Code Change Summary: Revised code language clarifies when the minimum required size of an equipment grounding conductor must be increased in size.

Note: The 2020 NEC^® book text in 250.122(B) shows this as a new code section which is incorrect. This is a revision of the existing section.

Often, the ungrounded conductors of a circuit must be increased in size in order to be able to sufficiently carry the load. The two most common reasons an ungrounded circuit conductor is increased in size are:

1. Voltage drop. Longer runs of wire results in greater impedance which means less current at the load end of the wire where the equipment is connected.
2.  Conditions of use (how the conductor is installed or used). This includes things like the ambient temperature around the wire and number of current carrying conductors in the raceway. These conditions may warrant the need to increase the size of a conductor.

In the 2011 NEC^®, the code language in 250.122(B) required that any time an ungrounded conductor of a circuit was increased in size (for any reason); the equipment grounding conductor of the circuit was also to be increased in size, the same proportional amount. This code language was restrictive and unnecessary which prompted a change in the 2014 code cycle.

In the 2014 NEC^®, the section was revised to read as follows:

2014 Code Language:

250.122(B). Where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors.

The key words in the above 2014 revision are "the minimum size that has sufficient ampacity for the intended installation".

Increasing an ungrounded conductor in size because of conditions of use does not necessarily create the need to also increase the equipment grounding conductor of the circuit. If it has been determined that a 6 AWG copper ungrounded circuit conductor is the smallest size wire that can carry a 20 amp load after applying adjustment factors for high ambient temperature, then the 6 AWG conductor is now "the minimum size that has sufficient ampacity for the intended installation".

According to the 2014 code language, if the 6 AWG conductor is connected to a 20-amp overcurrent device, the minimum required size copper equipment grounding conductor (12 AWG per Table 250.122) can still perform to carry 20 amps of ground fault current under these circumstances.

On the other hand, an increase in the size of an ungrounded circuit conductor because of voltage drop, creates the need to also increase the size of the equipment grounding conductor of the circuit. If there is significant impedance in an ungrounded conductor because of voltage drop at the end of a long run of wire, then it won’t perform well. The same issue occurs with the equipment grounding conductor. Installing an equipment grounding conductor of the proper size ensures an effective ground-fault current path. An increase in the size of the equipment grounding conductor means less resistance. This results in the overcurrent device tripping quicker during a ground-fault condition.

The above concept is the similar in the 2020 NEC^®, but with a few additions. Now, if ungrounded conductors are increased in size “for any reason” besides ambient temperature correction factors required in 310.15(B) or adjustment factors required when there are more than three current-carrying conductors per 310.15(C), wire-type equipment grounding conductors, must be increased in size proportionately to the increase in circular mil area of the ungrounded conductors. “For any reason” could apply to a situation where  the ungrounded conductors were increased in size from 2 AWG to 1 AWG because the electrician was out of 1 AWG wire.

The exception to this section is new in 2020 and allows equipment grounding conductors to be sized by a qualified person. Interesting. Likely the inspection authority will decide who that person is.

Below is a preview of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC^® edition of NFPA 70.