When incorporating larger feeders or branch circuits into equipment, many commercial and industrial electrical designs require use of parallel arrangements. Installing conductors in parallel for feeders means multiple conductors are electrically connected at both ends to create a single conductive path or conductor for each of the circuit’s ungrounded or grounded-phase conductors.


Installing parallel conductors becomes a necessity when larger circuit conductors are required for supplying switchboards and other large electrical equipment, because very large single conductors are not necessarily practical, economical or even available in many cases. Installing feeders or circuits in parallel requires compliance with Section 310.10(H) of the National Electrical Code (NEC), which means the conductors must be the same length, material and size; have the same insulation; and be terminated in the same manner.


Where the entire parallel arrangement of conductors is installed in one raceway, cable or cable tray, one equipment grounding conductor (EGC) is permitted. The metallic raceway or cable tray could also qualify as an EGC in accordance with 250.118. In this case, a wire-type EGC is not necessary to meet the minimum NEC requirements. 


However, according to 310.10(H)(3), the physical characteristics of each raceway in the parallel arrangement must have the same physical characteristics where parallel runs are installed. In other words, a parallel run could not be installed using PVC conduit, rigid conduit and intermediate metal conduit in the same parallel run. The impedances of the contained conductors in the conduit would be different and result in unbalanced division of current over the current-carrying conductors of the parallel run. Current will divide over all paths available when returning to its source. This applies to normal current and fault current.


Specific rules exist for wire-type EGCs installed with parallel conductor installations. Section 250.122(F) requires that, when conductors are run in parallel, any wire-type EGCs must be run in parallel. If wire-type EGCs are installed with the ungrounded feeder conductors installed in parallel, the EGCs must also be installed in parallel, but they can be smaller than 1/0. The reason is the EGCs are not being installed to create a single larger conductive path in the circuit. They are already fully sized according to Section 250.122, as required.


Where parallel arrangements of conductors are installed in separate raceways (such as PVC conduits) and wire-type EGCs are required, an EGC shall be installed in each of the separate raceways. In this type of installation, each EGC installed in parallel is required to be sized using Table 250.122 based on the rating of the fuse or circuit breaker protecting the entire parallel set.


It is important to remember Section 300.3(B) generally requires all circuit conductors, including EGCs, to be installed in the same raceway, cable or trench. In other words, in a parallel run, using separate PVC raceways, a wire-type EGC has to be installed in each raceway.


One reason for the requirement to include an EGC in each of the raceways is that the full-sized EGC prevents overloading and possible damage of a smaller, inadequately sized EGC if a ground fault occurs along one of the parallel branches.


Another reason is that it keeps impedance levels low during normal operation and during ground-fault events. Installing one EGC in one of the raceways of the parallel feeder separates the EGC from its associated ungrounded conductors of the same circuit. This is a violation of sections 310.10(H)(3), 250.122(F) and 300.3(B). Keeping the impedance in the effective ground-fault current path as low as possible ensures fast, effective operation of fuses or circuit breakers in the case of a ground fault.


Section 250.122(A) indicates that EGCs never have to be larger than the circuit’s ungrounded conductors. In a parallel feeder, the ungrounded conductors include all of the conductors in parallel that are added together to make one conductor per phase. For example, if four 600 kcmil conductors are installed in parallel to create a 1,600-ampere feeder circuit, the circuit conductors are 2,400 kcmil. The minimum size copper EGC required in each raceway is 4/0 AWG.


EGCs of the wire-type installed in parallel runs must be installed in each raceway of the parallel set, unless the raceway containing the run qualifies as an EGC in accordance with 250.118. The raceways enclosing conductors of a parallel arrangement must have the same physical characteristics in accordance with 310.10(H)(3). EGCs in parallel arrangements must also be installed in parallel, and each has to be sized using 250.122, based on the rating of the fuse or breaker protecting the entire parallel set. Each cable assembly in parallel installations must include an EGC sized based on 250.122 using the rating of the overcurrent device protecting the parallel feeder or branch circuit.