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Ampacities of Conductors in Cable Trays
(1) In ventilated and ladder-type cable trays, where the air space between conductors, cables, or both is maintained at greater than 100% of the largest conductor or cable diameter, the ampacity of the conductors or cables shall be the value specified in Paragraph (a) or (b):
(a) Single conductors, single-conductor metal-sheathed or armoured cable, and single-conductor mineral-insulated cable, as specified in Tables 1 and 3; and
(b) Multi-conductor cables as specified in Tables 2 and 4, multiplied by the correction factor in Table 5C for the number of conductors in each cable.
Intent for Rule 12-2212(1). When ventilated and ladder-type cable trays are used and when the spacing between single conductors and cables is maintained at greater than 100% of the largest conductor or cable diameter, we intend to allow Tables 1 and 3 ampacities to be used. No further derating is required, except for ambient temperature adjustment in compliance with Table 5A. If multi-conductor cables are used, the correction factor in Table 5C for the number of conductors in each cable shall be considered.
In ventilated and ladder-type cable trays, where the air space between conductors, cables, or both is less than 25%, and for any spacing in a non-ventilated cable tray, the ampacity of the conductors or cables shall be the value as specified in Table 2 or 4 multiplied by the correction factor specified in Table 5C for the total number of conductors in the cable trays.
Intent for Rule 12-2212(3).
When ventilated or ladder-type trays are used and the air space is less than 25% of the largest diameter cable, and for any spacing in a non-ventilated-type tray, we intend that the ampacity be the value specified in Table 2 or 4 multiplied by the correction factor of Table 5C for the total number of conductors in the tray.
Assume six No. 2/0 AWG copper, RW90 insulation, 4-conductor cables with random spacing. Assume a 40°C ambient.
Ampacity = 185 A (Table 2) × 0.70 (Table 5C) × 0.90 (Table 5A) = 117 A
In determining the total number of conductors in the cable tray in Subrule (3), Rule 4-004(7) shall apply.
Intent for Rule 12-2212(4).
We intend that the correction factors of Table 5C be applied for the number of conductors installed in a tray when spaced as described in Subrule (3). When computing the total number of conductors, consideration should be given to harmonic currents that are drawn by lamp ballasts and data processing equipment on 3-phase systems having a common neutral. These harmonics may result in a current in the neutral which can be as large as the phase current, even when the loads are perfectly balanced. Therefore, the neutral conductor of a 3–phase, 4-wire circuit supplying such loads should not be ignored because of Rule 4-004(3). In such cases, all four conductors should be counted in calculating the derating factor required by Table 5C. We do not, however, count bonding control or communication conductors in the determination of this derating factor.
Rule 4-004 Ampacity of Wires and Cables (see Appendix B)
(1) The maximum current which a copper conductor of a given size and insulation may carry shall be as follows:
(a) Single conductor, and single-conductor metal sheathed or armoured cable, in a free air run, as specified in Table 1; and
(b) 1, 2, or 3 conductors in a run of raceway, or 2- or 3-conductor cable, except as indicated in Subrule (1)(d), as specified in Table 2; and
(c) 4 or more conductors in a run of raceway or cable, as specified in Table 2 with the correction factors applied as specified in Table 5C; and
(d) Single conductor and 2-, 3-, and 4-conductor cables and single and 2-, 3-, and 4-conductor metal armoured and metal sheathed cables, in conductor sizes 1/0 AWG and larger, in an underground run, as calculated by the method of the IEEE Standard, Standard Power Cable Ampacity Tables, IEEE 835.
Electrical Safety Authority