllers, and motor control centers in Part III of this Article. This section specifies sizing and selecting overload devices that protect motors, motor-control apparatus, and motor branch circuit conductors against “excessive heating” due to motor overloads and failure to start.
Important terms to know
Familiarize yourself with these important terms to become more adept in the use of Art. 430. Unique terms are typically listed in the “.2 section” of an Article — right after scope. Here are a few terms listed in Art. 100 but referenced in Art. 430 and Art. 440.
Adjustable circuit breakers. This type of circuit breaker can be set to trip at various values of current, time, or both within a predetermined range; it has small dials above or below the poles.
Setting of circuit breakers. The value of current, time, or both, at which an adjustable circuit breaker is set to trip.
Instantaneous trip circuit breakers. No delay is purposely introduced in the tripping action of this circuit breaker.
Inverse time circuit breakers. There is a delay in the tripping action of this circuit breaker purposely introduced, which delay decreases as the magnitude of the current increases.
Nonadjustable circuit breakers. This circuit breaker has no adjustments or settings for the trip time (its response time is fixed).
Hermetic refrigerant motor-compressor. A combination consisting of a compressor and motor, both of which are enclosed in the same housing, with no external shaft or shaft seals, with the motor operating in the refrigerant.
Follow my column for more practical how-to articles and tips for applying electric basics concepts in the field Everyday Instructions for Electricians, Introduction to Commercial Service Calculations, Introduction to Residential Service Calculations, The Apprentice’s Guide to Non-Metallic Sheathed Cable, The Apprentice’s Guide to NEC Chapter 4, The Apprentice’s Guide to Art. 300, the Apprentice’s Guide to Art. 640, The Apprentices Guide to Special Equipment, and The Apprentice’s Guide to Service Entrance Cable. For more information on why a structured approach is so important to navigating the NEC and how to put its requirements into practice in real-world settings, read “The NEC for Newbies.”
Harold De Loach, a master electrician and electrical trainer/instructor, is the founder of The Academy of Industrial Arts (www.taia-school.com) in Philadelphia. With more than 30 years of experience in the field, he writes regular exclusive content for the E-Train and can be reached at [email protected].
