Grounding electrode conductor size calculator

How the conductor size is worked

The grounding electrode conductor connects the grounded conductor and the grounding electrode system to earth. NEC Table 250.66 sizes it from the largest ungrounded service-entrance conductor, or from the added area of parallel conductors, and not from the breaker. Four electrode types follow the table in different ways: a metal water pipe or building steel takes the table size, and a ground rod, a concrete-encased electrode and a ground ring each cap the size for the run to that electrode.

Equivalent area

A = n x (circular mils of one conductor)One conductor's area comes from NEC Chapter 9, Table 8: 3/0 AWG is 167,800 circular mils and 4/0 AWG is 211,600. A kcmil size is 1,000 circular mils per kcmil. Parallel conductors are added and treated as one: four 250 kcmil aluminum conductors are 1,000 kcmil, and three 600 kcmil copper conductors are 1,800 kcmil.

Table 250.66 row

the first row whose upper limit is at or above A, in the column of the service conductor materialCopper column, with the copper and aluminum GEC for each row, 2 or smaller: 8 AWG copper or 6 AWG aluminum; 1 or 1/0: 6 AWG copper or 4 AWG aluminum; 2/0 or 3/0: 4 AWG copper or 2 AWG aluminum; Over 3/0 through 350: 2 AWG copper or 1/0 AWG aluminum; Over 350 through 600: 1/0 AWG copper or 3/0 AWG aluminum; Over 600 through 1100: 2/0 AWG copper or 4/0 AWG aluminum; Over 1100: 3/0 AWG copper or 250 kcmil aluminum. The aluminum or copper-clad aluminum column uses the same GEC sizes against these service sizes: 1/0 or smaller; 2/0 or 3/0; 4/0 or 250; Over 250 through 500; Over 500 through 900; Over 900 through 1750; Over 1750. "Through" includes the upper size, so a 600 kcmil copper service is in the row for 1/0 AWG and a 700 kcmil service is in the next row, 2/0 AWG.

Rod, pipe or plate

GEC = lesser of the table size and 6 AWG copper (4 AWG aluminum)NEC 250.66(A): where the conductor goes to rods, pipes or plates and no further to an electrode that needs more, it need not be larger than 6 AWG copper or 4 AWG aluminum. A 3/0 AWG copper service has a table size of 4 AWG copper, and the limit makes it 6 AWG copper. A 2 AWG copper service has a table size of 8 AWG, and the limit does not raise it.

Concrete-encased electrode

GEC = lesser of the table size and 4 AWG copperNEC 250.66(B): no larger than 4 AWG copper, with the same condition on other electrodes. IAEI adds that aluminum conductors are not permitted for this connection. A 1,800 kcmil copper service (3/0 AWG to a water pipe) takes 4 AWG copper to the concrete-encased electrode.

Ground ring

GEC = lesser of the table size and the ground ring conductorNEC 250.66(C): no larger than the ground ring conductor, which is bare copper of 2 AWG at the smallest (250.52(A)(4)). A ring of 2 AWG copper limits the 1,800 kcmil example to 2 AWG. A 500 kcmil copper service on a 4/0 AWG ring keeps the table size, 1/0 AWG copper, because a limit never raises a size.

Not the equipment grounding conductor

GEC: service conductor size, Table 250.66. EGC: breaker rating, Table 250.122The equipment grounding conductor of a circuit or feeder is sized from the rating of the overcurrent device ahead of it. IAEI's example is a 200 A device, which takes 6 AWG copper from Table 250.122. The grounding electrode conductor for a 3/0 AWG copper service is 4 AWG copper. The two tables answer different questions.

Eight services through the same table

The first five rows are the worked examples of an IAEI Magazine article by the chair of NEC code-making panel 7, the sixth is the 1,200 A service of a Mike Holt column and the seventh is the example of an NFPA fact sheet. This page gives the same sizes. Those sizes apply to a metal water pipe or building steel, where the full table applies.

