NEC 250 — Grounding & Bonding (Washington)

NEC 250 — Grounding & Bonding (Washington Journeyman)

Learning Objectives

1.1 Scope, Performance Rules, and Article Organization (250.1, 250.4)

Section 250.1 limits Article 250 to installations not over 1000 volts ac, 1500 volts dc, nominal. Learn the map: Part III is the grounding electrode system and grounding electrode conductor, Part IV covers enclosure, raceway, and service cable connections, Part V is bonding, and Part VI is equipment grounding and equipment grounding conductors.

Section 250.4 states the goals. A grounded system must be connected to earth so that voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines is limited and the voltage to earth is stabilized, and materials enclosing conductors or equipment must be connected to earth and bonded to the supply source to create an effective ground-fault current path. Section 250.4(A)(5) defines that path: a low-impedance circuit must operate the overcurrent protective device or ground detector, must safely carry the maximum ground-fault current likely to be imposed on it, and the earth shall not be considered an effective ground-fault current path.

1.2 The Grounding Electrode System and Its Electrodes (250.50, 250.52)

Section 250.50 requires all grounding electrodes described in 250.52(A)(1) through 250.52(A)(7) that are present at each building or structure served to be bonded together to form the grounding electrode system; if none of those electrodes exists, one or more of the electrodes specified in 250.52(A)(4) through 250.52(A)(8) shall be installed and used.

Section 250.52(A) permits a metal underground water pipe in direct contact with the earth for 3.0 m (10 ft) or more and electrically continuous to the points of connection; a metal in-ground support structure with 3.0 m (10 ft) of vertical earth contact; a concrete-encased electrode of at least 6.0 m (20 ft) of rebar not less than 13 mm (1/2 in.) in diameter or bare copper not smaller than 4 AWG, encased by at least 50 mm (2 in.) of concrete within a footing or foundation in contact with the earth; a ground ring of at least 6.0 m (20 ft) of bare copper not smaller than 2 AWG encircling the building in contact with the earth; rod and pipe electrodes not less than 2.44 m (8 ft) in length, with rods at least 15.87 mm (5/8 in.) in diameter unless listed; plate electrodes exposing not less than 0.186 m2 (2 ft2), steel at least 6.4 mm (1/4 in.) thick and nonferrous metal at least 1.5 mm (0.06 in.) thick. Section 250.52(B) prohibits metal underground gas piping systems, aluminum, and the pool structures and structural rebar described in 680.26.

1.3 Installing the Grounding Electrode System (250.53)

Rod, pipe, and plate electrodes must be free of nonconductive coatings. Section 250.53(A)(2) requires a single rod, pipe, or plate electrode to be supplemented by an additional electrode of a type specified in 250.52(A)(2) through 250.52(A)(8), bonded to that electrode, the grounding electrode conductor, the grounded service-entrance conductor, a nonflexible grounded service raceway, or any grounded service enclosure. The exception is the number to memorize: if a single rod, pipe, or plate electrode has a resistance to earth of 25 ohms or less, the supplemental electrode is not required, so a driven rod is nearly always a two-electrode installation. Section 250.53(A)(3) requires multiple rod, pipe, or plate electrodes to be not less than 1.8 m (6 ft) apart.

Section 250.53(A)(4) requires at least 2.44 m (8 ft) of the rod in contact with the soil and the electrode driven vertically to a depth of not less than 2.44 m (8 ft); if rock bottom is encountered the rod may instead be driven at an oblique angle not exceeding 45 degrees from the vertical or buried in a trench at least 750 mm (30 in.) deep. Plate electrodes and ground rings under 250.53(F) go not less than 750 mm (30 in.) below the surface of the earth, and 250.53(B) requires each electrode of one grounding system to be not less than 1.83 m (6 ft) from any other electrode of another grounding system. Section 250.53(D) forbids relying on water meters or filtering devices for the continuity of a water pipe electrode.

