Article 110 in the National Electrical Code (NEC) 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]. Article 110 also covers voltages; conductors; wiring integrity; interrupting rating; selective coordination; mechanical execution of work; mounting and cooling of electrical equipment; electrical connections; arc flash hazard warning markings, which includes the new section for field-applied hazard markings; various requirements pertaining to disconnecting means; spacing around electrical equipment; and enclosure types. Article 110 contains installation requirements for systems at a rating threshold of 600 volts (V), nominal. In the 2014 edition of the NEC, the voltage threshold was changed from 600V to 1,000V in several hundred places throughout the Code. However, the voltage threshold of 600V did not change in Article 110.


Article 110 does not mention conductor ampacity very much, but when it does, it is significant. As 110.14(C) stipulates, the temperature rating associated with a conductor’s ampacity shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor or device. The main idea of this section is that a conductor’s temperature rating shall not exceed the lowest temperature rating for the connected equipment. As I stated last month, a conductor has at least two ends or terminations, and each termination has a temperature rating. Regardless of the conductor’s actual temperature rating, the lowest termination temperature rating along the conductor’s path determines the maximum ampacity. For example, a 90°C conductor will be installed on a circuit rated less than 100 amperes (A). The termination on one end has a temperature rating of 75°C, and the termination on the other end has a temperature rating of 60°C. Although this conductor has a temperature rating of 90°C, the temperature rating shall not exceed the lowest termmination temperature rating. Because one termination is only rated 60°C, the conductor’s ampacity shall not exceed the ampacity listed in the 60°C column of Table 310.15(B)(16) (see Figure 1).


Section 110.14(C) is divided into provisions for equipment and for separate connectors. Terminations for equipment are divided into two groups. The first group covers circuits rated 100A or less or marked for 14 AWG through 1 AWG conductors. The second group covers circuits rated over 100A or marked for conductors larger than 1 AWG. Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.15(B)(16) as appropriately modified by 310.15(B)(7). 


The first group contains four stipulations. When determining the ampacity of a 14 AWG through 1 AWG conductor or for conductors supplying circuits rated 100A or less, it is required to comply with one of the four stipulations. 


The first stipulation in 110.14(C)(1)(a) is to use a conductor rated 60°C (140°F). The maximum ampacity for a 60°C conductor is listed in the 60°C column of Table 310.15(B)(16). The first provision is very limited because the only conductors with a 60°C temperature rating are Types TW and UF. Type MTW (machine tool wiring) in a wet location also has a 60°C rating. The majority of conductor types are rated either 75°C or 90°C.


The second stipulation states that it is permissible to use a conductor with a higher temperature rating as long as the ampacity does not exceed the ampacity shown in the 60°C (140°F) column. If any termination is either 60°C or unknown, the conductor’s maximum ampacity is the ampacity listed in the 60°C column. 


For example, what is the maximum ampacity for a 4 AWG THHN conductor under the following conditions? One end of this conductor will have a 60°C termination, and the other end will have a 75°C termination. There will be a total of three current-carrying conductors and an equipment grounding conductor in this raceway. The maximum ambient temperature will be 30°C. Because of the 60°C terminations, the conductor’s ampacity shall not exceed the ampacity shown in the 60°C column. In accordance with Table 310.15(B)(16), the maximum ampacity for the 4 AWG THHN conductor in this example is 70A (see Figure 2).


The third stipulation in 110.14(C)(1)(a) states that conductors with higher temperature ratings can be installed if the equipment is listed and identified for use with such conductors. This means the ampacity of a 75°C conductor can be based on the 75°C column if all of the terminations are rated 75°C. 


For example, what is the maximum ampacity for an 8 AWG XHHW 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 other end will have a 60/75°C termination. A temperature rating of 60/75°C means the equipment has been listed for both 60°C and 75°C conductors; therefore, it is permissible to use the 75°C rating if the installed conductor is rated at least 75°C. 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.) In accordance with Table 310.15(B)(16), the maximum ampacity for the 8 AWG XHHW conductor in this example is 50A (see Figure 3).


A 90°C conductor can also be installed, but, if all of the terminations are rated 75°C, 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 3 AWG THHN conductor under the following conditions? The terminations on both ends of the conductor will have a temperature rating of 75°C. 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. Although this conductor has a temperature rating of 90°C, the 75°C terminations will limit the conductor’s ampacity to the ampacity listed in the 75°C column of Table 310.15(B)(16). The maximum ampacity for the 3 AWG THHN conductor in this example is 100A (see Figure 4).


The third stipulation in 110.14(C)(1)(a) also means the ampacity of a 90°C conductor can be based on the 90°C column if all of the terminations are rated at least 90°C. Be careful about using the 90°C column because no equipment is listed and identified for use with 90°C conductors other than individual lugs, terminal bars and equipment listed for use on circuits over 600V.


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