The NEC uses terms like equipment grounding conductor (EGC) but does not mean to actually provide grounding via that conductor. How can this be? Let’s first look at what the NEC considers to be suitable for use as an EGC [250.118]. It lists fourteen such items.
The first is a conductor. The next two are metallic conduits, followed by metallic tubing. Then two types of flexible metallic conduit followed by flexible metallic tubing. Then three types of metal-clad cable. The final four are cable trays (which must be used with the wiring methods listed in Table 392.10(A), cable bus framework (if bonded), other listed electrically continuous metal raceways and listed auxiliary gutters, and surface metal raceways listed for “grounding.”
You may notice that all of these EGC types are metallic and present extremely low impedance. You may also notice that “earth” or anything similar (e.g., a ground rod) is omitted from this list. That’s because earth presents an impedance that is not only far higher than that of metal but also varies considerably and thus does not eliminate differences in potential.
Now, let’s turn to Art. 100 to see the definition of “ground.” In the 2020 NEC, it was “the earth.” In the 2023 NEC, “ground” is no longer defined. But “grounded (grounding)” is. That definition is “Connected (connecting) to ground or to a conductive body that extends the ground connection.”
You have to look at the definitions of “ground fault” and “ground-fault current path” to find any mention of earth. But in those definitions, a connection to earth is one of several options.
This means you don’t get a straightforward definition of “ground” in the NEC. So how do you understand when the NEC means connecting to the earth versus making those metallic paths as per 250.118? Part of the answer lies in the definition of “bonding” in Art. 100: “Connected (so as) to establish electrical continuity and conductivity” (“so as” added by the author for clarity).”
The rest of the answer reveals itself as you come to understand Art. 250, Part V (bonding) and then the rest of Art. 250 (except Part IX). The short version of it is as follows.
The main job of grounding is to connect part of the power source to earth. You do not connect the utilization equipment to earth, but instead, you bond the metallic bodies of such equipment (not the current-carrying conductors) to the EGC and at some point that EGC connects to the grounding system. You may also have a grounded conductor (which is usually, but not necessarily, the neutral) that also is ultimately tied to ground.
An even simpler way to put it is you do not ground on the load side. Where the EGC essentially leaves the load side, you ground it there. If you have a grounded system, that system is grounded at the source (e.g., transformer) not at the utilization equipment.
Could you leave the EGC ungrounded? Yes, but then you’d have a difference of potential between it (plus all of the equipment connected to it) and the power source grounding system. The point of connecting the EGC to ground is not to “ground” the utilization equipment but to eliminate differences of potential between that equipment and the grounding means. It’s really bonding, when you think about it. This same principle is why we bond the ground rods of separate utility systems [Sec. 250.50].
Driving a ground rod next to the transformer supplying your lighting panel makes sense, since the transformer is a (separately derived) source. Driving a ground rod next to a motor makes no sense, since the motor is utilization equipment and the rod serves no electrical purpose.
