The other grounding table, and the one people reach for by mistake
Sized from the overcurrent device ahead of it, not from the conductor beside it, and not from the service size.
On this page
Which conductor this is
The equipment grounding conductor runs with the circuit conductors and bonds metal back toward the source. It is the fault-current path.
It is not the grounding electrode conductor, which runs from the service to an earth electrode and is sized completely differently.
Confusing the two is the most common error in Article 250, and it is easy to make because both are in the same article and both produce a wire size.
The test: ask what the conductor terminates on. An electrode means grounding electrode conductor. Equipment means equipment grounding conductor.
The input is the device rating
Size the equipment grounding conductor from the rating of the overcurrent device ahead of the circuit (NEC 250.122).
Not from the circuit conductor size. Not from the load. From the device.
That is deliberate: the conductor has to survive long enough for that device to open, so the device rating is the relevant quantity.
Look the value up in your own book. Table 250.122 is short and it is worth an individual tab.
When it has to be upsized
If the circuit conductors are increased in size for any reason, such as voltage drop, the equipment grounding conductor is increased proportionally.
That catches people. You upsize the phase conductors for a long run, feel finished, and leave the grounding conductor at the table value.
The reasoning is impedance: a longer or larger circuit has a longer fault path, and the grounding conductor has to keep up with it or the device will not see enough current.
Parallel runs
Where conductors are run in parallel in separate raceways, each raceway gets an equipment grounding conductor, and each is sized from the device rating rather than from a share of it.
Full size in each. Not divided.
That is counterintuitive and it is exactly why it appears on exams.
What this page cites
- NEC 250.122 Sizing equipment grounding conductors from the overcurrent device rating.
- NEC 250.66 The other table, for the grounding electrode conductor.
- NEC 250.4 Why the fault-current path has to be low impedance.