Advertisement

Advertisement

Sizing Conductors, Part XLIV

By Charles R. Miller | Jan 15, 2015
Figure 1

Advertisement

Advertisement

Advertisement

Article 210 in the National Electrical Code (NEC) covers branch circuits, with the exception of circuits that supply motor loads only. Article 430 covers these branch circuits, and Part II covers motor-circuit conductors. As stated in 430.21, Part II specifies ampacities of conductors that are capable of carrying the motor current without overheating under the conditions specified. Section 430.22 has specifications on sizing conductors that supply a single motor, and 430.24 has specifications on sizing conductors supplying several motors or a motor or motors and other load or loads.


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 full-load current (FLC) rating, as determined by 430.6(A)(1), or not less than specified in 430.22(A) through (G). Some of the rules pertaining to conductors supplying motor loads are applicable only to motor loads. Other rules pertaining to conductors supplying motor loads are applicable to motor loads and to nonmotor loads.


Rules for temperature limitations are applicable to motor loads as well as nonmotor loads. As stated in 110.14(C), the temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor or device.


The termination provisions are divided into two groups. The first group in 110.14(C)(1)(a) pertains to circuits rated 100 amperes (A) or less or marked for 14 AWG through 1 AWG conductors. With this provision, it is not required that the circuit size be rated 100A or less and the conductor be 14 AWG through 1 AWG. If the circuit size is 100A or less or the conductor is 14 AWG through 1 AWG, 110.14(C)(1)(a) is applicable. With motors, the branch-circuit short-circuit and ground-fault protective device could be rated more than 100A, and the conductor size could be smaller than 1 AWG. Section 110.14(C)(1)(a) contains four stipulations. The first three stipulations pertain to all loads, and the fourth stipulation pertains to various motors only.


For motors marked with design letters B, C or D, conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity. As long as the conductor selected has a temperature rating of 75°C or 90°C, the ampacity is not limited to the 60°C (140°F) column. Even if the conductor selected is in the 90°C column, the ampacity of that conductor shall not exceed the ampacity listed in the 75°C column.


For example, what is the minimum size THHN copper conductor required to supply the following motor? The motor is a 20-horsepower (hp), three-phase motor that will be supplied by three-phase, 230 volts (V). The nameplate on this motor shows that it will draw 49.6A when supplied by 230V. The motor has a duty rating of continuous. The nameplate also shows this is a design B motor. 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. The branch-circuit short-circuit and ground-fault protective device will be a 125A inverse-time breaker. Separate overload devices will be installed, and they will be sized in accordance with 430.32(A)(1). 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).


Start by selecting the FLC from Table 430.250. The FLC of a 20-hp, three-phase, 230V motor is 54A. Since this motor is rated continuous-duty, multiply the FLC rating of 54A by 125 percent. The minimum conductor ampacity for this 20-hp motor is 68A (54 × 125% = 67.5 = 68).


With this motor installation, the circuit size will be more than 100A, but the conductor will not be larger than 1 AWG. Therefore, 110.14(C)(1)(a) is applicable. Because this motor is marked with a design letter B, conductors having an insulation rating of 75°C or higher can be installed. A NEMA design B motor is the most common type of motor for industrial applications. Since the minimum ampacity required is 68A and the conductor will be a 90°C conductor, it would seem that the minimum size conductor would be a 6 AWG THHN conductor. While a 6 AWG conductor in the 90°C column of Table 310.15(B)(16) is good for 75A, installing this size conductor would be a violation. In accordance with 110.14(C)(1)(a)(4), the ampacity of the conductor shall not exceed the ampacity listed in the 75°C column. A 6 AWG conductor in the 75°C column is only good for 65A. Because of the ambient temperature and the number of current-carrying conductors, there is no need to adjust or correct the conductor ampacity. Select a conductor from the 75°C column of Table 310.15(B)(16) that has an allowable ampacity of at least 68A. The minimum size THHN conductor required to supply this motor is 4 AWG (see Figure 1).


