When complying with the requirements of Article 250, Grounding and Bonding, often we can get confused as to who, what, when, and where. Since the 2014 National Electric Code, sizing grounding conductors became a little bit more simplified, in my opinion, just by adding one new Table. I will attempt to share the complex by explaining to you how I see it.
Let us review some significant terms as we explore this topic. Knowing what a grounding and bonding conductor is called can help you apply the correct installation requirements and make a safer electrical system.
First the Code’s definition is located in Article 100.
“Grounding Electrode Conductor (GEC): A conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system.”
This is the wire that connects a service or a transformer of a grounded system to the earth and attaches to the earth through a ground rod, water pipe, or “Ufer” ground as applicable. To size this conductor, you simply use Table 250.66. If the connection is to a ground rod it is not required to be larger than a 6 AWG copper. If you are connecting to a concrete encased rebar or a Ufer, it is not required to be larger than a 4 AWG copper. If you connect to a ground ring, then it is typically not larger than the conductor used for the ground ring except it must be at least a 2 AWG. All other electrodes require the use of the table and the largest size in all cases would be 3/0 AWG copper.
The next word from the 2023 edition of the NEC.
“Equipment Grounding Conductor (EGC): A conductive path(s) that is part of the effective ground-fault current path and connects normally non-current carrying metal parts of equipment together and to the grounded conductor (neutral) or to the grounding electrode conductor, or both.”
This is permitted to be a wire, a metal raceway or conduit, metal armor of a cable assembly, and the metallic enclosures that contain electrical parts. 250.118 contains a list of qualifying EGC’s. This includes the green wire often known to us in the field as “the ground.” These conductors’ bond metallic parts together as part of the effective ground-fault path back to the source of power. When a ground-fault occurs, the current is temporarily routed on this path and when the current become high enough the fuse or circuit breakers open quickly and renders the system safe. These conductors are required for feeders and branch circuits. To size this type of conductor, we use Table 250.122. The key to properly using this Table is to make sure you insert the words “Not Exceeding” when applying the table to the OCPD size protecting the circuit.
If the conductor is not one of these then you are working with a conductor that must comply with 250.102 and the table contained there. This would include main and system bonding jumpers, supplied-side bonding jumpers, and the minimum size grounded (neutral) conductor required to carry ground faults back to the source. Once you calculate the neutral conductor load in accordance with Article 220, it must be compared to the minimum size as required in Table 250.102(C)(1).
This table looks very similar to Table 250.66 except when the total circular mills of the ungrounded conductors exceed 1100 kcmil. Then, Note 1 applies and you are required to apply the 12.5 % rule and multiple the kcmil of the largest ungrounded conductors by 12.5 % to calculate the size of the bonding jumpers required.
So, to summarize this in a simple application.