For my December 2005 column in ELECTRICAL CONTRACTOR, I wrote about the installation of parallel conductors in a metal wireway and the subsequent heating effects. These heating effects occur whether the parallel conductors are installed in a metal or nonmetallic wireway or in an auxiliary gutter. In the 2014 National Electrical Code (NEC) process, Code-makers attempted to insert requirements in 310.10(H) to minimize the heating effects for parallel conductors. 


However, locating the requirement in the new 310.10(H)(6), as proposed, would have created a correlation issue with 300.3(B)(1), Exception. This exception permits parallel conductors installed in underground-located nonmetallic raceways to have all of one phase or grounded conductors in a raceway if the raceways are close to each other and the conductors comply with 300.20(B). The proposals and comments dealing with this issue were rejected on the floor of the NFPA annual meeting. 


While these proposals and comments did not result in changes to the 2014 NEC, safety issues for the installation of parallel conductors in metal wireways (Article 376), nonmetallic wireways (Article 378) and auxiliary gutters (Article 366) still remain. A number of public inputs have been submitted to the 2017 NEC process for changes in articles 366, 376 and 378. These public inputs have passed ballot in the first stage of the new 2017 NEC process. 


The history of this issue is still the same as I wrote about in my December 2005 article. Making the appropriate changes to Article 376 for metal wireways, Article 378 for nonmetallic wireways, and Article 366 for auxiliary gutters would not create a correlation issue because the parallel conductors are being addressed in wireways and gutters, not in raceways as covered in 300.3(B)(1), Exception. 


A particular installation prompted me to delve into parallel conductor heating in a wireway or gutter. It involved the failure of a service entrance bus duct supplying power to a major grocery store data center and corporate facility. This facility was the 24-hour support system for all store transactions for a major grocery store chain; therefore, maintaining power to the data center and the corporate complex is critical for the business’s continued operation. The distance from the utility company service point at the tap box to the service equipment in the building was approximately 50 feet with 10 600-kcmil THHN/THWN copper conductors installed in parallel, for each phase and grounded conductor of the 277/480-volt, three-phase, 4-wire system, in a metal wireway installed in accordance with Article 376 in the 2005 National Electrical Code (NEC). 


The installation of parallel conductors in the auxiliary gutter involved all of phase A in one group, phase B in another group, phase C in a group and the neutrals in a group. The service conductors were re-energized. Conductor current readings were taken on all conductors connected to the system. High current readings were noted on some conductors within each phase group and low current on other conductors within the same phase group.


Remember,
Section 310.4 of the 2005 NEC and 310.10(H) in the 2014 NEC require each paralleled phase, polarity (for direct current circuits), neutral or grounded conductor be the same length, the same conductor material, the same size in circular mil area, the same insulation type, terminated in the same manner, and physically the same. Where installed in separate raceways or cables, the cables or raceways must have the same number of conductors, all of which must have the same characteristics. Metal and nonmetallic wireways are still considered to be raceways based on a change that occurred to the definition of raceways in Article 100 in the 2014 NEC and to the defined terms in 376.2 for metal wireways and 378.2 for nonmetallic wireways. Auxiliary gutters are not considered to be a raceway; however, 310.10(H) still applies.


The cables in the grocery installation were removed and then reinserted into the gutter by bundling the cables with phases A, B and C and grounded conductor in each bundle. The service was re-energized, the individual cables were monitored for two days, and an infrared scan was performed on the cables. Not only were the current readings on the conductors relatively the same but also the system operated much cooler than previously noted. This parallel installation is the necessary proof to justify the changes that are submitted for the 
2017 NEC.