Often in electrical design and installations, there is a desire to install feeder conductors in parallel applications. When feeders are large in size and subsequent ampacity, they quickly reach a point where it becomes impractical to install them using only one conductor per phase and one for the grounded (neutral) conductors. This is where the provisions for feeder conductors installed in parallel come under much needed review. The National Electrical Code [NEC] requirements for parallel conductors are explained in section 310.10(H). Additional requirements for equipment grounding conductors for parallel conductors are examined in sections 300.3(B)(1), 310.10(H)(5), and 250.122(F)(1) and (F)(2).
The focus of this article will be on the 2017 NEC Code Revision to Section 250.122(F)(2) as it pertains to Multiconductor Cable assemblies in parallel, such as Type TC (Tray Cable), Type MC (Metal-Clad Cable).
Section 250.122(F)(2)(a) explains that as a general rule, when multiconductor cables are installed in parallel, the equipment grounding conductors (EGC) in each cable assembly shall be connected in parallel. Where multiconductor cables are installed in parallel in the same raceway, auxiliary gutter, or cable tray supporting system, a single equipment grounding conductor, sized in accordance with section 250.122 shall be permitted in combination with the equipment grounding conductors provided within the multiconductor cable assembly as long as the single equipment grounding conductor installed and the equipment grounding conductors in the multiconductor cables are connected together at both ends of the installation. It is also beneficial to note that Cable Trays complying with 392.60(B), and metal raceways in accordance with section 250.118, and auxiliary gutters shall be permitted as the equipment grounding conductor.
Notice - Except as aforementioned for applications of a parallel multiconductor cable assemblies in a single raceway, auxiliary gutter, or cable tray supporting system, traditionally rules apply which requires the sizing of the equipment grounding conductor for each multiconductor cable assembly to be sized in accordance with section 250.122 based on the size of the overcurrent protective device (OCPD) for the feeder or branch circuit.
This revision within the 2017 NEC® under section 250.122(F)(2) is designed to provide flexibility to the use of standard sized multiconductor cable assemblies with their relevant UL standard sized equipment grounding conductors, which is not traditionally sized for use in parallel installations.
Paul W Abernathy, CMI, CMECP®