1.4 Grounding Electrode Conductor Installation and Sizing (250.64, 250.66)

Under 250.64(A) aluminum and copper-clad aluminum grounding electrode conductors have corrosion and concrete-contact restrictions, and terminations inside outdoor enclosures listed for the environment are permitted within 450 mm (18 in.) of the bottom of the enclosure. Under 250.64(B) a 6 AWG or larger conductor not exposed to physical damage may be run along a surface without metal covering, but exposed to physical damage it needs rigid metal conduit, Schedule 80 PVC, RTRC-XW, electrical metallic tubing, or cable armor. Section 250.64(C) requires one continuous length without splices or joints, except as provided in 250.30(A)(5), 250.30(A)(6), 250.30(B)(1), and 250.68(C).

Section 250.66 sizes the grounding electrode conductor, and the bonding jumpers for connection of grounding electrodes, from the largest ungrounded service-entrance or feeder conductor, subject to Table 250.66 and the exceptions in 250.66(A) through 250.66(C):

Worked values: 400 kcmil copper requires 2 AWG copper, 600 kcmil copper requires 1/0 AWG copper, and 1200 kcmil copper requires 3/0 AWG copper. Under 250.66(A) a conductor connected to a rod, pipe, or plate electrode that does not extend on to other types of electrodes requiring a larger conductor need not be larger than 6 AWG copper wire or 4 AWG aluminum, which is why a tap to a ground rod is 6 AWG copper no matter how large the main conductor is. Under 250.66(B) a concrete-encased connection need not be larger than 4 AWG copper wire.

1.5 Connecting the Grounding Electrode Conductor to the Electrode (250.68, 250.70)

Section 250.68(A) requires all mechanical elements used to terminate a grounding electrode conductor or bonding jumper to a grounding electrode to be accessible. Exception No. 1 exempts an encased or buried connection to a concrete-encased, driven, or buried grounding electrode; Exception No. 2 exempts exothermic or irreversible compression connections at terminations attached to fireproofed structural metal.

Section 250.68(C) lists the locations that may extend the connection to an electrode: interior metal water piping electrically continuous with a metal underground water pipe electrode and located not more than 1.52 m (5 ft) from the point of entrance to the building, measured along the water piping; the metal structural frame of a building, secured by hold-down bolts to a concrete-encased electrode complying with 250.52(A)(3); and a rebar-type concrete-encased electrode installed per 250.52(A)(3) with an additional rebar section extended to an accessible location not subject to corrosion, with rebar never used as a conductor to interconnect the electrodes of a grounding electrode system.

Section 250.70 requires ground clamps to be listed for both electrode and conductor materials and permits only one conductor per single clamp unless it is listed for multiple conductors; permitted means under 250.70(A)(1) are exothermic welding, listed lugs, listed pressure connectors, listed clamps, and other listed means, and 250.70(A)(2) requires clamps on pipes, rods, or other buried electrodes to be listed for direct soil burial or concrete encasement.

1.6 Main Bonding Jumper, System Bonding Jumper, and Supply-Side Bonding Jumpers (250.28, 250.102)

Section 250.28 governs the jumper that ties the grounded conductor to the equipment grounding conductor at the source. Main and system bonding jumpers must be copper, aluminum, copper-clad aluminum, or other corrosion-resistant material, and must be a wire, bus, screw, or similar suitable conductor; a screw used alone must be identified with a green finish visible with the screw installed. Section 250.28(D)(1) sizes them from Table 250.102(C)(1), and 250.28(D)(3) sizes each system bonding jumper of a separately derived system supplying more than one enclosure on the largest ungrounded feeder conductor serving it.

Section 250.102(A) permits bonding jumpers to be a wire, bus, screw, nonflexible metal raceway and fittings, or similar suitable conductor. The wire-type supply-side bonding jumper shall not be smaller than Table 250.102(C)(1), and where ungrounded supply conductors are paralleled in two or more raceways or cables, 250.102(C)(2) permits an individual jumper per raceway sized on the largest ungrounded supply conductor there, or a single jumper sized on the sum of the circular mil areas. Section 250.102(D) sizes the equipment bonding jumper on the load side of an overcurrent device per 250.122, and 250.102(E) caps an outside jumper at 1.8 m (6 ft).

