The first National Electrical Code (NEC) was developed in 1897. The National Fire Protection Association (NFPA) became the developer and publisher of the National Electrical Code (NEC) in 1911, and the NFPA continues to develop and publish the Code today. Every page in the NEC is numbered at the bottom, preceded by the number 70 and a dash (70-). For example, Article 110 in the 2014 edition (except for the handbook) starts on page 70-36. The NEC is the NFPA’s document number or code number 70; therefore, the NEC is also known as NFPA 70.
The NFPA develops, publishes and disseminates more than three hundred consensus codes and standards. The NFPA’s Standard for Electrical Safety in the Workplace is document number 70E. Similar to the NEC, all of the page numbers are preceded by the number 70, a capital E and a hyphen (70E-).
In accordance with the NFPA, the NEC is the most widely used code for the built environment in the world, establishing the standard for safe electrical installations. Nearly every state in the United States, many territories and several countries have adopted the NEC. Moreover, in the countries that have not adopted the NEC, it is not unusual to find the Code being used in plants, factories, buildings, campuses, etc. A number of companies that have locations in the United States have elected to comply with requirements in the Code for all of their locations, even outside of the United States.
As more companies comply with the NEC, the installations will be safer. Some of the provisions considered essential to a safe installation pertain to selecting and installing the correct size conductors. One of the first things to consider when sizing conductors is the temperature ratings of the equipment at the conductor terminations. 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 [110.14(C)]. It is important to know the lowest temperature rating of the equipment because that will help determine the conductor’s maximum ampacity, shown in Table 310.15(B)(16). For many editions, Table 310.15(B)(16) was numbered Table 310.16.
In the electrical trade, it is vital to have a good understanding of this table because it is one of the most referenced in the NEC. Table 310.15(B)(16) provides maximum ampacities of insulated copper and aluminum conductors rated up to and including 2,000 volts (V). Ampacities for copper-clad aluminum conductors are in the same columns as aluminum conductors. Conductor sizes range from 18 AWG to 2,000 kcmil. Each size copper conductor has three ampacities listed for conductor sizes 14 AWG and larger. Each aluminum or copper-clad aluminum conductor has three ampacities listed for conductor sizes 12 AWG and larger. The different ampacities are based on the conductor’s temperature rating. The temperature ratings are 60°C (140°F), 75°C (167°F), and 90°C (194°F). Besides the temperature ratings, the ampacities are also based on not having more than three current-carrying conductors in a raceway, cable or earth (directly buried) and an ambient temperature of 30°C (86°F) (see Figure 1).
For each size copper conductor in Table 310.15(B)(16), three ampacities are listed. The same is true for aluminum and copper-clad aluminum conductors. The different ampacities are listed because of the different temperature ratings of insulated conductors. The conductor’s insulation and temperature rating determines the ampacity listed in the table. For example, a 2 AWG TW copper conductor has an allowable ampacity of 95 amperes (A) because Type TW is in the 60°C column. A 2 AWG THWN copper conductor has an allowable ampacity of 115A because Type THWN is in the 75°C column. A 2 AWG THHN copper conductor has an allowable ampacity of 130A because Type THHN is in the 90°C column (see Figure 2).
Two of the insulations are listed in two different columns. Types THHW and XHHW are listed in both the 75°C column and the 90°C column. To select the ampacity out of the correct column, look at the conductor application information in Table 310.104(A). The temperature rating for both THHW and XHHW (not XHHW-2) conductors depend on the location where the conductor is installed. With both conductors, if the installed location is a wet location, the maximum operating temperature (or temperature rating) is 75°C (167°F). With a Type THHW conductor, if the installed location is dry, the maximum operating temperature is 90°C (194°F). With a Type XHHW conductor, if the installed location is dry or damp, the maximum operating temperature is 90°C (194°F).
Besides the insulation type letters and the maximum operating temperature, other helpful information can also be found in Table 310.104(A), such as the trade name, application provisions, insulation, insulation thickness, and outer covering (if any) (see Figure 3). Conductor insulation information for conductors rated 2,000V and higher are in tables 310.104(B) through (E).
Just because a conductor is a 90°C conductor does not mean the overcurrent protective device can be based on the ampacity shown in the 90°C column of Table 310.15(B)(16). A conductor with Type THHN insulation is a common conductor used throughout the electrical industry. This conductor has a maximum operating temperature rating of 90°C, and, although it is in the 90°C column, it may not be permissible to use the ampacity shown in that column. One factor that determines a conductor’s ampacity is the temperature rating of the termination (or connection) points. As previously mentioned, the temperature rating associated with a conductor’s ampacity shall be selected and coordinated so the lowest temperature rating of any connected termination, conductor or device is not exceeded.
Conductor temperature limitations can be compared to the strength of a chain. A chain is only as strong as its weakest link. If every link on a chain (except one) has a weight capacity of 1,000 pounds and the one link has a weight capacity of 100 pounds, the maximum weight capacity for this chain is only 100 pounds. Conductor terminations and the conductor itself can be thought of as links to a chain, so it is important to find the weakest link. A conductor has at least two ends or terminations, and each has a temperature rating. The conductor also has a temperature rating. Therefore, with the installation of every conductor, there are at least three temperature ratings. The conductor’s maximum ampacity shall not exceed the ampacity listed in the column with the temperature rating of the lowest termination. For example, a 90°C conductor will be installed. The termination on one end of this conductor has a temperature rating of 60°C. The termination on the other end of the conductor has a temperature rating of 75°C. The temperature rating of the weakest link, or lowest rated termination, in this example is the 60°C termination. Even though it is a 90°C conductor, the maximum ampacity is not the ampacity shown in the 90°C column. In this example, the maximum ampacity is not even in the 75°C column. Because of the 60°C termination, the conductor’s ampacity shall not exceed the ampacity shown in the 60°C column (see Figure 4).
Next month’s column continues the discussion of conductor temperature limitations.
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.