Article 430, Part XIII provides the NEC requirements for motor grounding. This is a problem because motors are loads, not power sources. Therefore, they are bonded, not grounded. There is, in fact, nothing in Part XIII about grounding.
Does this seem confusing? If so, refer to the Art. 100 definition of “ground,” which is the earth. What is meant here is bonding; more specifically, equipment bonding. This means a metallic pathway between equipment. Part XIII clarifies this in its last section, 430.25 Method of Grounding. There, it says the connection to the equipment grounding (bonding) conductor must be done per Part VI of Art. 250. When you read those requirements, you see things like “bonding jumpers” and “permanent wiring methods,” but you do not see anything about ground rods or any other kind of earth connection. Further insight is gained by reading 250.118 Type of Equipment Grounding Conductors. You see metallic raceways but not anything that is stuck into the ground.
The “equipment grounding conductor” is actually a system of bonding conductors, and this system is ultimately connected to the grounding system at some point. However, there is no direct connection of equipment to ground because the high impedance of the earth does not provide a sufficient path back to the source. Also, because of this impedance, it is physically impossible to create an equipotential plane via grounding.
You can have a lethal voltage between two ground rods only 20 ft apart, but you will not have a voltage of any significance between two properly bonded cabinets that are 100 ft apart. This fact is why, when you have multiple utility ground rods at a service (one for power, one for telephone, one for gas, etc.), you always bond those ground rods together. This, not grounding, prevents a difference of potential between them.
Also, note that load-side grounding connections are generally prohibited [250.24(A)(5)].
You must bond a stationary motor if any of the four conditions in 430.242(1) through (4) apply. For example, the voltage of any terminal is over 150V to ground. If a portable motor operates at over 150V to ground, ensure it’s bonded or guarded.
And here you see we have yet another use of the word ground; this can also be confusing. In this case, we are considering the system voltage where there’s a grounded conductor.
