Welcome to the eighth installment of this continuing deep dive into understanding one of the most often misinterpreted parts of NFPA 70, National Electrical Code (NEC). It is time to examine the requirements in place for the central focus of a grounded electrical system: the grounded conductor.

Editor's Note: Click here to read part 7 of this series.

The roles of the grounded conductor

The whole point of grounding an electrical system is to connect one of the circuit conductors to the earth. This takes us to Section 250.4, where we learned that one reason why we ground an electrical system is to stabilize the voltage to earth during normal operation. Having a grounded conductor in the system establishes a solid reference point that ensures the voltage between any ungrounded conductor and the grounded conductor, or anything bonded to it through the main bonding jumper, will be the same throughout the system.

Having a grounded conductor in the system establishes a solid reference point that ensures the voltage between any ungrounded conductor and the grounded conductor, or anything bonded to it through the main bonding jumper, will be the same throughout the system.

The grounded conductor plays another critical role in the system. You’ve probably heard the misconception about how electricity tries to get to ground, but in reality, it is really only trying to get back to the source. The ground serves as the return path to the power source for fault current, and if it is used as a neutral conductor, this will be the path for any unbalanced current between phases from line-to-neutral loads.

Section 250.24(D)

Once again, we will start our grounded conductor journey by heading to Section 250.24. In 250.24(D), we find requirements for the grounded conductor for grounded AC systems under 100V. This section states that if an AC system operating at 1,000V or less is grounded at any point, then a grounded conductor must be brought to each service disconnecting means. This must be done regardless of whether the system contains any line-to-neutral loads. The grounded conductor must be routed with the ungrounded conductors to each service disconnecting means, where it will be connected to the service disconnect enclosure by the main bonding jumper.

Now, let’s look at how the grounded conductor must be installed. For this article, I will focus solely on the grounded conductor requirements and not incorporate any neutral conductor ones. The installation requirements for the grounded conductor can be found in 250.24(D)(1)–(4). This is where we will find guidance on how to size our grounded conductors to be the final phase of the effective ground-fault current path. However, the sizing requirements are different for grounded conductors installed in a single raceway or cable and those installed in parallel.

Table 250.102(C)

Section 250.24(D)(1) requires that grounded conductors in a single raceway or cable must not be smaller than specified by Table 250.102(C). If this table number seems familiar, it should. This is the same table we used to size our main and system bonding jumpers in last month’s article (“Jumping on System Bonding Jumpers”). Simply put, this table specifies the size of the grounded conductor based on the size of the ungrounded conductors used in the system, or the equivalent area of ungrounded conductors installed in parallel. It lists ungrounded conductor sizes up to 1,100-kcmil copper and 1,750-kcmil aluminum or copper-clad aluminum (CCA). Above these values, size the grounded conductor to be 12.5% of the size of the ungrounded conductors.

Section 250.24(D)(2) applies when the parallel ungrounded conductors are in two or more separate raceways or cables. The first thing we need to know is that in this installation, the grounded conductor must be paralleled and in all parallel raceways.  Since the minimum size conductor that can be in parallel is 1/0 AWG, this puts a minimum size on the grounded conductor when in this position.

Next, there are two methods we can use to determine the size. One commonly used way when the parallel installation has one set of ungrounded conductors per parallel raceway or cable is to size the grounded conductor based on the ungrounded conductors in each of the raceways or cables, as in Table 250.102(C).

However, if the installation consists of parallel raceways containing multiple sets of parallel ungrounded conductors, the grounded conductor is still sized in accordance with Table 250.102(C), but based on the equivalent area of the largest paralleled conductors in each raceway. 

For example, if two parallel raceways contain two sets of 250-kcmil conductors in parallel, the grounded conductor can be installed as a single conductor based on the 500 kcmil equivalent area of the ungrounded conductors. While this is permitted, it is far less common than installing a single set of ungrounded conductors in each raceway.

Just as we discussed with sizing main and system bonding jumpers, don’t forget about Note 2 to Table 250.102(C), which applies when selecting the size of a grounded conductor that is a different material than the ungrounded conductors. For ungrounded conductor sizes up to 1,100-kcmil copper or 1,750-kcmil aluminum or CCA, use the column listed in the table based on the grounded conductor’s material.

When the size of the ungrounded conductors exceeds 1,100-kcmil copper or 1,750-kcmil aluminum or CCA, however, you must determine what size of ungrounded conductors of the same material as the grounded conductor would achieve the same ampacity and then calculate 12.5% of that circular-mil area to determine the grounded conductor size. See last month’s column for an example of how to do this with the main bonding jumper.

Exceptions

The final two subsections in 250.24(D) apply to special circumstances. Section 250.24(D)(3) applies to sizing the grounded conductor of a corner-grounded delta-connected system. Since this type of system grounds a phase conductor and supplies only three-phase loads, the grounded conductor will carry the same amount of current as the ungrounded conductors and therefore must be the same size and ampacity. 

Section 250.24(D)(4) applies to the installation of impedance grounded systems and sends users to specific sections on these types of installations. I will cover special grounding circumstances in a future installment of this column.

The key to proper installation of the grounded conductor relies on the installer understanding how to properly size it based on its function. As mentioned, while we didn’t explore how the grounded conductor gets sized when serving as a neutral conductor, that also plays an important role for grounded conductors that fit that part. Remember that every grounded system requires a grounded conductor to be run with the ungrounded service conductors to each service disconnecting means and sized at a minimum to handle the fault current on its journey back to the source.

Next month’s column will dissect installation requirements and the purpose of another cornerstone of grounding and bonding: the equipment grounding conductor. Until then, stay safe and remember to always test before you touch!