Grounding Electrode Conductor Sizing Card

Why this matters

On a service change, the grounding electrode conductor is the wire inspectors check with a gauge in hand, and it is the one most often left at its old size when the service gets bigger. A 100 A service upgraded to 200 A keeps its 8 AWG copper to the water pipe, and the job fails final. Going the other way, electricians buy and run 4 AWG to a pair of ground rods because "the table says 4," when 6 AWG is all the code asks for on that run. This card is the sizing table for the GEC, the two caps that matter in houses, and the line between the GEC and the two conductors it gets confused with.

Three conductors, three different jobs

Name the conductor before you size it, because each is sized from a different number.

• Grounding electrode conductor (GEC). Connects the service's grounded conductor (at the service equipment) to the grounding electrode system: the water pipe, the rods, the concrete-encased electrode. It is sized from the size of the service-entrance conductors under Table 250.66. The breaker rating does not enter into it.
• Main bonding jumper (MBJ). Connects the grounded (neutral) conductor to the equipment grounding terminal bar and the enclosure inside the service disconnect. It is sized from the same service-conductor size under 250.28(D), using Table 250.102(C)(1), whose rows match Table 250.66 up to 1100 kcmil copper and then switch to a 12.5 percent rule above it.
• Equipment grounding conductor (EGC). Runs with each circuit and feeder to bond equipment and carry fault current back to the source. It is sized from the overcurrent device rating under Table 250.122. That table belongs to its own card: see related 'Equipment Grounding Conductor Sizing (NEC 250.122)'.

The usual error is using the EGC logic, "200 A breaker, so 6 AWG," on a GEC. On a 200 A service with 2/0 copper conductors, Table 250.66 says 4 AWG copper to the water pipe, and the breaker never comes into it.

The card

NEC 2023 (2020 where the section moved), as adopted by your jurisdiction, local amendments checked. The left columns are the largest service-entrance conductor, or the equivalent area of parallel conductors added together. The right column is the copper GEC.

TABLE 250.66 - COPPER GEC FROM SERVICE CONDUCTOR SIZE
COPPER SERVICE          ALUMINUM SERVICE       CU GEC
2 AWG or smaller        1/0 or smaller         8 AWG
1 or 1/0                2/0 or 3/0             6 AWG
2/0 or 3/0              4/0 or 250 kcmil       4 AWG
over 3/0 to 350 kcmil   over 250 to 500        2 AWG
over 350 to 600 kcmil   over 500 to 900        1/0
over 600 to 1100 kcmil  over 900 to 1750       2/0
over 1100 kcmil         over 1750              3/0
--------------------------------------------------------
CAPS ON THE PORTION TO ONE ELECTRODE, 250.66(A)-(B):
 rod, pipe or plate only:   need not exceed 6 AWG Cu
 concrete-encased only:     need not exceed 4 AWG Cu
COMMON DWELLING SERVICES (310.12 sizes, as adopted):
 100 A  4 Cu or 2 Al      -> 8 AWG Cu GEC
 150 A  1 Cu or 2/0 Al    -> 6 AWG Cu GEC
 200 A  2/0 Cu or 4/0 Al  -> 4 AWG Cu GEC

The bottom block gives the GEC for the service conductor sizes 310.12, as adopted, permits for 100, 150 and 200 A dwelling services. If the service was installed with larger conductors than the minimum, size the GEC from the conductors actually installed.

The caps, and why they exist

The table sizes a GEC for the worst case: an electrode with a low resistance to earth that could carry real fault or lightning current, such as a long run of metal underground water pipe. Some electrodes cannot draw that much current, so the code stops requiring a larger conductor to them.

• Rods, pipes and plates: 6 AWG copper maximum required (250.66(A)). A driven rod's resistance to earth is high enough that a conductor larger than 6 AWG copper adds nothing. This cap applies to the portion of the GEC that is the sole connection to the rod, pipe or plate. The jumper between two rods is in the same position.
• Concrete-encased electrode: 4 AWG copper maximum required (250.66(B)). A footing electrode can carry more current than a rod, so the cap is one size larger.
• What the caps do not do. They do not apply to the GEC to a metal underground water pipe. If one GEC runs from the service to the water pipe and then on to the rods, the run to the water pipe takes the full table size; only the portion that serves solely the rods may drop to 6 AWG.

The caps are "need not exceed," not "must not exceed." Running 4 AWG to a rod is legal, just more copper than the code asks for. The violation is going under the table where the cap does not apply.

Worked example: a 100 A to 200 A service change

An existing house has a 100 A overhead service with 4 AWG copper service-entrance conductors. Its electrode system, as found: 8 AWG copper GEC to the metal underground water pipe within 5 ft of where it enters the house, and one ground rod with its own 8 AWG copper conductor. The house is on a slab with no accessible footing steel.