Some of the rules pertaining to conductors supplying motor loads are applicable to all loads. If the ambient temperature is higher than 26–30°C and/or the number of current-carrying conductors is more than three, it will be necessary to apply adjustment and/or correction factors to the conductor.


For example, what is the minimum size THHN copper conductor required to supply the motor in Figure 1 if everything is the same except the ambient temperature? The ambient temperature for the motor in Figure 1 will now be 48°C. It had already been determined that the minimum ampacity required for the motor in Figure 1 was 68A. Therefore, the conductor must be good for at least 68A after the ambient temperature correction factor has been applied. As previously mentioned, the ampacity of the conductor shall not exceed the ampacity listed in the 75°C column of Table 310.15(B)(16). But, in accordance with the last sentence of 110.14(C), conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both. This means that instead of using the ampacity in the 75°C column, it is permissible to use the ampacity in the 90°C column because the conductor is a 90°C conductor. The Table 310.15(B)(2)(a) correction factor, in the 90°C column, for an ambient temperature of 48°C is 0.82. Since the minimum size THHN conductor required to supply the motor from Figure 1 was 4 AWG, start with a 4 AWG conductor to see if would be good for at least 68A after applying the ambient-temperature correction factor. The allowable ampacity for a 4 AWG THHN conductor is 95A. The maximum ampacity after applying the correction factor is 78A (95 × 0.82 = 77.9 = 78). A 4 AWG THHN conductor is good for more than 68A even after applying the correction factor of 0.82. Also, 68A does not exceed the ampacity listed in the 75°C column. Therefore, the minimum size THHN conductor required to supply the 20-hp motor in Figure 1 in an ambient temperature of 48°C is 4 AWG (see Figure 2).


While the ampacity of continuous loads and most motor loads is multiplied by 125 percent, the calculation procedures are not the same. When sizing conductors for most loads, the conductors are sized to carry the larger of 210.19(A)(1)(a) or (b). Section 210.19(A)(1)(a) pertains to continuous loads at 125 percent and 210.19(A)(1)(b) pertains to adjustment and correction factors. With motors, it is not a matter of comparing the conductor sizes after performing two different calculations and then selecting the largest conductor. If a motor is used in a continuous-duty application, the conductor must carry 125 percent of the motor’s FLC after correction and/or adjustment factors have been applied.


For example, what is the minimum size THHN copper conductor required to supply the 20-hp motor in Figure 1 if everything is the same except the ambient temperature and the number of current-carrying conductors? The ambient temperature for the motor will be 45°C. With this installation, there will be six current-carrying conductors in the raceway. The minimum conductor ampacity for this motor is 68A (54 × 125% = 67.5 = 68). The conductor must be good for at least 68A after the correction and adjustment factors have been applied. The Table 310.15(B)(2)(a) correction factor in the 90°C column for an ambient temperature of 45°C is 0.87. The Table 310.15(B)(3)(a) adjustment factor for six current-carrying conductors is 80 percent or 0.80. Since the minimum size THHN conductor required to supply the motor from Figure 1 was 4 AWG, start with a 4 AWG conductor to see if would be good for at least 68A after applying the correction and adjustment factors. The allowable ampacity for a 4 AWG THHN conductor is 95A. The maximum ampacity after applying the adjustment factor is only 66A (95 × 0.87 × 0.80 = 66.12 = 66). Since the conductor must be good for at least 68A after the correction and adjustment factors have been applied, a 4 AWG conductor is not the correct size. The next larger size conductor is 3 AWG with an allowable ampacity of 115A. The maximum ampacity after applying the adjustment factor is 80A (115 × 0.87 × 0.80 = 80.04 = 80). The minimum size THHN conductor required to supply the 20-hp motor in this example is 3 AWG (see Figure 3). Note that the minimum conductor ampacity could have been determined by dividing 68A by the adjustment and correction factors.


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.

featured Video

;

Vive Pico Wireless Remote

The Pico wireless remote is easy to install, it can be wall-mounted or mounted to any surface, and includes a ten-year battery life. See how this wireless wall control makes it simple to add lighting control wherever you need it.

Advertisement

Related Articles

Advertisement