Table 250.102(C)(1) serves the grounded conductor and every bonding jumper, and its values match Table 250.66:

Worked values: 250 kcmil copper requires a 2 AWG copper main bonding jumper, 600 kcmil copper requires 1/0 AWG copper, and a 1/0 AWG copper ungrounded service conductor requires 8 AWG copper. Note 1 sets a minimum area of 12½ percent of the largest ungrounded supply conductor where paralleled supply conductors exceed 1100 kcmil copper or 1750 kcmil aluminum, and Note 2 caps the jumper at the largest ungrounded conductor.

1.7 Grounding Separately Derived Systems (250.30)

The notes to 250.30 carry the definitional trap: an alternate source such as an on-site generator is not a separately derived system if the grounded conductor is solidly interconnected to a service-supplied system grounded conductor, as happens when the transfer equipment does not switch the grounded conductor.

Section 250.30(A)(1) requires an unspliced system bonding jumper complying with 250.28(A) through 250.28(D), made at any single point from the source to the first system disconnecting means or overcurrent device, or at the source when there is none. At the source it connects the grounded conductor to the supply-side bonding jumper and the enclosure; at the first disconnecting means it adds that enclosure and the equipment grounding conductors. Exception No. 3 is the small-transformer rule: the system bonding jumper for a system supplying a Class 1, Class 2, or Class 3 circuit and derived from a transformer rated not more than 1000 volt-amperes shall not be smaller than the derived ungrounded conductors and shall not be smaller than 14 AWG copper or 12 AWG aluminum.

Section 250.30(A)(2) requires a supply-side bonding jumper from the source enclosure to the first disconnecting means enclosure when the two are separate enclosures; it need not be larger than the derived ungrounded conductors and may be of nonflexible metal raceway type or of the wire or bus type. Section 250.30(A)(3) requires the grounded conductor, when the system bonding jumper is not at the source, to be not smaller than Table 250.102(C)(1) for a single raceway and not required to be larger than the derived ungrounded conductors, or when paralleled to be sized on the largest derived ungrounded conductor in each raceway or the sum of the circular mil areas, but not smaller than 1/0 AWG.

Section 250.30(A)(4) requires the building or structure grounding electrode system to be used, with connection to structural steel following 250.68(C)(2) or 250.68(C)(3). Section 250.30(A)(5) sizes the grounding electrode conductor for a single separately derived system per 250.66 for the derived ungrounded conductors and requires that connection at the same point on the system where the system bonding jumper is connected. Section 250.30(A)(6) permits a common grounding electrode conductor for multiple separately derived systems, the common conductor being a wire-type conductor not smaller than 3/0 AWG copper or 250 kcmil aluminum, a metal water pipe per 250.68(C)(1), or the metal structural frame per 250.68(C)(2).

1.8 Bonding Metal Piping, Structural Metal, and Lightning Protection (250.104, 250.106)

Section 250.104(A)(1) requires metal water piping systems installed in or attached to a building or structure to be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor if of sufficient size, or one or more grounding electrodes used on the same condition. The jumpers are installed per 250.64(A), 250.64(B), and 250.64(E), their points of attachment must be accessible, and they are sized per Table 250.102(C)(1) except that they need not be larger than 3/0 copper or 250 kcmil aluminum. Section 250.104(A)(3) covers buildings supplied by a feeder or branch circuit, bonded to the disconnecting means enclosure, the equipment grounding conductor run with the supply conductors, or one or more grounding electrodes used, sized per 250.102(D).