The same 1,800 kcmil copper service as the sixth row, on a daisy chain of electrodes: 3/0 AWG copper to the water pipe, 2 AWG copper to a 2 AWG ground ring, 4 AWG copper to the concrete-encased electrode and 6 AWG copper to the ground rods. IAEI notes that if that sequence is changed, the step-down is not available.

Standards and data sources

• NEC 2023, 250.66 and Table 250.66, 250.64, 250.52(A) and 250.53 - the table of grounding electrode conductor sizes, the limits for rod, concrete-encased and ground ring electrodes, and the installation rules. The NEC text was read as NFPA, IAEI and EC&M print it, not in NFPA's viewer, and the sources are the 2005, 2017 and 2020 editions. None shows a different table. Check the edition your AHJ adopted.
• NFPA fact sheet, Grounding and Bonding: Using the Tables in Article 250 of the NEC (June 2020) - the seven rows of Table 250.66 for copper and for aluminum or copper-clad aluminum, the three notes under the table, and the example of two 500 kcmil copper conductors per phase, 1,000 kcmil, which gives 2/0 AWG copper. The Tucson Electric Power sheets SR-453 and SR-601 print the same seven rows.
• L. Keith Lofland in IAEI Magazine, Sizing of Conductors Related to Grounding & Bonding (2021) - the 8 AWG copper and 6 AWG aluminum minimum, the 3/0 copper and 250 kcmil aluminum maximum, the five worked examples on this page, the limits for rods, concrete-encased electrodes and ground rings, the 2017 change to the wording of those limits, and the contrast with the equipment grounding conductor.
• Mike Holt in EC&M, Code Q&A: Sizing the GEC (2017) - the rule that a water pipe or building steel electrode is sized from Table 250.66 and that rods and concrete-encased electrodes can use 250.66(A) and (B), with 6 AWG and 4 AWG copper as the largest.
• Mike Holt in EC&M, Grounding vs Bonding, Part 7 of 12 (2005) - the 1,200 A service on three parallel 600 kcmil copper conductors per phase, 1,800 kcmil, which gives 3/0 AWG copper, and the ground ring of 20 ft of 2 AWG copper. It is the 2005 NEC. The installation notes on this page use the 2020 text from the 2021 column below.
• Mike Holt in EC&M, Stumped by the Code? NEC Requirements for the Installation of GECs (2021) - the 2020 text of 250.64(A) for aluminum and copper-clad aluminum (corrosive conditions, direct contact with concrete, no termination within 18 in. of the earth outside) and of 250.64(B) for protection, used for the installation notes on this page.

