250.122(B) Size of Equipment Grounding Conductors. Increased in Size.
Code Change Summary: New text was added to clarify when the size of an equipment grounding conductor must be increased in size. The new text requires a careful read and a bit of deep thought in order to fully grasp the impact of the change.
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:
- 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.
- Conditions of use (how the conductor is 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.
The previous 2011 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 too restrictive and unnecessary.
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.
Increasing an ungrounded conductor in size because of conditions of use does not always 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 new 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.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).
2011 Code Language:
250.122(B) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors.
2014 Code Language:
250.122(B) Increased in Size. 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.