Sizing grounding electrode conductors

Grounding electrode conductor sizing is the most reliable source of points on a master exam's grounding section, and also one of the most reliable ways to lose them. The method takes thirty seconds once you know it: find the largest ungrounded service conductor, read one row of one table, and then ask whether the conductor goes only to a rod, a concrete-encased electrode or a ground ring, because each of those has a cap. Dana Whitfield missed four sizing questions on her first practice set. Two came from using the service rating instead of the conductor size. One came from forgetting to add parallel conductors. The fourth came from sizing a conductor to a rod at 2 AWG when the code never requires more than 6 AWG for that run. This lesson makes the method automatic.

The rules are in 250.62 through 250.68 of Part III, with Table 250.66 at the center. As always, we describe the table's structure and use illustrative values stated as this course's examples. The table itself is in your code book, and you should confirm every value you use.

Material and the basic sizing rule

Section 250.62 lets the grounding electrode conductor be copper, aluminum, copper-clad aluminum, or the items listed in 250.68(C), such as structural steel and interior water pipe within the limits that section sets. It may be solid or stranded, insulated, covered or bare. Most installations use bare or insulated copper, because aluminum and copper-clad aluminum carry installation restrictions near earth and masonry that this lesson explains later.

Section 250.66 sizes the conductor. The size depends on the size of the largest ungrounded service-entrance conductor, or the equivalent area for parallel conductors, and is read from Table 250.66. The table has columns for copper and for aluminum or copper-clad aluminum service conductors on the left, and for copper and for aluminum or copper-clad aluminum electrode conductors on the right. You find the row that covers your largest ungrounded service conductor and read across to the electrode conductor material you are using. Note what does not appear: the rating of the service overcurrent device. A 400 ampere service can have very different conductors depending on whether they are copper or aluminum, single or paralleled, and the table follows the conductors.

Where service-entrance conductors are run in parallel, you add their areas to find the equivalent area. Where there are multiple sets of service-entrance conductors supplying separate service disconnects, as permitted by 230.40, you use the equivalent area of the largest service-entrance conductor of each set added together. In both cases, Chapter 9, Table 8 gives the circular mil areas you need for AWG sizes.

The caps for rods, concrete-encased electrodes and rings

Section 250.66(A), (B) and (C) put upper limits on the size needed for certain runs. Where the grounding electrode conductor, or the portion of it, or a bonding jumper, connects only to a rod, pipe or plate electrode, it need not be larger than 6 AWG copper or 4 AWG aluminum. Where it connects only to a concrete-encased electrode, it need not be larger than 4 AWG copper. Where it connects only to a ground ring, it need not be larger than the conductor used for the ring.

These caps make physical sense. A rod has a resistance to earth of many ohms, so the current through it is limited by that resistance, and a conductor larger than 6 AWG would never be loaded. The concrete-encased electrode has lower resistance and so gets a slightly larger cap. The caps apply only to the run, or the portion of the run, that goes solely to that electrode. A conductor that continues from the rod to a water pipe, or that serves several electrodes in series, must be sized for the most demanding electrode it serves.

The word only matters on the exam. A question might describe a service with 500 kcmil copper conductors and a grounding electrode conductor that runs to a rod and then on to the metal water pipe. Because the conductor serves the water pipe too, the 6 AWG cap does not apply to the portion between the service and the water pipe, and you size it from the table, perhaps at 1/0 AWG copper. The short jumper from the water pipe connection out to the rod could be capped at 6 AWG.

Worked examples

The examples below use the following illustrative rows, stated as this course's examples for copper service conductors and copper electrode conductors: 2 AWG or smaller gives 8 AWG; 1 or 1/0 AWG gives 6 AWG; 2/0 or 3/0 AWG gives 4 AWG; over 3/0 through 350 kcmil gives 2 AWG; over 350 through 600 kcmil gives 1/0 AWG; over 600 through 1,100 kcmil gives 2/0 AWG; and over 1,100 kcmil gives 3/0 AWG. Look these up in your own table and correct them if your edition differs.

The last example highlights a contrast that exam writers love. Table 250.66 stops growing at its last row: no matter how large the service, the grounding electrode conductor never needs to exceed the last row's size, because the electrode's resistance limits the current anyway. Table 250.102(C)(1), which sizes main and supply-side bonding jumpers, has a 12.5% rule instead, because those jumpers carry the full fault current back to the source. If you remember why, you will not confuse the two.

