Photo 1 indicates ten each 4/0, 3-phase bundled conductors in parallel with ground, in cable tray, mounted in floor, void of a Mobile 2 MW, 480-volt, 3-phase generator trailer. There is a removable cover above the cable tray compartment. Cable tray is totally enclosed with the exception of removing top cover plate. Cover is to be removed (if someone remembers) during operation of the generator.
Photo 2 indicates ten each 4/0, 3-phase bundled conductors in parallel with one separate 500 MCM ground in cable tray, mounted in floor, void of a Mobile 2 MW, 480-volt, 3-phase generator trailer. There is a removable cover above the cable tray compartment. Cable tray is totally enclosed with the exception of removing top cover plate. Cover is to be removed (if someone remembers) during operation of the generator.
Can the parallel conductor amperage be calculated according to 392.11(B)(4) Table 310.20? I would presume that 392.11(B)(4) pertains to a cable tray installation in open air and not one that is enclosed.
Is each parallel bundled conductor arrangement required to have its own grounding conductor and sized in accordance with Table 250.122? Or arranged as indicated in photo 2 with only one grounding conductor for the 10 each 4/0, 3-phase bundled conductors in parallel per Table 250.122?
Would the bundled paralleled conductors be classified as a cable and have to adhere to Article 250.122(F)(1) as noted in the NEC Handbook commentary?
Your prompt reply would greatly be appreciated. — B.G.
The multiple questions concerning the installation as described by the submitter involve a number of considerations that make this installation very interesting (see photos 1-3). The submitter does not reference which NEC is being used, so we will base our answers upon the 2005 edition.
The first question describes a cable tray mounted in the floor of a generator trailer where the cable tray is totally enclosed with a removable top cover. Instructions on the cover require the cover to be removed during operation. The question is, Would this not be classified as a raceway? To answer, we must turn to Article 100 and look at the definition of a raceway, which is defined as "an enclosed channel of metal or nonmetallic materials designed expressly for holding wires, cables or busbars, with additional functions as permitted in the Code.” Next, we turn to 392.2 that defines a cable tray system as "a unit or an assembly of units or sections and associated fittings forming a structural system used to securely fasten or support cables and raceways.” In short, a cable tray is a support system that may be used to support raceways and cables; however, it is not itself a raceway.
This raises the question, Is this wiring method as described by the submitter a cable tray, and does it comply with the installation requirements for cable tray? Section 392.6(H) requires that a cable tray "be exposed and accessible except as permitted by 392.6(G).” Article 100 defines exposed (as applied to wiring methods) as "on or attached to the surface or behind panels designed to allow access.” As the referenced cable tray is installed in the floor cavity, compliance with this requirement seems questionable. Cable trays are permitted only to pass vertically through floors as per 392.6(G). The cable tray referenced by the submitter apparently travels horizontally in the floor cavity, which does not appear to meet the requirement of 392.6(G).
Where single paralleled conductors are tripled or quadrupled together into circuit groups, they may be installed only in uncovered cable trays as per 392.11(B)(4). Removal of a cover during operation does not change the type of cable tray from the original design type. One might also show concern that the conductors contained in the fully enclosed cable tray installed in the floor cavity may be exposed to physical damage if the cover is removed during operation of the generator. The enclosure as described seems to be defined more accurately by 390.3(C) as a trench-type raceway also known as trench duct, which is indeed a raceway.
The second question was, Can the parallel conductor amperage be calculated according to 392.11(B)(4), Table 310.20? The information provided in the question leads me to conclude the answer is no, based on the premise that we are actually dealing with trench duct referenced in 390.3(C). We, therefore, may not use Article 392 or any sections therein for this application. The ampacity of the conductors in a trench-type raceway would be based on Table 310.16. Furthermore, 390.17 reminds us that the ampacity adjustment factors, in 310.15(B)(2) apply to conductors installed in underfloor raceways. Accordingly, the maximum ampacity of each of the ten 4/0 conductors per phase (10 sets) is 117 amperes per conductor based on 110.14(1)(b), using 90ºC conductors, assuming that the neutral carries only the unbalanced load, and applying the ampacity adjustment factors of 310.15(B)(2) of 45 percent for 21–30 current-carrying conductors in a raceway or cable.
