110-14(c) Temperature Limitations

Last month's In Focus introduced the topic of temperature limitation requirements. The groundwork for a better understanding began with a preliminary discussion of several relevant subjects, which included: conductor ampacity and ambient temperature correction factors [Table 310-16], adjustment factors wherever more than three current-carrying conductors are used [Table 310-15(b)(2)(a), and conductor overcurrent protection [240-3]. Proper sizing of conductors and overcurrent protection depends upon the application of the requirements outlined in all of these sections.

All too often, Section 110-14(c) is overlooked when choosing a conductor. The temperature rating associated with a conductor's ampacity must be selected and coordinated so that the lowest temperature rating of any connected termination, conductor, or device is not exceeded. [110-14(c)] Conductor temperature limitations can be compared to the strength of a chain. A chain is only as strong as its weakest link.

For conductors, every termination (or connection) point is a potential weak link. The fact that a conductor's insulation has a 90 degrees C temperature rating does not mean that the ampacity is automatically selected from the 90 degrees C column. The lowest temperature rating of the termination points, along the conductor's path, determines the maximum ampacity. Temperature limitations fall into two categories: (1) circuits rated 100 amperes or less, and (2) circuits rated over 100 amperes.

110-14(c)(1) Circuits Rated 100 Amperes or Less

Equipment termination provisions for circuits rated 100 amperes or less, or marked for Nos. 14 through 1 conductors, must comply with Section 110-14(c)(1)(a), (b), (c), or (d). Equipment, as defined in Article 100, is a generic term that includes material, fittings, devices, appliances, fixtures, apparatus, etc. Any and all connection points along the conductor's entire length must be considered. The termination point with the lowest temperature rating may be the determining factor for selecting the conductor's ampacity. Generally, circuits with a rating of 100 amperes or less cannot exceed the ampacities listed in the 60 degrees C (140 degrees F) column for the same-size conductor. [110-14(c)(1)(b)] If a termination point's temperature rating is unknown, the conductor's ampacity must be selected from the 60 degrees C column regardless of the insulation type. Likewise, if any connection point has a temperature rating of 60 degrees C, the conductor's ampacity must be selected from the 60 degrees C column.

A higher temperature rating is permitted if the equipment is listed and identified for use with such conductors. [110-14(c)(1)(c)] Where a conductor has a 90 degrees C temperature rating (such as THHN), and the lowest temperature rating of the termination points is 75 degrees C (or 60/75 degrees C), the conductor's ampacity can be selected from the 75 degrees C column.

The ampacity from the 90 degrees C column is appropriate only when all termination points, along that 90 degrees C conductor, have a temperature rating of 90 degrees C.

Example - Three No. 3 THHN copper conductors are installed in a conduit feeding a remote panelboard in a commercial occupancy. If all termination points have a 75 degrees C (or 60/75 degrees C) rating, the maximum-size overcurrent protection for that conductor is 100 amperes.

A 90-ampere overcurrent protection device is the maximum if any termination point has a 60 degrees C rating. Remember that if the conductor's ampacity does not correspond with a standard-size (ampere rating) fuse or circuit breaker, the next-higher size is permitted. [240-3(b)]

110-14(c)(1) Circuits Rated Over 100 Amperes

Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than No. 1, must comply with Section 110-14(c)(2)(a) or (b). Generally, circuits having a rating over 100 amperes cannot exceed the ampacities listed in the 75 degrees C (167 degrees F) column for the same-size conductor. If any termination point has a rating of 75 degrees C, the conductor's ampacity is selected from the75 degrees C column. The 90 degrees C column's ampacity rating is only permitted if all termination points are temperature rated at 90 degrees C (194 degrees F).

Example - what size paralleled THHN aluminum conductors are required for a 500-ampere service in a commercial occupancy? (The termination points are rated 75 degrees C.) The ampacity rating for 250 kcmil aluminum, from Table 310-16's 90 degrees C column, is 230 amperes.

Because the conductors are paralleled, the total ampacity is 460 amperes. (Conductors connected in parallel are electrically joined at both ends, thereby forming a single conductor.) Since this ampacity does not match a standard-size fuse or breaker listed in Section 240-6(a), the next- higher standard size is permitted, which in this case is 500 amperes. Disregarding Section 110-14(c), it would appear that a parallel set of 250 kcmil aluminum conductors could be installed. However, this is not correct. Because the termination points have a temperature rating of 75 degrees C, the ampacity must be selected from the 75 degrees C column. Each 250-kcmil conductor (from that column) has an ampacity of only 205 amperes. The combined ampacity of the parallel set is then 410 amperes. These conductors must have overcurrent protection no greater than 450 amperes.

A parallel set of 300 kcmil aluminum conductors has a combined rating of 460 amperes (230 (2). Therefore, the minimum size paralleled set of THHN aluminum conductors required for a 500-ampere service is 300 kcmil.

Example - what size THHN copper conductors are required to feed a 1,200-ampere switchboard with termination points rated 75 degrees C? (Use three paralleled sets. Three paralleled sets mean three conductors connected to each phase, as well as to the neutral.) The ampacity for each 500-kcmil copper conductor from the 75 degrees C column is 380 amperes. The total combined ampacity for each phase is 1,140 amperes. Where the conductor does not match a standard-size fuse or circuit breaker, the next-higher size is permitted, unless the rating is over 800 amperes. Section 240-3(c) stipulates that where the overcurrent device is rated over 800 amperes, the ampacity of the protected conductors must be equal to, or greater than, the rating of the overcurrent device.

Since the overcurrent device in this example is 1,200 amperes, the minimum conductor ampacity rating is 1,200 amperes. The minimum-size THHN copper conductors required for this example is 600 kcmil.

As previously mentioned, other factors affect the selection process when matching conductors with the proper size overcurrent protection: ambient temperature, the number of conductors within the raceway or cable, and whether the load is continuous or noncontinuous to name a few. When using conductors with temperature ratings higher than those specified for terminations, the higher ampacity can be used for ampacity adjustment, correction, or both. [110-14(c)] This means that if a conductor has a 90 degrees C rating and the termination points are only rated 75 degrees C, the higher ampacity can be used in the derating process, provided the final rating is no greater than the ampacity listed in the 75 degrees C column.

MILLER, owner of Lighthouse Educational Services, teaches custom-tailored classed and conducts seminars covering various aspects of the electrical industry. He is the author of Illustrated Guide to the National Electrical Code. For more information, visit his Web site at www.charlesRmiller.com. He can be reached by phone at (615) 333-3336, or via e-mail at charles@charlesRmiller.com.