250-122 (F) Equipment-grounding Conductors for Parallel RunsPublished: May 2011By Michael JohnstonFor Electrical Contractor Magazinehttp://www.ecmag.com/section/safety/side-side-equipment-grounding-conductors

Many electrical designs incorporate parallel arrangements. The National Electrical Code (NEC) requires parallel conductors when supplying large switchboards and other large electrical equipment because large single conductors are not practical, economical or even available in many cases.

Installation criteriaInstalling conductors in parallel for feeders or branch circuits means multiple conductors are electrically connected at both ends of the circuit to create a single larger conductor. Installing feeders or circuits in parallel requires compliance with Section 310.10(H), which means the conductors must be the same length, consist of the same material, be the same size, have the same insulation and be terminated in the same manner. The cable assembly sheaths or raceways must also have the same physical characteristics. For example, if an installation uses polyvinyl chloride (PVC) conduit, it must be physically the same in each run in the overall parallel set.

The same rule applies to metallic raceways and cable assemblies. When the entire parallel arrangement of conductors is installed in a single raceway, cable assembly or cable tray, the NEC permits the installation of a single equipment-grounding conductor. The metallic raceway or cable tray could also qualify as an equipment-grounding conductor in accordance with Section 250.118. In this case, a wire-type equipment-grounding conductor (EGC) is not necessary to meet the minimum NEC requirements.

Sizing wire-type EGCs for parallel runsThere are specific rules for wire-type EGCs or equipment-bonding jumpers installed with parallel runs for feeders or branch circuits. Section 250.122(F) states that, when conductors are run in parallel in separate raceways or cables, any wire-type EGCs also are required to be run in parallel in their respective raceway or cable. If wire-type EGCs are installed with the paralleled feeder conductors, the EGCs must also be installed in parallel, but they can be smaller than 1/0. This is because multiple EGCs are not being installed to share ground-fault current in the occurrence of a ground-fault event but to provide a low-impedance path regardless of where the fault occurs. They are already fully sized according to Section 250.122, as required.

When parallel arrangements of conductors are installed in separate raceways, such as in multiple conduits, and the NEC requires wire-type EGCs, 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 using Table 250.122 based on the rating of the fuse or circuit breaker protecting the entire parallel set.

It is important to remember that Section 300.3(B) generally requires all conductors of the circuit, including the EGCs, to be installed in the same raceway, cable or trench. 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 should a ground fault occur in one of the parallel branches. Another reason is that it keeps impedance levels low during normal operation and during ground-fault events. Installing an EGC in only one of the raceways separates the EGC from its associated ungrounded conductors of the same circuit and is a violation of Sections 250.122(F), 300.3(B) and 310.10(H). 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.

Sizing examplesQuestions about sizing EGCs for parallel installations are common. Installing EGCs in parallel arrangements is not complicated. The following addresses a couple of basic sizing examples for paralleled equipment-grounding conductors.

Example 1: A 4,000-ampere (A) feeder is installed in 10 PVC conduits in a parallel arrangement, each containing four 750 copper conductors. What is the minimum size copper EGC required in each conduit?

Answer: The NEC requires a 500 kcmil copper EGC in each raceway based on the 4,000A overcurrent device in accordance with Table 250.122.

Example 2: If an 800A feeder is installed in (2) raceways in a parallel arrangement, each containing (4) 750 copper conductors, what is the minimum size wire-type EGC in each raceway of this circuit?

Answer: The NEC requires a 1/0 AWG copper EGC in each raceway.

Sizing wire-type equipment- bonding jumpers in parallel runsWhere wire-type equipment-bonding jumpers (supply side or load side) are installed with parallel runs, the NEC requires them to be installed according to 310.10(H) and sized according to 250.102. Section 250.102(C) includes sizing rules for supply--side bonding jumpers and equipment--bonding jumpers installed on the load side of an overcurrent device. They must be sized according to 250.102(D). The difference is simple. Supply-side bonding jumpers are sized using Table 250.66 or the 12.5 percent rule where the largest ungrounded conductor exceeds the sizes provided in Table 250.66. Load-side equipment-bonding jumpers are sized based on the rating of the overcurrent device using Table 250.122. Good examples of supply-side bonding jumpers are those installed with parallel secondary conductors from separately derived systems or parallel service-entrance conductors.

Sizing examplesExample 1: A 2,000A service is installed in six rigid metal conduits in a parallel arrangement, each containing four 600 kcmil copper conductors. The conduits rise into the bottom of a service switchboard. What is the minimum size copper supply-side bonding jumper from each conduit busing to the equipment-grounding terminal bar in the service equipment?

Answer: The NEC requires a 1/0 copper equipment-bonding jumper from each raceway based on the values in Table 250.66. Note that if the equipment-bonding jumper was installed in a “daisy-chain” fashion, connecting all the conduits together and to the equipment-grounding terminal bar with a single bonding jumper through lay-in lugs, the equipment-bonding jumper must be a minimum of 450 kcmil or the next higher size in Table 8, which is 500 kcmil. Note: 6 600 kcmil = 3,600 kcmil 12.5% = 450 kcmil.

Example 2: If an 800A service is installed in two raceways in a parallel arrangement, each containing four 750 copper conductors, what is the minimum size equipment-bonding jumper required?

Answer: The NEC requires a 4/0 AWG copper equipment-bonding jumper from each raceway to the enclosure. Note: 750 kcmil 2 = 1,500 kcmil 12.5% = 187,500 cm and round up to the next higher size in Table 8, Chapter 9.

Cable assemblies installed in parallel arrangementsCable assemblies, such as metal-clad cable (Type MC), are manufactured in standard conductor size configurations unless they are a special-order item. The EGC in a standard cable is typically sized adequately for single circuit use and might not be adequate for all parallel circuit installations. If cable assemblies are installed in large-capacity parallel circuits, it is necessary to verify that each EGC is sized as Table 250.122 requires, based on the size of the fuse or circuit breaker protecting the entire parallel circuit. Installing cable assemblies in parallel arrangements may necessitate a special order that includes sizing the EGC in each cable large enough to allow the cable to be Code-compliant in a parallel installation. Some cable manufacturers produce cable assemblies with larger EGCs when parallel arrangements are included in the project design. Section 250.122(A) indicates that EGCs never have to be larger than the ungrounded conductors of the circuit. Remember, in a paralleled feeder run, the ungrounded conductors of the circuit include all of the conductors in parallel that are added together to make a single conductor.

SummarySection 250.118 of the NEC specifies EGC types. Wire-type EGCs must be sized according to Section 250.122 based on the rating of the overcurrent device protecting the circuit. When circuits are installed in parallel arrangements in multiple raceways of cable assemblies, the NEC requires a separate EGC in each raceway or cable assembly. The minimum size required for each EGC must not be smaller than the sizes Table 250.122 specifies. Where equipment-bonding jumpers are installed with circuits in parallel, they must meet the sizing requirements contained in Sections 250.102(C) for supply-side bonding jumpers and 250.102(D) for load-side bonding jumpers. The note following Table 250.122 is not optional. EGCs must provide an effective ground-fault current path as described in Section 250.4. The requirements in the NEC are the minimum.

JOHNSTON is NECA’s executive director of standards and safety. He is a former director of education, codes and standards for IAEI; a member of the IBEW; and an active member of the NFPA Electrical Section, Education Section and the UL Electrical Council. Reach him at mjohnston@necanet.org.