Installation rules for the conductor

Section 250.64 governs installation, and several of its rules appear on exams. Aluminum or copper-clad aluminum grounding electrode conductors must not be in direct contact with masonry or the earth or exposed to corrosive conditions, and outside a building they must not be terminated within 18 inches of the earth. Copper has no such limits, which is one reason it dominates.

Protection depends on size. A 4 AWG copper or aluminum conductor, or larger, must be protected only where it is exposed to physical damage. A 6 AWG conductor that is free from exposure to physical damage may be run along the surface of the building without metal covering or protection, securely fastened to the construction; otherwise it must be protected in rigid metal conduit, intermediate metal conduit, Schedule 80 PVC, reinforced thermosetting resin conduit of the heavy-wall type, electrical metallic tubing or cable armor. Conductors smaller than 6 AWG must always be protected in one of those raceways or armor.

The conductor must be installed in one continuous length without a splice or joint, with three exceptions in 250.64(C): splices made with irreversible compression-type connectors listed as grounding and bonding equipment, splices made by the exothermic welding process, and connections to busbars sized and installed as 250.64(F) describes. Standard split bolts and mechanical lugs used as splices are not on the list.

One rule catches experienced electricians. Where a grounding electrode conductor runs in a ferrous metal raceway or enclosure, such as steel conduit, 250.64(E) requires the raceway to be electrically continuous from the point of attachment to the cabinet or equipment to the grounding electrode, and bonded at each end to the conductor inside. The reason is inductance: a steel conduit around a single conductor acts like a choke during a surge or fault, raising the impedance sharply, unless the conduit itself is bonded at both ends so it becomes part of the path. PVC conduit has no such problem, which is why many electricians protect grounding electrode conductors in Schedule 80 PVC.

Multiple disconnects and connections to electrodes

Where a service has more than one disconnecting means in separate enclosures, 250.64(D) offers three ways to make the grounding electrode connection. You may run a common grounding electrode conductor with taps to each enclosure, the common conductor sized for the total service and each tap sized from the conductors serving that enclosure. You may run an individual grounding electrode conductor from each disconnect, each sized from the conductors serving it. Or you may connect at a common location, such as a busbar of at least 1/4 inch by 2 inches, in the service equipment or an accessible wireway.

Section 250.68 adds rules for the connections to electrodes. The connection must be accessible, unless it is to a buried or concrete-encased electrode, in which case it need not be. Bonding jumpers must be installed around insulated joints and around equipment likely to be disconnected for repairs, such as water meters and filters. And 250.70 controls how the connection is made: by exothermic welding, listed lugs, listed pressure connectors, listed clamps or other listed means, never by solder alone, and with no more than one conductor under a ground clamp unless the clamp is listed for more.

A calculator you can edit

The listing below sizes a copper grounding electrode conductor from a set of service conductors and applies the caps. Its table holds this lesson's illustrative rows only; check each row against Table 250.66 before trusting it.

ROWS = [(66.36, "8 AWG"), (105.6, "6 AWG"), (167.8, "4 AWG"), (350, "2 AWG"),
        (600, "1/0 AWG"), (1100, "2/0 AWG"), (10 ** 9, "3/0 AWG")]   # illustrative
ORDER = ["8 AWG", "6 AWG", "4 AWG", "2 AWG", "1/0 AWG", "2/0 AWG", "3/0 AWG"]
CAPS = {"rods only": "6 AWG", "concrete-encased only": "4 AWG"}

def gec(set_kcmil, sets=1, electrode="water pipe or steel"):
    area = set_kcmil * sets
    size = next(s for limit, s in ROWS if area <= limit)
    cap = CAPS.get(electrode)
    if cap and ORDER.index(cap) < ORDER.index(size):
        size = cap
    return area, size

for label, k, n, e in [("Brandt, rods only", 133.1, 1, "rods only"),
                       ("Brandt, water pipe", 133.1, 1, "water pipe or steel"),
                       ("Delgado, steel", 167.8, 2, "water pipe or steel"),
                       ("Delgado, footing", 167.8, 2, "concrete-encased only"),
                       ("Commons, 4 x 500", 500, 4, "water pipe or steel")]:
    area, size = gec(k, n, e)
    print(f"{label}: {area:.0f} kcmil -> {size} copper")

Brandt, rods only: 133 kcmil -> 6 AWG copper
Brandt, water pipe: 133 kcmil -> 4 AWG copper
Delgado, steel: 336 kcmil -> 2 AWG copper
Delgado, footing: 336 kcmil -> 4 AWG copper
Commons, 4 x 500: 2000 kcmil -> 3/0 AWG copper