Welcome back to the series on Article 250, “Grounding and Bonding” in NFPA 70, National Electrical Code. This article will continue the discussion on properly sizing wire-type equipment grounding conductors (EGC). Past articles have covered how to size an EGC based on the overcurrent protective device (OCPD) protecting the circuit and Table 250.122. However, there are some special circumstances where the rules are slightly different. One such scenario is when the circuit conductors are installed in parallel.

Remember that the main functions of the EGC are to bond together the normally non-current-carrying conductive surfaces together, and in the event of a fault in the circuit, the EGC provides the effective ground-fault current path and facilitates the OCPD’s operation. 

Parallel circuit conductors

Parallel circuit conductors are commonly used with large ampere-rated circuits to make circuit conductor installation a little easier. For instance, a 500A circuit is easier to install using two 4/0 AWG copper conductors per phase than to use a single 700-kcmil copper conductor per phase. 

Parallel conductors are commonly found in raceways, cable trays, gutters and multi­conductor cables. Depending on how the parallel circuit conductors are installed, there are different rules for putting in the EGC. If the parallel circuit conductors are installed in a raceway, a couple of configurations can be used. Depending on the size of the parallel circuit conductors, it might be possible to install a single raceway containing all parallel circuit conductors and the EGC. However, keep in mind that the smallest size conductor permitted to be installed in parallel is a 1/0 AWG, so the raceway will need to be large if using only one. For this reason, a more common configuration for parallel circuit conductors is separate raceways.

Single or separate raceways

If a single raceway is used, the rules for sizing the EGC are relatively simple. A single EGC also is installed in the raceway based on the size of the OCPD rating and Table 250.122. Basically, this is the same as the original method for EGC sizing. All circuit conductors are contained in the same raceway, and, regardless of which conductor sees a fault, the EGC is sized properly to facilitate the OCPD’s operation. 

This is the same for parallel conductors in the same cable tray or gutter. However, keep in mind that Article 392 sets the minimum size of EGC in a cable tray at 4 AWG if a single-conductor EGC is installed in the cable tray to provide the effective ground-fault current path.

However, if the parallel circuit conductors are installed in separate raceways, we need to take a slightly different approach to ensure an effective ground-fault current path is provided. When parallel conductors are installed in separate raceways, there is the possibility that a fault occurs in a single one. The EGC in the single raceway where the fault occurs must still be able to provide an effective ground-fault current path and make it possible for the OCPD protecting the circuit to clear the fault. Therefore, section 250.122(F) requires that in each of the separate raceways, the EGC installed must be sized in accordance with Table 250.122 based on the rating of the OCPD protecting the circuit. The key here is to understand that the EGC is based on the OCPD’s size, not the conductors’. Just because we can split the normal circuit load current between parallel circuit conductors doesn’t mean that the fault current will play by the same rules. 

Multiconductor cables

Another common way to install parallel circuit conductors is to use multiconductor cables. In general, multiconductor cables follow the same process for sizing the EGC as raceways. However, with multiconductor cables, it is not as common to see parallel conductors in the same cable. It is far more common to see parallel conductors installed using separate multiconductor cables. Similar to wire-type EGCs installed with parallel conductors in separate raceways, there is the possibility that a fault occurs in only one cable when parallel conductors are installed in separate multiconductor cables. Again, the EGC in the cable where the fault occurs needs to facilitate the OCPD’s operation to clear the fault. 

This brings up an interesting issue when installing parallel circuit conductors using multiconductor cables. Multiconductor cable manufacturers don’t know the cable application when they build it. Typical multi­conductor cables are constructed with an EGC based on the largest OCPD that the ungrounded conductors are likely to be connected to based on their ampacity. For instance, one manufacturer lists a 250-kcmil, three copper conductor cable as having a 4 AWG copper insulated EGC in the cable. The ampacity of a 250-kcmil conductor per the 75°C column in Table 310.16 is 255A and, therefore, the largest OCPD this cable can connect to is 300A. 

The 4 AWG size for the wire-type EGC in the cable is based on Table 250.122 using a 300A rating for an OCPD. However, if four of these cables are installed in parallel to provide a 1,000A feeder circuit, a 4 AWG conductor is not sufficient to provide the EGC in accordance with Table 250.122. A properly sized wire-type EGC in accordance with Table 250.122 for a circuit supplied by a 1,000A OCPD would require a minimum of a 2/0 AWG copper wire-type EGC.

Therefore, think carefully when selecting the correct cable. Just like when installing an EGC for parallel circuit conductors in separate raceways, you need to know that the EGC in each cable must be the full size required by Table 250.122 based on the size of the OCPD protecting the circuit. To use multiconductor cables for parallel circuit conductors, it is often required to special-order cables with the correct size EGC.

Why is an EGC important?

The most important thing to remember about the EGC is that it plays a critical role in the safety of the electrical installation. If an EGC is not properly sized, the internal resistance of the smaller wire size can restrict the fault current and slow the upstream OCPD’s operation. During fault conditions, every fraction of a second counts. Even a slight restriction in the amount of fault current can delay the OCPD’s operation. In the case of parallel conductor circuits, if the EGC is mistakenly sized based on wire size in each separate raceway or cable, it is entirely possible that the OCPD never clears the fault.

This just about brings us to the end of our discussion on the EGC. Having talked about what can serve as an EGC, what needs to be connected to an EGC, and how to size the EGC, all that remains is to cover methods of EGC connections, which will bring this topic to a close.

Until then, stay safe and remember to always test before you touch!

Victor / stock.adobe.com