Widen Pipe, or Reduce the Juice

Last month, I wrote about Informational Annex I in the 2011 National Electrical Code (NEC), which provides information on 110.14 for electrical connections and the requirements for proper torquing of electrical connections within and to electrical equipment.

Section 110.14 provides requirements for conductor terminals in 110.14(A), conductor splicing in 110.14(B), and, in 110.14(C), temperature limitations and ratings associated with the ampacity of a conductor relating to the determination of termination provisions for electrical equipment in 110.14(C)(1), and separately installed pressure connectors in 110.14(C)(2). The temperature rating of the connected conductors often restrict the sizing of conductors, specific equipment marking and any listing information, so take care to follow the general requirements in 110.14(C), in addition to any permissive requirements elsewhere in the NEC. Wiring methods in Chapter 3 may permit conductor ampacity values that are higher than the terminals, and the terminations are rated according to 110.14(C) based on the listing of the electrical equipment, inadvertently resulting in a violation of the NEC.

Section 110.14(C) in the 2011 NEC states that 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. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.”

In other words, the installer or designer must consider the lowest temperature rating of the termination at both the load and the supply end as well as meet the requirements in 310.15(A)(3) concerning the operating temperature of the conductor and its insulation. The temperature rating of the conductor is considered to be the maximum temperature at any location along its length that the conductor can withstand over a prolonged period of time without serious degradation of the insulation. The operating temperature of a conductor is primarily determined by the ambient temperature along the conductor length, heat generated internally in the conductor by current flow, the rate that any generated heat within the conductor or at the conductor termination can dissipate, and the heat generated by any adjacent current-carrying conductors that may heat sink into the conductor.

Based on 110.14(C)(1)(a), electrical equipment terminations, at the point of connection to the circuit, are based on 60°C if the equipment is rated for circuits of 100 amperes (A) or less or are marked for conductor sizes of No. 14 AWG through No. 1 AWG. Where the electrical equipment exceeds 100A or is marked for conductors larger than No. 1 AWG, the terminations are based on 75°C. These basic termination rules apply with some minor modifications in 110.14(C)(1)(a)(1) through (4) and 110.14(C)(1)(b)(1) and (2). The qualification for these termination requirements, however, are based on the last sentence in 110.14(C)(1), which states, “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).” This last sentence is often overlooked where dealing with conductor ampacities in accordance with the requirements in some articles within Chapter 3.

For example, 392.80(A)(2), covering the ampacity of single-conductor cables, permits the ampacities for 600 kcmil and larger single-conductor cables in uncovered cable trays to have an ampacity not exceeding 75 percent of the allowable ampacities in Table 310.15(B)(17) and Table 310.15(B)(19), where these cables conform to the installation requirements of 392.22(B) for cable tray fill for single-conductor cables. Table 310.15(B)(17) provides the allowable ampacity for single insulated conductors installed in free air. In the 75°C column in the table, a 600 kcmil single conductor cable has a rated ampacity of 690A. Using the 75 percent allowable ampacity permitted for an open conductor in an uncovered cable tray would provide an ampacity for the cable of 517.5A. Based on the last sentence in 110.14(C)(1), as I described above, the allowable ampacity for the maximum equipment termination in the 75°C column in Table 310.15(B)(16) is 420A. The difference between the allowable ampacity of 517.5A for the cable tray calculation and the maximum permitted termination ampacity of 420A is 97.5A. In addition, the second sentence in 392.80(A)(2) also states that the adjustment factors, as required in 310.15(B)(3)(a), for the number of current-carrying conductors in a raceway or cable, does not apply.

Since the terminal and the busbar termination in the equipment based on 110.14(C)(1)(b) are not rated for the higher ampacity levels permitted for a single conductor cable installation in a cable tray, the cable tray installation must use the lower ampacity level, thus requiring a larger cable or a lower ampacity level to be in compliance with the NEC.

ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and mark.c.ode@us.ul.com.

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