How this calculation works
Equipment grounding conductor, Table 250.122
The EGC is the fault-clearing path. It runs with the circuit conductors and its only job is to carry fault current back to the source fast enough and hard enough to open the breaker. Because its job is defined by the breaker, it is sized from the breaker — not from the load, and not from the circuit conductors.
A 100 amp feeder gets an 8 AWG copper EGC whether the conductors are 3 AWG or 1/0.
The upsizing rule. 250.122(B) says that if you increase the ungrounded conductors for any reason — voltage drop is the usual one — you have to increase the EGC proportionally by circular mil area. Go from 3 AWG to 1/0 for a long run and the 8 AWG ground goes up too. This gets missed constantly and inspectors look for it.
The EGC also never has to be larger than the circuit conductors it runs with.
Grounding electrode conductor, Table 250.66
The GEC connects the system to earth. It is sized from the largest ungrounded service-entrance conductor, and then — this is the part that matters — it gets capped depending on what electrode it lands on.
- To a rod, pipe or plate electrode: never required to be larger than 6 AWG copper.
- To a concrete-encased electrode: never required to be larger than 4 AWG copper.
- To a ground ring: never required to be larger than the ring conductor itself.
Those caps in 250.66(A) through (C) are why a 200 amp house with a driven rod takes 6 AWG regardless of what the main table shows for 2/0 service conductors.
The distinction that matters
Earth does not clear faults. Soil resistance is far too high to carry enough current to open a breaker. The GEC's job is to reference the system to earth and drain lightning and surge energy. Fault clearing is entirely the EGC's job, back through the bonding path to the source.
People who conflate the two end up believing a ground rod protects them from a fault. It does not.
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Frequently asked questions
Why is the EGC sized from the breaker and not the wire?
Because its job is to carry enough fault current to open that specific breaker, fast. The conductor size tells you about the load; the breaker tells you how much current has to flow to clear a fault. Table 250.122 is built from the breaker for exactly that reason.
Do I always need to upsize the ground when I upsize the wire?
Whenever you increase the ungrounded conductors above the minimum required — voltage drop, future capacity, anything — 250.122(B) requires a proportional increase in the EGC by circular mil area. It is one of the most commonly missed items on a rough-in inspection.
Is 6 AWG really enough for a 200 amp service ground rod?
Yes. 250.66(A) caps the grounding electrode conductor at 6 AWG copper where the sole connection is to a rod, pipe or plate electrode, regardless of service size. Earth cannot carry enough current to justify anything larger.
Can the metal raceway be the equipment ground?
For several wiring methods, yes. EMT, IMC, RMC and properly listed MC and AC cable are all recognized as equipment grounding conductors under 250.118. PVC and ENT are not — those require a separate wire-type EGC pulled with the circuit.
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