What size grounding electrode conductor do I need for ground rods on a 400 amp service?

6 AWG copper. NEC 250.66(A) caps the conductor to a rod, pipe or plate electrode at 6 AWG copper or 4 AWG aluminum regardless of service size.

What size GEC goes to the cold water pipe on a 400 amp service?

There is no cap for a metal underground water pipe electrode, so it comes from Table 250.66. A 400 amp service using two parallel sets of 4/0 copper equals 422,000 circular mils and requires 1/0 copper.

What is the largest GEC size needed for a 400 amp residential service to ground rods?

Q: What size conductor goes to a concrete encased electrode?

NEC 250.66(B) caps the conductor to a concrete encased electrode at 4 AWG copper.

6 AWG copper. Section 250.66(A) in NEC 2023 states that where the grounding electrode conductor is connected to a rod, pipe or plate electrode, or to any part of the grounding electrode system that is a rod, pipe or plate electrode, that portion of the conductor is not required to be larger than 6 AWG copper or 4 AWG aluminum. That cap applies no matter how large the service is, so a 400 amp residential service and a 100 amp service get the same 6 AWG run to the ground rods.

Why the cap exists

The size of a grounding electrode conductor in Table 250.66 scales with the service entrance conductors because the conductor may have to carry fault current. A driven rod cannot carry meaningful fault current. Even a well installed rod pair typically presents 25 ohms or more of resistance to earth, which at 240 volts limits current to under 10 amps. Running 1/0 copper to a rod does not improve anything, so the code caps it where the physics stop paying off.

What the rest of the electrode system needs

The cap is electrode specific, so the other connections in the same grounding electrode system may need larger conductors. Section 250.66(B) caps the conductor to a concrete encased electrode at 4 AWG copper. Section 250.66(C) caps the conductor to a ground ring at the size of the ring conductor, which is 2 AWG minimum under 250.52(A)(4). For a metal underground water pipe electrode there is no cap, so the conductor comes straight out of Table 250.66 based on the service entrance conductor size. For a 400 amp service built on two parallel sets of 4/0 copper, the equivalent area is 422,000 circular mils, which lands in the over 350 through 600 kcmil row and calls for 1/0 copper.

That is the answer to the common pairing of this question. Ground rods get 6 AWG, the cold water pipe bond and grounding electrode connection gets the full Table 250.66 size, typically 1/0 copper on a 400 amp service with parallel 4/0.

Installation details that get cited

A 6 AWG grounding electrode conductor run to a rod must be protected from physical damage or run along the building surface without protection, per 250.64(B). It must be continuous without splice except by exothermic welding or irreversible compression connectors under 250.64(C). Two rods are required unless a single rod is tested at 25 ohms or less, per 250.53(A)(2). The connection at the rod must be listed for direct earth burial, which is what the acorn clamp marking DB means. Ferrous raceway protecting the conductor must be bonded at both ends under 250.64(E) or the choke effect defeats the connection.

Field takeaway

6 AWG copper is the largest grounding electrode conductor required to ground rods on any service. Size the water pipe and concrete encased connections separately, since those follow different rules in 250.66.