The temperature rating associated with the ampacity of a conductor must be selected and coordinated so that the lowest temperature rating of any conductor, equipment termination, or electrical device is not exceeded.
Initially, it seems easy enough to comply with this requirement; however, there may be some confusion in the industry about what ampacities should be used in determining these temperature ratings.
The temperature characteristics of electrical equipment and device terminations depend upon the proper sizing of the electrical conductor and the rating of the connection. The ambient temperature surrounding the termination and any heat generated by the electrical equipment must also be taken into account. If the conductor is sized too small, too much current is allowed to flow in the system, the ambient temperature is too high, or if the termination cannot dissipate the heat generated during operation of the electrical system, then failure could result.
Since the conductor is such an important part of the connection, selecting the proper conductor ampacity table is critical in determining the conductor’s ampacity rating for a particular termination. Electrical equipment rated at 600 volts or less is evaluated for temperature characteristics at conductor termination points using conductor ampacities based on Table 310-16 in the National Electrical Code (NEC).
The general rule in Section 110-14(c)(1) is that termination provisions of equipment for circuits rated 100 amperes or less or marked for Nos. 14 through 1 are based on the use of conductors rated at 60 degrees Celsius (140 degrees Fahrenheit). Conductors with a higher temperature rating can also be used, providing the conductor ampacity is determined based on the 60 degrees Celsius ampacity column of Table 310-16. of the conductor size used.
For example, a No. 8 THWN CU (75 degrees Celsius) conductor could be used as long as the ampacity of the circuit does not exceed 40 amperes. A higher-rated conductor can be used where the terminations are listed and identified at this higher temperature rating for both the supply and load end.
Motors marked with design letters B, C, D, or E and designed to NEMA standards are temperature-evaluated using conductors based on the 75 degrees Celsius ampacity. Section 110-14(c)(1)(d) was added in the 1999 NEC to permit the ampacity of the conductors for these motors to be rated at the 75 degrees Celsius ampacity, even if the motors are rated at less than 100 amperes.
Section 110-14(c)(2) permits termination provisions of equipment for circuits rated over 100 amperes or marked for conductors larger than No. 1, based on the use of the 75 degrees Celsius ampacity. Con-ductors with a higher temperature rating can also be used where the ampacity of the conductor is determined based on the 75 degrees Celsius ampacity. For example, a No. 1/0 THHN CU (90 degrees Celsius) conductor is permitted as long as the circuit’s ampacity does not exceed 150 amperes (75 degrees Celsius ampacity rating). Again, a higher-rated conductor can be used where the terminations are listed and identified at this higher temperature rating at both the supply and load end. These ampacities are based on Table 310-16 for 600-volts-and-less-rated electrical equipment.
Section 110-14(c)(3) permits separately installed pressure connectors to be used at the rating of the listed and identified temperature of the connector and the conductor, provided the terminations at the supply and load ends are not exceeded. For example, if a No. 8 THWN CU (75 degrees Celsius) conductor is spliced using an end-to-end (butt), 75 degrees Celsius-rated crimp, and the supply and load end terminations are both rated at 75 degrees Celsius, then the 75 degrees Celsius rating of 50 amperes is permitted. Again, Table 310-16 is used for this rating where the equipment is rated at 600 volts or less.
Confusion may occur in a situation where the ampacities from Table 310-16 are not used for the conductor ampacity and termination to 600-volts-or-less-rated electrical equipment or devices are necessary. Assume the use of 1/0 THHN, USE-2, or Mineral Insulated (MI) CU cable in a free air application (not bundled or stacked). All of these conductors have a temperature rating of 90 degrees Celsius. If the ampacity of Table 310-17 is used, these 1/0 conductors have an ampacity rating of 260 amperes. However, since these conductors must be terminated at both a load and a supply end to equipment terminations based upon Table 310-16, a 1/0 CU conductor of these same insulation types in Table 310-16 is only rated at 150 amperes, not 260 amperes.
Since the termination at the electrical equipment is based upon the 75 degrees Celsius column in Table 310-16, a 300-kcmil (285-ampere) conductor would be large enough to handle the higher ampacity of the 1/0 in Table 310-17, while still complying with Section 110-14(c)(2) and (3) termination requirements.
If a 75 degrees C butt splice kit is used, then the ampacity of the 1/0 is 230 amps from Table 310-17 and a 4/0 (230 amperes) from Table 310-16 could be used for the termination at the equipment. The text in Section 110-14(c) and in Article 330 for the 2002 NEC should provide this necessary reference to Table 310-16 and help clarify this issue.
ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at firstname.lastname@example.org.