Many commercial and industrial electrical designs incorporate larger feeders or branch circuits to supply equipment; these high capacity circuits necessitate the use of parallel conductor arrangements. Installing parallel conductors becomes a necessity when larger circuit conductors are required for supplying switchboards and other large electrical equipment, simply because very large single conductors are not practical, economical, or even available in many cases.
Installing feeders or circuits in parallel requires compliance with 310.4, which means the conductors must be the same length, material, and size; must have the same insulation; and must be terminated in the same manner. Installing conductors in parallel for feeders means that multiple conductors are electrically connected at both ends to create a single conductive path or conductor. Where the entire parallel arrangement of conductors is installed in a single raceway, cable, or cable tray, a single equipment grounding conductor (EGC) is permitted to be installed. 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. There are specific rules for wire-type EGCs installed with parallel conductor installations. Section 250.122(F) indicates that where conductors are run in parallel, any installed wire-type EGCs are also required to be run in parallel.
Wire-Type Equipment Grounding Conductors
If wire-type EGCs are installed with parallel feeder circuit conductors, the EGCs also must be installed in parallel, but they can be smaller than 1/0. The EGCs are not being installed to create a single larger conductive path in the circuit; they are already fully sized as required by 250.122. Where parallel arrangements of conductors are installed in separate raceways such as nonmetallic conduits, and wire-type EGCs are required, an EGC must be installed in each of the separate raceways. In this type of installation, each EGC installed in parallel is required to be sized according to Table 250.122 based on the rating of the fuse or circuit breaker protecting the ungrounded conductors installed in parallel.
It is important to remember that 300.3(B) generally requires all conductors of the circuit, including the EGCs, to be installed in the same raceway, cable, or trench. This means that 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 the overloading and possible damage that a smaller, inadequately-sized EGC might sustain should a ground fault occur along one of the parallel branches. Another reason is that the full-sized EGC keeps impedance levels low during normal operation and during ground-fault events. Installing a single EGC in one of the raceways of the parallel feeder separates the EGC from its associated ungrounded conductors of the same circuit. Such installations do not comply with the requirements of 310.4, 250.122(F), and 300.3(B). Operation of fuses or circuit breakers to quickly interrupt the circuit in the case of a ground fault is facilitated by keeping the impedance in the effective ground-fault current path as low as possible.
Sizing Examples
The following examples answer some of the more common questions related to sizing EGCs installed in parallel. Installing EGCs in parallel arrangements is not as complicated as it first appears, and these examples cover some typical electrical installations.
Example 1: A 4000-ampere feeder is installed in parallel using ten nonmetallic conduits. Each conduit contains four 750 kcmil copper circuit conductors. What is the minimum size copper EGC required in each conduit?
Answer: Based on the 4000 ampere overcurrent protective device rating or setting and using Table 250.122, a 500 kcmil copper EGC is required to be installed in each of the ten raceways.
Example 2: An 800-ampere feeder is installed in parallel using two rigid metal conduits. Each conduit contains four 750 kcmil copper circuit conductors and the design document requires that a wire-type EGC be installed in all raceways. What is the minimum size copper wire-type EGC required in each conduit?
Answer: Based on the 800 ampere overcurrent protective device rating or setting and using Table 250.122, a 1/0 AWG copper EGC is required in each raceway.
Cable Assemblies in Parallel
Cable assemblies, such as MC cable, are manufactured in standard conductor size configurations. The EGC in a cable is typically sized adequately for single-circuit use but not necessarily for all parallel circuit installations. If cable assemblies are installed in large-capacity parallel circuits, it is necessary to verify that the EGC in each of the individual cables of the parallel set is sized as required by Table 250.122, based on the rating or setting of the fuse or circuit breaker protecting the parallel circuit. Installing cable assemblies in parallel arrangements might necessitate a special cable order that includes sizing the EGC in each individual cable so that it meets the requirements of Table 250.122 based on the size of the fuse or circuit breaker used for the specific parallel conductor installation.
Section 250.122(A) indicates that EGCs never have to be larger than the ungrounded conductors of the circuit. In a parallel feeder, the ungrounded conductors of the circuit include all the conductors in parallel, which are added together to make a single conductor. As an example, if four 600 kcmil conductors are installed in parallel for a 1600-ampere feeder, the ungrounded circuit conductor is the total of the four conductors installed in parallel, or a 2400 kcmil conductor. The minimum size EGC required in each raceway is a 4/0 AWG copper, based on the 1600 ampere overcurrent protective device.
Summary
Equipment grounding conductors of the wire type, installed in parallel runs, must be installed in each raceway of the parallel set. EGCs in parallel arrangements must also be installed in parallel, and each has to be sized according to 250.122, based on the rating or setting 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. According to 250.122(B), a proportional increase in size of EGCs is necessary when the circuit conductors are increased in size.
Michael Johnston is NECA’s Executive Director of Standards and Safety. Mike served on NEC CMP-5 in the 2002, 2005, and 2008 cycles and is currently the chair of CMP-5, representing NECA for the 2011 NEC cycle. An active member of IAEI, the NFPA Electrical Section, Education Section, the UL Electrical Council, and National Safety Council, Mike is a member of the IBEW and has experience as an electrical journeyman wireman, foreman, and project superintendent.