Last month’s article covered the basics of cable tray installation requirements, so this month, I will provide specific information on how to determine the ampacity of cables rated at 2,000V or less installed in cable trays.

Electrical designers, electrical contractors and electricians must understand where cable trays are permitted to be installed and what wiring methods are allowed within cable trays. These applications are often misapplied. Once determining that the cable tray installation complies with 392.10 for uses permitted, the next step is to calculate the permissible ampacity of conductors and cables in the tray and make decisions on the termination of the cables and conductors.

Conductor ampacity

Ampacity of conductors in cable trays is covered in 392.80(A) applying to cables rated at 2,000V or less. The first application in 392.80(A)(1) applies to the ampacity of multiconductor cables, which are required to be installed in accordance with the number of multiconductor cables in 392.22(A). 

Once the appropriate number of cables are installed in the cable tray, the ampacity of the multiconductor cables is required to comply either with Table 310.16 [Ampacities of Insulated Conductors with Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried) at an ambient temperature of 30°C (86°F)] or ampacity Table 310.18 [Ampacities of Insulated Conductors with Not More Than Three Current-Carrying Conductors in Raceway or Cable at an ambient temperature of 40°C (104°F)]. These ampacity tables apply to three current-carrying conductors and may require ambient temperature correction factors to be applied.

Section 392.80(A)(1)(a) states that “the adjustment factors of 310.15(C)(1) apply only to multiconductor cables with more than three current-carrying conductors. Adjustment factors are limited to the number of current-carrying conductors in each cable and not to the number of conductors in the cable tray.” 

The reason for this application of Table 310.15(C)(1) for only the number of conductors within each cable assembly is the appropriate separation of the cables within the cable tray so heat buildup between each cable assembly is not an issue.

Section 392.80(A)(1)(b) states, “where cable trays are continuously covered for more than 6 feet with solid unventilated covers, not over 95 percent of the ampacities of Table 310.16 and Table 310.18 are permitted for multiconductor cables.” 

Section 392.80(A)(1)(c) states that “where multiconductor cables are installed in a single layer in uncovered trays, with a maintained spacing of not less than one cable diameter between cables, the ampacity must not exceed the ambient temperature-­corrected ampacities of multiconductor cables, with not more than three insulated conductors rated 0 through 2,000 volts in free air, in accordance with 310.14(B).”

Remember that 310.14(B) applies to Equation 310.14(B), which can only be applied under engineering supervision. The reason for this application of Table 310.15(C)(1) for only the number of conductors within each cable assembly is the appropriate separation of the cables within the cable tray so heat build-up between each cable assembly is not an issue.

Single-conductor cables

Section 392.80(A)(2) applies to single-­conductor cables. The basic requirement in (A)(2) for single-conductor cables states, “The ampacity of single-conductor cables shall be as permitted by 310.14(A)(2).” Section 310.14(A)(2) applies “where more than one ampacity applies for a given circuit length, the lowest value shall be used.” 

The adjustment factors of 310.15(C)(1), for the number of current-carrying conductors shall not apply to the ampacity of cables in cable trays. The reason for this application of Table 310.15(C)(1) for only the number of conductors within each single cable in a triplex (three conductors) configuration or installed as a quadruplex (four conductors) configuration is the appropriate separation of each group of cables in the tray so heat buildup between each cable grouping is not an issue.

The ampacity of single-conductor cables, or single conductors cabled together nominally rated 2,000V or less, must comply with 392.80(A)(2)(a) through (A)(2)(d). Based on the size of the individual single conductors, these subsections permit an ampacity of 65% of the ampacity provided in Table 310.17 for 1/0 AWG through 500 kcmil single conductors; an ampacity of 75% of the ampacity provided in Table 310.17 for conductors of 600 kcmil and larger; and where single conductors are installed in a single layer in uncovered cable trays, with a maintained space of not less than one cable diameter between individual conductors, the ampacity of 1/0 AWG and larger cables shall not exceed the ampacities in Table 310.17.

Since there are major requirements for conductors used in cable tray systems, ensure terminations are in compliance with 110.14(C)(1). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16. Table 310.17 only applies to the cables and conductors in the cable tray.

Holly Sauer