The new service is 200 A with 4/0 aluminum SE cable, the size 310.12 permits for a 200 A dwelling, as adopted.

1. Find the row. 4/0 aluminum falls in the "4/0 or 250 kcmil" aluminum row, so the table GEC is 4 AWG copper.
2. The water pipe. The water pipe is not a rod, pipe or plate electrode in the 250.66(A) sense (that cap is for made electrodes, 250.52(A)(5) and (A)(7)), so the full table applies. The 8 AWG GEC to the water pipe is now two sizes too small and gets replaced with 4 AWG copper. Keep it continuous from the service to the clamp: 250.64(C) allows splices only by irreversible compression connectors or exothermic welding (or at a busbar), in substance, so a split bolt in the middle of the run is not acceptable.
3. The rods. One rod by itself must have a resistance to earth of 25 ohms or less, or a second rod is required; in practice most shops drive the second rod rather than test, at least 6 ft from the first (250.53(A)(2) and (A)(3), as adopted). The conductor serving only the rods may be 6 AWG copper under the 250.66(A) cap. The old 8 AWG to the existing rod is replaced: it was below 6 AWG and would need protection anyway, and the second rod is joined with a 6 AWG copper bonding jumper. Driving and connecting the rods is a HowTo: see related 'How to Drive and Connect a Pair of Ground Rods'.
4. The water pipe as electrode needs a supplement. 250.53(D)(2) requires the metal water pipe electrode to be supplemented by another electrode, which the rods provide.
5. Interior water piping bond. Separately from the electrode, the interior metal water piping is bonded under 250.104(A), sized from Table 250.102(C)(1) off the service conductors: 4 AWG copper for 4/0 aluminum. On many houses the same 4 AWG conductor and clamp near the entrance serve both purposes. See related 'How to Bond the Water Pipe at the Service'.
6. The main bonding jumper. In a panel listed as service equipment, the MBJ is usually the screw or strap the manufacturer supplies; 250.28(D) and Table 250.102(C)(1) give 4 AWG copper equivalent for 4/0 aluminum if a wire-type jumper is used.

The before-and-after on that house: water-pipe GEC 8 AWG to 4 AWG copper; rod conductor 8 AWG to 6 AWG copper; one rod to two rods with a 6 AWG copper jumper.

This work happens on a service with the utility involved. The service conductors upstream of the main stay energized until the utility disconnects them, and a meter pull is the utility's call, not the electrician's, in most territories. The full sequence is in the existing service SOPs; this card covers only the conductor sizes.

Protection and routing of the GEC

The size you pick also decides how the GEC may be run, under 250.64(B), as adopted:

• 6 AWG copper and larger may run along the building surface without protection if it is securely fastened and not exposed to physical damage; where exposed, it goes in rigid metal conduit, IMC, Schedule 80 PVC, EMT or cable armor (the permitted list varies slightly by edition).
• 8 AWG copper is always protected by one of those raceways or armor.

A metal raceway enclosing a GEC must be bonded at both ends so it does not act as a choke on fault current (250.64(E)). Use a bonding bushing or a listed fitting at each end, or skip the metal raceway and use PVC of the schedule your edition permits where the GEC needs protection.

What changes the answer

• Parallel service conductors. Add the areas of the parallel sets and read the table at the equivalent size. Two sets of 3/0 copper total about 335 kcmil equivalent, which falls in "over 3/0 to 350 kcmil," so a 2 AWG copper GEC to the water pipe.
• Aluminum or copper-clad aluminum GEC. Table 250.66 has an aluminum GEC column with its own sizes and 250.64(A) restricts where aluminum may be run (not in direct contact with masonry or earth, not in corrosive conditions, and terminated a minimum distance above earth outdoors, in substance). This card is copper.
• A detached building or a separately derived system. A building fed by a feeder gets its own electrode system sized under 250.32, from the feeder conductors; a transformer or generator that is a separately derived system follows 250.30. See related 'The Detached Garage with a Bonded Neutral'.

References

• NFPA 70, National Electrical Code, 2023 edition as adopted by your jurisdiction: 250.28(D), 250.30, 250.32, 250.52(A), 250.53, 250.64, 250.66, Table 250.102(C)(1), 250.104(A), 310.12
• See related: Equipment Grounding Conductor Sizing (NEC 250.122)
• See related: Grounding + Bonding Fundamentals (NEC Article 250)
• See related: How to Drive and Connect a Pair of Ground Rods; How to Bond the Water Pipe at the Service