When utilizing cable trays, 392.11(B)(4) permits the use of Table 310.20 where single conductors are installed in triangular or square circuit groups in uncovered cable trays. Section 392.6(A) requires that cable trays shall be installed as a complete system. Disassembly of a portion(s) of the cable tray assembly during times of use does not change the cable tray type with respect to whether the tray is the covered or uncovered type. Section 392.11(B) permits the application of the adjustment factors of 310.15(B)(2) to be omitted when using uncovered cable trays. Keep in mind that the question references the use of what is described as a covered cable tray, therefore we would not be able to omit the adjustment factors. It is also important to note that 392.11(B)(3) directs that solid bottom cable trays shall have the ampacity of single conductors and single conductor cables determined by 310.15(C).
The third question, "Is each parallel bundled conductor arrangement required to have its own equipment grounding conductor sized in accordance with Table 250.122, or would only one equipment grounding conductor sized per Table 250.122 for the 10 4/0 AWG per phase paralleled bundled conductors be necessary?” Section 300.3(B) requires that all conductors of the same circuit, including the equipment grounding conductor, be contained within the same raceway, auxiliary gutter, cable tray, etc. If using an underfloor raceway (trench duct), a single equipment grounding conductor sized as per 250.122 is permitted. The equipment grounding conductor is sized per Table 250.122, based on the size of the overcurrent device protecting the circuit. In this case, the submitter indicates that a 3200-ampere breaker is being used; therefore, a 500-kcmil copper or an 800-kcmil aluminum conductor would be required. Section 250.118(13) permits "listed electrically continuous metal raceways” to serve as an equipment grounding conductor. Article 390 does not require this type of raceway to be listed, so the suitability of the raceway to serve as the equipment grounding conductor is dependent on whether the raceway is listed. The metal enclosure must be grounded in either event, as per 250.86.
With respect to cable tray installations, we must remember that 250.118(11) allows metal cable tray as permitted in 392.3(C) and 392.7 to be used as an equipment grounding conductor. Section 392.7 requires that "metallic cable trays that support electrical conductors shall be grounded as required for conductor enclosures in accordance with section 250.96.” Accordingly, the tray may qualify as an equipment grounding conductor in its own right and may negate the need for additional equipment grounding conductor installation. When the conductors are all contained within the same metal cable tray as described in 300.3(B), a single conductor would be permitted. If paralleled circuit conductors are installed in multiple cable trays then 300.3(B)(1) would apply the requirement that all conductors be installed in the same raceway, auxiliary gutter, cable tray, etc., to each portion of the paralleled installation. Accordingly, the equipment grounding conductors for each cable tray shall comply with the provisions of 250.122.
The fourth question, "Would the bundled paralleled conductors be classified as a cable and have to adhere to Article 250.122(F)(1) as noted in the NEC Handbook commentary?” The installation as described by the submitter would be one of bundling conductors into circuit groups in the underfloor raceway. Section 390.17 reiterates the general requirement that 310.15(B)(2) is applied to current-carrying conductors installed within the raceway whether or not the conductors are bundled into circuit groups.
Section 392.11 differentiates between multiconductor cables in part (A) and single conductor cables in part (B). Single conductors or single conductor cables installed in a triangular or square configuration as per 392.11(B)(4) would not change the definition of each phase set to that of a multiconductor cable. It would constitute a conductor or cable configuration within the cable tray. In short, conductors configured or bundled together into circuit groups within either trench duct or cable tray would not re-classify these bundled circuit groups as cables and, accordingly, would not trigger requirements of 250.122(F), as all of the conductors are contained in the same raceway or cable tray.
The fifth question, "Does the short 3600-amp 90ºC bus attached to the 75ºC 3200-amp breaker allow the use of 90ºC rated conductors calculated at their 90ºC current rating?” (See photo 3).
As indicated by photo 3, does the short 3600-amp 90°C bus attached to the 75°C 3200-amp breaker allow the use of 90°C rated conductors calculated at their 90°C current ratings?
We first turn to 110.14(C)(1)(b) which limits "termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG,” to 75ºC, or up to the ampacity of the conductor "if the equipment is listed and identified for use with such conductors.” The limiting factor here is the 75ºC rating of the 3200 ampere breaker. We may, accordingly, use these conductors only at their 75ºC rating; however, we are given permission to use the actual 90ºC conductor rating for purposes of applying adjustment factors due to high ambient temperatures, or to meet the requirements of Table 310.15(B)(2).
The answers to the five questions presented are based on the installation of the conductors in what appears to be an underfloor raceway as described by Article 390. In an attempt to answer more thoroughly the submitter’s questions, Article 392, Cable Trays, is referenced. However, the installation as described by the submitter does not appear to conform to the installation requirements of Article 392, Cable Trays. Refer to NEC 445 for additional information concerning generator applications.
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