Motors, Motor Circuits and Controllers, Part III: Article 430

Article 430 in the National Electrical Code (NEC) covers motors, motor branch-circuit and feeder conductors and their protection, motor overload protection, motor control circuits, motor controllers, and motor control centers. The article is in Chapter 4, Equipment for General Use. There are 22 articles in Chapter 4, and like the rest of the NEC, most of the articles are divided into parts. Article 430 has 14 parts.

Part I in Article 430 contains general requirements pertaining to motors, motor circuits and motor controllers. Some of the headings in Part I are Scope, Definitions, Ampacity and Motor Rating Determination, Marking on Motors and Multimotor Equipment, Marking on Controllers, Terminals, Motor Terminal Housings, Location of Motors, and Highest Rated or Smallest Rated Motor. The sections in Part I include 430.1 through 430.18.

Section 430.6, Ampacity and Motor Ratings Determinations, is an important one. The first sentence instructs the user to select conductors supplying equipment covered by Article 430 from the allowable ampacity tables in accordance with 310.15(B) or to calculate them in accordance with 310.15(C). The sections given for selecting conductors are the same sections used to select conductors for general wiring.

The first section referenced in 430.6 is 310.15(B), which pertains to ampacities for conductors rated 0 to 2,000 volts (V). This section includes three tables used often for selecting conductors. The first table in 310.15(B) is Table 310.15(B)(2)(a), Ambient Temperature Correction Factors Based on 30°C (86°F). The second table used often for selecting conductors is Table 310.15(B)(3)(a), Adjustment Factors for More Than Three Current-Carrying Conductors.

The table probably referenced more than any other table in the NEC is Table 310.15(B)(16). This table provides allowable ampacities of insulated conductors rated up to and including 2,000V, 60°C through 90°C (140°F through 194°F). Besides the voltage limitations, other parameters must be met when using this table to select the appropriate size conductor. Table 310.15(B)(16) is based on not more than three current-carrying conductors in raceway, cable or earth (directly buried). When there will be more than three current-carrying conductors, the ampacity shown in Table 310.15(B)(16) will have to be adjusted or derated.

Table 310.15(B)(3)(a) contains adjustment factors that shall be applied to the ampacity of a conductor when there are more than three current-carrying conductors. Table 310.15(B)(16) also is based on an ambient temperature of 30°C (86°F). When the ambient temperature is something other than 30°C, the ampacity shown in Table 310.15(B)(16) will have to be corrected. Table 310.15(B)(2)(a) contains correction factors that shall be applied to the ampacity of a conductor when the ambient temperature is something other than 30°C. Table 310.15(B)(16) shows allowable ampacities for copper and aluminum (or copper-clad aluminum) insulated conductors.

The temperature ratings are based on the conductor types. Both copper and aluminum conductors are divided into three temperature ratings, which are 60°C (140°F), 75°C (167°F) and 90°C (194°F). The second section referenced in 430.6 is 310.15(C), which permits calculating conductor ampacities under engineering supervision. Whether selecting conductors for general wiring or conductors supplying equipment covered by Article 430, select the conductors in accordance with 310.15(B) or calculate them in accordance with 310.15(C) (see Figure 1).

Section 430.6(A) states the current ratings for general motor applications shall be determined based on 430.6(A)(1) and (A)(2). Other than for motors built for low speeds (less than 1,200 revolutions per minute [RPM]) or high torques, and for multispeed motors, the values given in Tables 430.247 through 250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate [430.6(A)(1)].

Tables 430.247, 430.248, 430.249, and 430.250 provide full-load currents (FLC) in amperes for direct-current motors, single-phase alternating-current (AC) motors, two-phase AC motors and three-phase AC motors. Motors built for low speeds (less than 1,200 RPM) or high torques may have higher FLCs than those shown in the tables in the back of Article 430. Multispeed motors will have FLCs that vary with speed changes. With any of these motor types, use the nameplate current ratings for all calculations.

For motors not listed in the tables, it will be necessary to use the nameplate currents in amperes for the calculations. For example, a 350-horsepower (hp), 4,160V, three-phase motor will be installed in an industrial facility. The nameplate shows this motor will draw 45 amperes (A). The RPM of this motor is 1,785. Because this motor is not built for low speed—which would be less than 1,200 RPM—and because this motor is not a multispeed motor, it will be necessary to look for the FLC in one of the tables in the back of Article 430.

Since this is a three-phase, AC motor, look in Table 430.250. Table 430.250 shows FLCs in amperes for 350-hp motors, but the table only goes up to 2,300V. Since the 350-hp motor in this example is rated for 4,160V, it will be necessary to use the nameplate rating for all calculations associated with the installation of this motor. The nameplate rating for this motor is 45A (see Figure 2).

Except for motors built for low speeds, motors with high torques, multispeed motors, and motors not listed in the tables, it will be necessary to use the FLC ratings in Article 430 for most calculations pertaining to motors. Use the FLC values given in Tables 430.247 through 250 when calculating ampere ratings of switches, branch-circuit conductors, branch-circuit protection, feeder conductors and feeder protection.

As stated in 430.6(B), it shall be permissible to use the motor nameplate current rating when sizing separate motor overload protection. Field terms for motor overload protection include the terms heaters and overloads.

For example, a 10-hp, 208V, three-phase, AC motor will be installed in an industrial facility. The amps, or full-load amperes (FLA), shown on the nameplate for this motor is 26.2A when connected to 208V. When selecting branch-circuit conductors for this motor, it would be a Code violation to use the nameplate amps for the conductor sizing calculation. Therefore, it is necessary to look for the FLC in one of the tables in the back of Article 430. Since this is a three-phase, AC motor, look in Table 430.250. As shown in Table 430.250, the FLC in amperes for a 10-hp, 208V, three-phase, AC motor is 30.8A. Instead of using 26.2A when calculating the branch-circuit conductors for this motor, it is required to use 30.8A (see Figure 3).

The values shown in the FLC tables in the back of Article 430 are for motors that are not very efficient. A motor with a low efficiency will draw more amperes than a motor with higher efficiency. Using the FLCs in the tables instead of using the nameplate amperes when selecting switches, conductors, and protection for motors will help when motors are replaced.

For example, an industrial plant will be built and all the motors in this plant will be high-efficiency motors. If nameplate amperes are used to size conductors and equipment, there could be a problem in the future if the high-efficiency motors are replaced with low-efficiency motors.

Next month’s column continues the discussion of requirements for motors, motor circuits and controllers.

About the Author

Charles R. Miller

Code Contributor

Charles R. Miller, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and NFPA’s “Electrical Reference.” He can be reached at 615...

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