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Sizing Conductors, Part XLIII

By Charles R. Miller | Dec 15, 2014
Figure 1

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Article 430 of the National Electrical Code (NEC) covers motors, motor branch-circuit and feeder conductors, motor branch-circuit and feeder protection, motor-overload protection, motor-control circuits, motor controllers, and motor-control centers. Many factors must be considered when sizing conductors for motors. To select the correct conductor size, it is necessary to reference several sections throughout the Code. Before referencing the NEC, certain information is needed. Gather it by answering some key questions.

What is the lowest temperature rating of any connected termination, conductor or device? What is the full-load current (FLC) of the motor? Will the conductors supply a motor used in a continuous-duty application? What will be the temperature rating of the conductors? Will the conductors supply a single motor, more than one motor, or one or more motors and one or more other loads? What will be the maximum ambient temperature? How many current-carrying conductors will be in the raceway or cable?

Reference 430.22 when sizing conductors for a single motor. Conductors that supply a single motor used in a continuous-duty application shall have an ampacity of not less than 125 percent of the motor FLC rating, as determined by 430.6(A)(1), or not less than specified in 430.22(A) through (G). While this section provides provisions for many different types of motors, the most common type is a motor with a continuous-duty rating. For example, what is the minimum size THWN copper conductor required to supply the following motor?

The motor is a 15-horsepower (hp), three-phase motor that will be supplied by three-phase, 230 volts (V). The nameplate shows the motor will draw 34 amperes (A) when supplied by 230V. The nameplate also shows this motor has a continuous-duty rating. The branch-circuit conductors supplying this motor will be installed in a raceway. The maximum ambient temperature will be 30°C, and there will not be more than three current-­carrying conductors in the raceway. The voltage drop will not exceed the NEC recommendation. All of the terminations will be rated 75°C. As stated in 430.22, conductors that supply a single motor used in a continuous-duty application shall have an ampacity of not less than 125 percent of the motor FLC rating, as determined by 430.6(A)(1). Other than for motors built for low speeds (less than 1,200 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)].

Since this example is looking for the minimum conductor ampacity, select the FLC from the appropriate table in the back of Article 430. In accordance with Table 430.250, the FLC of a 15-hp, three-phase, 230V motor is 42A. Do not make the mistake of multiplying the full-load ampere (FLA) shown on the motor’s nameplate by 125 percent and then selecting a conductor. Since this motor is rated continuous-duty, multiply the FLC rating of 42A by 125 percent. The minimum conductor ampacity for this 15-hp motor is 53A (42 × 125% = 52.5 = 53). Because of the ambient temperature and the number of current-carrying conductors, there is no need to adjust or correct the conductor ampacity. Since the terminations will be rated 75°C and THWN conductors are rated 75°C, select a copper conductor from the 75°C column that has an allowable ampacity of at least 53A. The minimum size THWN conductor required to supply this motor is 6 AWG (see Figure 1).

Since 430.22 uses the term “continuous duty,” it is essential to understand what it means. It is important to note that the term is not the same as “continuous load.” As defined in Article 100, a continuous load is a load where the maximum current is expected to continue for three hours or more. Continuous duty does not have a three-hour time condition. Continuous duty, also defined in Article 100, is operation at a substantially constant load for an indefinitely long time. Most motors have a duty or time rating of continuous, and if that is the case, the motor shall be marked continuous. In accordance with 430.7(A), time rating is just one of many items that must be marked on a motor. Some of the other items required to be marked on the motor include manufacturer’s name; rated volts and FLC; rated frequency and number of phases if an alternating-current (AC) motor; rated full-load speed; rated temperature rise or the insulation-system class and rated ambient temperature; time rating; rated horsepower if it’s ⅛ hp or more; code letter or locked-rotor amperes if it’s an AC motor rated ½ hp or more; and design letter for design B, C or D motors. While there are 15 items listed in 430.7(A), not all of these items are required to be on every motor. For example, if a motor is provided with a thermal protector complying with 430.32(A)(2) or (B)(2), it shall be marked “thermally protected” [430.7(A)(13)].

This section has an alternative provision for motors rated 100 watts or less. Thermally protected motors rated 100W or less and complying with 430.32(B)(2) shall be permitted to use the abbreviated marking “T.P.” Some swimming-pool- pump motors are provided with a thermal protector and the motor shall be marked “thermally protected” if the thermal protector complies with 430.32(A)(2) or (B)(2) (see Figure 2).

When sizing circuit conductors for a single motor, the FLC of a continuous-duty motor shall be multiplied by 125 percent before selecting the conductor. A continuous-duty motor is just one type of motor covered in 430.22. Some of the other types of motors in this section are DC motors operating from a rectified power supply; multispeed motors; wye-start, delta-run connected motors; and part-winding-connected motors. When the motor is not a continuous-­duty motor, 430.22(E) is applicable. Conductors for a motor used in a short-time, intermittent-, periodic-, or varying-duty application shall have an ampacity of not less than the percentage of the motor nameplate current rating shown in Table 430.22(E), unless the authority having jurisdiction grants special permission for conductors of lower ampacity [430.22(E)]. See Article 100 (under “Duty”) for the definitions of short-time, intermittent-, periodic- and varying-duty. As stated in the footnote under Table 430.22(E), any motor application shall be considered as continuous-duty unless the nature of the apparatus it drives is such that the motor will not operate continuously with a load under any condition of use. As previously mentioned, the time rating is one of the items required to be marked on the motor. It pertains to the amount of time a motor will be operated. In accordance with 430.7(A)(6), the time rating shall be 5, 15, 30 or 60 minutes, or continuous. A 5-minute rated motor will operate for 5 minutes and then will not operate with load for 55 minutes. A 15-minute rated motor will operate for 15 minutes and then will not operate with load for 45 minutes. A 30-minute rated motor will operate for 30 minutes and then will not operate with load for 30 minutes.

For example, what is the minimum size THWN copper conductor required to supply the following motor? The motor will be used in an intermittent-duty application and has a time rating of 15 minutes. The nameplate on this three-phase motor shows the motor will draw 120A when supplied by 460V. The branch-circuit conductors supplying this motor will be installed in a raceway. The maximum ambient temperature will be 30°C, and there will not be more than three current-carrying conductors in the raceway. The voltage drop will not exceed the NEC recommendation. All of the terminations will be rated 75°C. Because this motor will be used in an intermittent-duty application, multiply the motor nameplate current rating by the appropriate percentage in Table 430.22(E). The percentage for an intermittent-duty motor with a 15-minute time rating is 85 percent. It is permissible to use the nameplate rating instead of the FLC rating when using Table 430.22(E). The minimum conductor ampacity for this motor is 102A (120 × 85% = 102). Because of the ambient temperature and the number of current-carrying conductors, there is no need to adjust or correct the conductor ampacity. Since the terminations will be rated 75°C and THWN conductors are rated 75°C, select a copper conductor from the 75°C column of Table 310.15(B)(16). The conductors must have an allowable ampacity of at least 102A. The minimum size THWN conductor required to supply this motor is 2 AWG (see Figure 3).

Next month’s column continues the discussion of sizing conductors.

About The Author

Charles R. Miller, owner of Lighthouse Educational Services, teaches custom-tailored seminars on the National Electrical Code and NFPA 70E. He is the author of “Illustrated Guide to the National Electrical Code” and “Electrician's Exam Prep Manual.” He can be reached at 615.333.3336 and [email protected]. Connect with him on LinkedIn.

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