In addition to the engineering basics of the effective ground-fault current path, the sizing rules in the National Electrical Code for equipment grounding conductors (EGCs) are also important.

The driving text of Section 250.122 is that the minimum size required for wire-type EGCs is not to be less than the values in Table 250.122. This rule states that in no case are the EGCs required to be larger than the circuit conductors supplying the equipment.

Parallel arrangements

It is important to mention parallel arrangements and their relationship to the circuit. Where circuit conductors are installed in parallel to create the equivalent of a larger circuit conductor, the size of the circuit conductor is the total area of all individual conductors in parallel that create the larger circuit conductor using multiple parallel paths. The NEC does not permit conductors to be installed in parallel to create an EGC. Each wire-type EGC in a parallel installation must generally be sized based on Table 250.122.

Section 250.122 also indicates that raceways, cable sheaths and trays serving as EGCs have to meet the performance requirements in 250.4(A)(5) or (B)(4) as applicable. For multiconductor cable, the EGC within the cable assembly can be sectioned where the combined circular mil area of the sectioned EGC meets the size requirement in Table 250.122. Using this table requires knowing the rating of the overcurrent device protecting the branch circuit or feeder. 

Once this value is known, the rating (or a rating that does not exceed the value in the left column of the table) should be found. The reader can then move across the table horizontally from left to right to determine the minimum size EGC expressed in AWG or circular mils. Make sure to use the appropriate column for aluminum or copper-clad aluminum as compared to copper EGCs.

Increases in EGC size

EGCs are required to increased in size to address voltage drop. A proportionate increase of the associated ungrounded circuit conductors is required. If the ungrounded conductors of a circuit are increased in size for any reason other than as required in 310.15(B) or 310.15(C), the wire-type EGCs must also be increased proportionately according to the circular mil area of the ungrounded conductors. This can be verified by following these steps, using a 400A feeder as an example:

The 400A feeder (420 allowable ampacity) is generally installed using 600 kcmil copper circuit conductors and a 3 AWG copper EGC. For voltage drop reasons or circuit capacity needs, the 600 kcmil conductor is increased in size from 600 kcmil to two paralleled 400 kcmil copper conductors for each ungrounded phase conductor and the neutral conductor. The circular mil values are added together (400 plus 400) to result in 800 kcmil copper now required for the circuit conductors in this installation. The adjusted size (800 kcmil) is then divided by the originally required size (600 kcmil) to determine the proportionate value of circular mil area adjustment (1.3).

It is always best to perform this simple calculation to verify that the adjusted size of the EGC meets or exceeds the minimum requirements. Simply increasing the equipment grounding conductor size is not adequate in all cases.

The minimum size wire-type EGC for a 400A feeder is normally a 3 AWG copper, according to Table 250.122. The circular mil value of a 3 AWG conductor is 52,620 circular mils, as provided in NEC Table 8, Chapter 9. Take the value of 52,620 circular mils and multiply by 1.3 to come up with 68,406.

Take this value back to NEC Table 8, Chapter 9 and round up to the next-higher value (in the third column) to determine the minimum size EGC as adjusted proportionately to the increase in size for the ungrounded phase conductors of the circuit. The next-higher circular mil value in Table 8 is 83,690. Therefore, the new minimum size required for this EGC is a 1 AWG copper, based on the adjustment.

It is always best to perform this simple calculation to verify that the adjusted size of the EGC meets or exceeds the minimum requirements. Simply increasing the EGC size is not adequate in all cases.

Multiple circuits in the same raceway

It is common for installers to economize in construction installation methods. One way is to combine electrical circuits into a single raceway, rather than running individual conduits or cables. This combination offers the advantage of requiring only a single EGC in the run. 

If multiple circuits are installed in a single raceway, cable or cable tray, a single EGC is permitted. The sizing requirement is based on Table 250.122 using the rating of the largest overcurrent protective device ahead of any circuit in the raceway, cable, trench or tray.