Limitations of this calculator

• This page gives preliminary numbers for estimating, for checking a plan and for studying the table. A licensed electrician sizes and installs the grounding system under the permit and the inspection of the local authority having jurisdiction (AHJ). Nothing here is an approval of an installation.
• The NEC text was read as an NFPA fact sheet, IAEI Magazine, EC&M columns and two utility sheets print it, not in NFPA's own viewer. They cover the 2005, 2017 and 2020 editions and agree on the table. The 2023 text was not read directly, and this page names no change in it. Check the edition your AHJ adopted.
• The page works the grounding electrode conductor of a service, of a building supplied by a feeder, or of a separately derived system, from the ungrounded conductor size. It does not size the equipment grounding conductor (Table 250.122), the grounded conductor, the main or system bonding jumper (Table 250.102(C)(1)), or a supply-side bonding jumper.
• Table 250.66 has notes. Note 1 sizes from the largest sum of the areas when several sets of service-entrance conductors connect to one service drop or lateral, and the page does that only if you enter the largest sum as parallel conductors. Note 2 applies where there are no service-entrance conductors: the size is the equivalent of the largest service-entrance conductor the load requires, which you have to work out and enter.
• The smaller sizes of 250.66(A), (B) and (C) are for the conductor that ends at that electrode. From the 2017 NEC on, they apply where no other electrode that needs a larger conductor is connected on beyond it. Earlier editions allowed them only for the part of the conductor that is the sole connection to the electrode. A run that goes to a ground rod first and then to a water pipe is sized from Table 250.66 all the way. The page sizes one run to one electrode type.
• Under 250.64(A) of the 2020 NEC, as Mike Holt printed it in EC&M in 2021, bare or covered aluminum or copper-clad aluminum without an extruded polymeric covering is not installed where subject to corrosive conditions or in direct contact with concrete, and outside a building or enclosure it is not terminated within 18 in. of the earth. Older editions also kept it off masonry and the earth. IAEI (Lofland) adds that aluminum is not permitted for the connection to a concrete-encased electrode, so the page returns copper there. Whether a given aluminum conductor can be used on a ground ring is for the inspector.
• Installation rules are not checked. For the 2020 NEC, Holt prints that a GEC of 6 AWG or larger may run exposed along the building surface if securely fastened and not exposed to physical damage, one exposed to physical damage goes in RMC, IMC, Schedule 80 PVC, RTRC-XW, EMT or cable armor, and one smaller than 6 AWG always does (250.64(B)). The rules for splices, taps, termination and bonding the raceway are in 250.64 and 250.70.
• Electrode requirements are not checked. A ground ring is at least 20 ft of bare 2 AWG copper in contact with the earth, and a metal water pipe electrode needs a supplemental electrode (250.53(D)(2), as Holt printed it for 2005). The page does not size the electrodes, the soil resistance or the number of rods.
• Multiple-enclosure services (250.64(D)), services over 1000 V and DC systems are not covered. The sizes are minimums. A larger grounding electrode conductor is allowed.

Common mistakes with grounding electrode conductors

1. Sizing the grounding electrode conductor from the breaker. Table 250.66 goes by the size of the service conductors, not the breaker or the amps. A 3/0 AWG copper service carries 200 A in the 75°C (167°F) column of Table 310.16, and its grounding electrode conductor is 4 AWG copper. The equipment grounding conductor for a 200 A device is another table, 250.122, and gives 6 AWG copper (IAEI). Using one answer for the other undersizes or oversizes the wire.
2. Using the 6 AWG ground rod allowance for a run that goes on to a water pipe. The 6 AWG copper limit of 250.66(A) holds for the conductor that ends at the rod. If the same conductor is run to the rod first and on to a metal water pipe, it is sized from Table 250.66 as if it went to the pipe. For the 600 kcmil x 3 copper service that is 3/0 AWG copper, not 6 AWG.
3. Putting 6 AWG or 4 AWG on a water pipe or building steel. The step-down is only for rods, pipes, plates, concrete-encased electrodes and ground rings. A metal water pipe or building steel electrode takes the full table. A 500 kcmil copper service needs 1/0 AWG copper to a water pipe, and the 6 AWG copper that is enough for a rod is not.
4. Using aluminum on a concrete-encased electrode or in the ground. Aluminum is not permitted for the connection to a concrete-encased electrode, so a 3/0 AWG copper service with an Ufer ground takes 4 AWG copper, never 2 AWG aluminum. Aluminum also has installation limits near concrete, corrosion and the earth under 250.64(A).
5. Sizing from one conductor when the service is paralleled. Parallel conductors are added. A single 250 kcmil aluminum conductor is 250,000 circular mils and gives 4 AWG copper or 2 AWG aluminum. Four of them are 1,000 kcmil and give 2/0 AWG copper or 4/0 AWG aluminum. Three 600 kcmil copper conductors are 1,800 kcmil and give 3/0 AWG copper, not the 1/0 AWG of one.
6. Reading the top of a row as the start of the next one. "Over 350 through 600" includes 600 kcmil. A 600 kcmil copper service takes 1/0 AWG copper, and a 700 kcmil copper service takes 2/0 AWG. An aluminum service of 250 kcmil is in the "4/0 or 250" row, 4 AWG copper, and 300 kcmil is already in the next row.

Common questions about grounding electrode conductors

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