Grounding Electrode Conductor Size: How to Read NEC Table 250.66

The grounding electrode conductor is sized from the largest hot service-entrance conductor, using Table 250.66 of the 2020 NEC (NFPA 70). A 200-amp service on 2/0 copper needs a 4 AWG copper grounding electrode conductor. If it runs only to ground rods, 6 AWG copper is the most the NEC requires. To a concrete-encased electrode, the most is 4 AWG copper.

Grounding electrode conductor size calculator (Table 250.66)

Table 250.66 works from your largest ungrounded (hot) service conductor. The bigger that conductor, the bigger the grounding electrode conductor, topping out at 3/0 copper or 250 kcmil aluminum. For parallel sets, the calculator adds the areas of one phase's conductors.

Sizes are AWG or kcmil. The table also applies at a building fed by a feeder or branch circuit, and at a separately derived system.

When can the conductor be smaller than the table?

The table size applies to any electrode, such as a metal water pipe. Three electrode types cap it lower, but only if the conductor stops there:

• Rods, pipes and plates: never more than 6 AWG copper or 4 AWG aluminum, per 250.66(A). These are the 8-foot rods and pipes and 2-square-foot plates of 250.52(A)(5) and (A)(7).
• Concrete-encased electrode: never more than 4 AWG copper, per 250.66(B). See the Ufer section below.
• Ground ring: never more than the ring wire, per 250.66(C). A ring is at least 20 feet of bare copper, 2 AWG or larger.

Concrete-encased electrode (Ufer) GEC size

A concrete-encased electrode is often called a Ufer ground. The grounding electrode conductor to it never has to be larger than 4 AWG copper, even on a large service, under 250.66(B). That holds only if the conductor stops there and doesn't continue on to an electrode that needs a larger size.

• What counts, 250.52(A)(3): at least 20 ft of bare, galvanized or other conductive-coated rebar ½ in. or larger, or 20 ft of bare copper 4 AWG or larger.
• Where: covered by at least 2 in. of concrete, in a footing or foundation that's in direct contact with the earth. Concrete sitting on insulation or a vapor barrier doesn't count.
• Required if present, 250.50: bond it into the grounding electrode system. In an existing building, you can skip it if you can't reach the rebar without disturbing the concrete.

Grounding electrode conductor size for a 200-amp service

A house has a 200-amp service on 2/0 copper (from Table 310.12). It has a metal water pipe, two ground rods and a concrete-encased electrode.

1. Row: 2/0 copper is in "2/0 or 3/0."
2. Size: 4 AWG copper or 2 AWG aluminum.
3. Water pipe: no cap, so 4 AWG copper.
4. Ground rods: the jumper serves only the rods, so 6 AWG copper.
5. Concrete-encased electrode: capped at 4 AWG copper, the same as the table here.

On 4/0 aluminum service conductors, the "4/0 or 250" row gives the same answer.

Example: parallel service conductors

Parallel sets are sized by their combined area. Two sets of 350 kcmil copper per phase is 2 × 350 = 700 kcmil. That's the "over 600 through 1100" row: 2/0 copper or 4/0 aluminum. A jumper only to the rods is still 6 AWG copper.

Installation rules

• Protection, 250.64(B): 6 AWG or larger can run uncovered if not exposed to damage. Smaller goes in RMC, IMC, Schedule 80 PVC, RTRC-XW, EMT or cable armor.
• Several electrodes, 250.64(F): size for the largest conductor any connected electrode needs.
• Aluminum, 250.64(A): outdoors, don't terminate within 18 inches of the earth.

Common Table 250.66 exam traps

• Sizing by the breaker. That's Table 250.122, for equipment grounding conductors.
• Using Table 250.66 for the main bonding jumper. See JCR's main bonding jumper guide.
• Using the 6 AWG rod cap on a run that continues to the water pipe. The cap applies only if it stops at the rods.

Sizing the service conductors first? Start with JCR's dwelling service load calculation.

Grounding electrode conductor questions

What size grounding electrode conductor do I need for a 200 amp service?

With 2/0 copper or 4/0 aluminum service conductors, 4 AWG copper or 2 AWG aluminum. A jumper only to ground rods can be 6 AWG copper. Larger service conductors may need more.

What size wire goes to a ground rod?

Never more than 6 AWG copper or 4 AWG aluminum, under 250.66(A). That holds if the conductor stops at the rods. A small service may need less, such as 8 AWG copper.

What size grounding electrode conductor goes to a Ufer ground?

Never more than 4 AWG copper, under 250.66(B), if the conductor stops at the concrete-encased electrode. With service conductors of 2 AWG copper or smaller, the table already gives 8 AWG copper.

What size grounding electrode conductor for a 100 amp service?

Table 310.12 puts a 100-amp dwelling service on 4 AWG copper or 2 AWG aluminum. That's the top row of Table 250.66: 8 AWG copper or 6 AWG aluminum. An 8 AWG conductor must be in a raceway or cable armor.

Can a grounding electrode conductor be spliced?

Only with irreversible compression connectors listed as grounding and bonding equipment, or exothermic welding. Section 250.64(C) also covers busbar sections, structural steel and metal water piping connections.

Is the grounding electrode conductor the same as the equipment grounding conductor?

No. The grounding electrode conductor connects the service to the earth electrodes and is sized from Table 250.66. The equipment grounding conductor runs with each circuit and is sized from Table 250.122 by the breaker or fuse rating.

Related guides: Main bonding jumper size (250.28) · 200 amp service wire size (Table 310.12) · Dwelling service load calculation

More worked examples: equipment grounding conductor size (Table 250.122) and all NEC Answers.