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General Installation Requirements, Part IX

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

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Article 110 in the National Electrical Code (NEC), Requirements for Electrical Installations, covers general requirements for the examination and approval, installation and use, access to and spaces about electrical conductors and equipment; enclosures intended for personnel entry; and tunnel installations [110.1]. While the scope mentions several topics covered in the article, it does not come close to mentioning the vast number of topics in Article 110. 


There are 26 main sections in the first two parts Article 110, a number of which contain subsections. Part I contains general provisions, and Part II contains provisions for electrical systems rated 600 volts (V), nominal, or less. 


Section 110.26, Spaces About Electrical Equipment, has six subsections (A through F), and half of those are divided into even more subsections. Although 110.14 contains only three subsections, it is no less important than 110.26. The three subsections in 110.14 are (A) Terminals, (B) Splices and (C) Temperature Limitations. The provisions in 110.14(C) are essential when sizing conductors and determining maximum ampere ratings for overcurrent protective devices. 


As stated in 110.14(C), the temperature rating of a conductor shall not exceed the lowest temperature rating of any connected termination, conductor or device. Section 110.14(C) is divided into provisions for equipment and provisions for separate connectors. The determination of equipment-termination provisions shall be based on 110.14(C)(1)(a) or 
(C)(1)(b). Section 110.14(C)(1)(a) covers circuits rated 100 amperes (A) or less or marked for conductors 14 AWG through 1 AWG. Section 110.14(C)(1)(b) covers circuits rated over 100A or marked for conductors larger than 1 AWG.


Last month’s column covered the first three of four provisions for circuits rated 100A or less or marked for conductors 14 AWG through 1 AWG. The discussion continues this month.


The fourth provision in 110.14(C)(1)(a) pertains to motors marked with design letters B, C or D. Conductors with an insulation rating of 75°C (167°F) or higher can be used to supply these motors as long as the ampacity of the conductors does not exceed the 75°C (167°F) ampacity. In Table 310.15(B)(16), the conductor temperature ratings are 60°C (140°F), 75°C (167°F) and 90°C (194°F). If a 75°C conductor is installed to supply power to a motor marked with a design letter B, C or D, the conductor’s ampacity can be based on the ampacity shown in the 75°C column. This section states conductors with an insulation rating of 90°C can also be installed, but the ampacity shall not exceed the ampacity in the 75°C column. 


For example, conductors are needed to supply power to a motor marked with a design letter B. The terminations on the opposite end from the motor will have a temperature rating of 75°C. The motor will be a 20-horsepower (hp) motor, and the voltage supplying the motor will be 208V, three-phase. The nameplate on the motor shows it will draw 51.6A when connected to 208V, three-phase. What is the minimum size THHN conductors required to supply power to this motor? 


In accordance with 430.22, the ampacity shall not be less than 125 percent of the motor full-load current (FLC) rating as determined by Table 430.250. In Table 430.250, the FLC of a 20-hp, 208V, three-phase motor is 59.4A. Multiply the motor’s FLC by 125 percent (59.4 × 125% = 74.25 = 74). The minimum ampacity for the conductors supplying power to this motor is 74A (see Figure 1).


Since the minimum ampacity is known for the motor in Figure 1, select the minimum size THHN conductors. The allowable ampacity of a 6 AWG THHN conductor (in the 90°C column) is 75A. Although conductors with a temperature rating of 90°C can be installed for motors marked with design letters B, C or D, the ampacity shall not exceed what is listed in the 75°C column. The ampacity of a 6 AWG conductor in the 75°C column is 65A. Because this motor requires a minimum ampacity of 74A, installing 6 AWG conductors is not permitted. The allowable ampacity of a 4 AWG conductor in the 75°C column is 85A. Therefore, conductors supplying power to the motor in Figure 1 shall be at least 4 AWG (see Figure 2).


Temperature-limitations provisions for equipment are divided into two groups. As previously discussed, the first group pertains to equipment for circuits rated 100A or less or marked for conductors 14 AWG through 1 AWG. The second group pertains to equipment for circuits rated over 100A or marked for conductors larger than 1 AWG. There are only two provisions for the second group, and conductor selection must be based on one of the two provisions. The first stipulation states to use a conductor rated 75°C (167°F). The maximum ampacity for a 75°C conductor is listed in the 75°C column of Table 310.15(B)(16). This first provision is somewhat limited because there are only seven conductors with a 75°C temperature rating. For example, what is the maximum ampacity for a 2/0 AWG THHW conductor under the following conditions? 


There will be a total of three current-carrying conductors and an equipment grounding conductor in this raceway. The raceway will be installed in a wet location. The maximum ambient temperature will be 30°C. One end of this conductor will have a 75°C termination, and the termination on the other end is unknown. Because this conductor will be installed in a wet location, the conductor’s temperature rating is 75°C. See Table 310.104(A) for conductor applications. Even though one termination’s temperature rating is unknown, selecting the ampacity from the 75°C column is permitted because the conductor is larger than 1 AWG. In accordance with Table 310.15(B)(16), the maximum ampacity for the 2/0 AWG THHW conductor in this example is 175A (see Figure 3).


In accordance with the second provision in 110.14(C)(1)(b), a 90°C conductor can be installed, but if any of the terminations are rated 75°C or less, the conductor’s maximum ampacity shall not exceed the ampacity listed in the 75°C column of Table 310.15(B)(16). For example, what is the maximum ampacity for a 2 AWG THHN copper conductor fed from a 125A circuit breaker? 


There will be a total of three current-carrying conductors and an equipment grounding conductor in this raceway. The raceway will be installed in a dry location. The maximum ambient temperature will be 30°C. Both ends of this conductor will have a 75°C termination. To comply with 110.14(C)(1)(b)(2), do not exceed the 75°C ampacity for this conductor. Although this conductor is smaller than 1 AWG, the overcurrent device is rated more than 100A; therefore, using the ampacity in the 75°C column is permitted. The maximum ampacity for the 2 AWG THHN conductor in this example is 115A (see Figure 4).


When it is necessary to apply adjustment and/or correction factors to the conductor’s maximum ampacity, the last sentence of 110.14(C) is very useful. Adjustment factors apply when there are more than three current-carrying conductors in a raceway or cable. Correction factors apply when the ambient temperature is something other than 30°C. Note that applying adjustment and correction factors is sometimes referred to as “derating.” Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both [110.14(C)]. If a 90°C conductor is installed, it is permissible to use the ampacity in the 90°C column as the starting point for conductor derating as long as the final ampacity complies with 110.14(C). 


Next month’s column continues the discussion of conductor temperature limitations.


Correction: In Figure 2 of the August column, General Installation Requirements, Part VII, the copper conductors should be labeled TW (60°C), THW (75°C) and THHN (90°C). We regret the confusion caused by this error.

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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