Section 250.104(B) covers other metal piping, including gas piping, likely to become energized, bonded to the equipment grounding conductor for the circuit likely to energize it, the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor if of sufficient size, or one or more grounding electrodes used on the same condition, with the sizing basis now Table 250.122 using the rating of that circuit. Section 250.104(C) covers exposed structural metal interconnected to form a metal building frame and likely to become energized, bonded to the service equipment enclosure, the grounded conductor at the service, the disconnecting means for buildings supplied by a feeder or branch circuit, or one or more grounding electrodes used, sized per Table 250.102(C)(1) but not required to be larger than 3/0 AWG copper. Sections 250.104(D)(1) and (D)(2) bond the grounded conductor of each separately derived system to the metal water piping and structural metal in the area served. Section 250.106 bonds lightning protection system ground terminals to the building or structure grounding electrode system.

1.9 Equipment Grounding and Equipment Grounding Conductors (250.110, 250.112, 250.118, 250.122, 310.16)

Section 250.110 requires exposed, normally non-current-carrying metal parts of fixed equipment supplied by or enclosing conductors likely to become energized to be connected to an equipment grounding conductor where they are within 2.5 m (8 ft) vertically or 1.5 m (5 ft) horizontally of ground or grounded metal objects and subject to contact by persons, in a wet or damp location and not isolated, in electrical contact with metal, in a hazardous (classified) location, supplied by a wiring method that provides an equipment grounding conductor, or operating with any terminal at over 150 volts to ground. Section 250.112 adds equipment that must be grounded regardless of voltage, including switchgear frames, motor frames, elevator equipment, luminaires, and skid-mounted equipment.

Section 250.118(A) permits as equipment grounding conductors a copper, aluminum, or copper-clad aluminum conductor, solid or stranded, insulated, covered, or bare, as a wire or busbar; rigid metal conduit; intermediate metal conduit; electrical metallic tubing; listed flexible metal conduit meeting all conditions of the section, including termination in listed fittings, circuit conductors protected by overcurrent devices rated at 20 amperes or less, size not exceeding metric designator 35 (trade size 3/4 through 1-1/4), and a combined length of flexible metal conduit, flexible metallic tubing, and liquidtight flexible metal conduit in the same effective ground-fault current path not exceeding 1.8 m (6 ft); and listed liquidtight flexible metal conduit. Section 250.118(B) excludes grounding electrode conductors and the structural metal frame of a building.

Section 250.122 sizes wire-type equipment grounding conductors from the rating or setting of the automatic overcurrent protective device ahead of the equipment:

Read the table by device rating, not by load. Worked values: a 200-ampere device requires 6 AWG copper; 400 amperes requires 3 AWG copper or 1 AWG aluminum; 600 amperes requires 1 AWG copper; 800 amperes requires 1/0 AWG copper; 1200 amperes requires 3/0 AWG copper or 250 kcmil copper; and 2000 amperes requires 350 kcmil aluminum.

Section 250.122(A) caps the requirement, since equipment grounding conductors are not required to be larger than the circuit conductors supplying the equipment, and 250.122(D), carried as 250.122(B) in earlier editions, requires wire-type equipment grounding conductors to be increased in size proportionately when ungrounded conductors are increased in size for any reason other than as required in 310.15(B) or 310.15(C). Section 310.16 gives the ampacity of insulated conductors in a raceway, cable, or earth.

1.10 Service Raceways, Enclosures, and Bonding at the Service (250.80)

Section 250.80 requires metal enclosures and raceways for service conductors and equipment to be connected to the grounded conductor if the electrical system is grounded, or to the grounding electrode conductor for systems that are not grounded; on the supply side the grounded conductor, not an equipment grounding conductor, performs the bonding function. Metal components installed in a run of underground nonmetallic raceway(s) and isolated from possible contact by a minimum cover of 450 mm (18 in.) need not be connected to the grounded conductor, supply-side bonding jumper, or grounding electrode conductor.

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Summary

Section 250.4 states what the system must accomplish, limited voltage to earth during lightning, line surges, and unintentional contact with higher-voltage lines; an effective ground-fault current path that will operate the overcurrent device; and a firm rule that the earth is never that path. Section 250.1 locates the rules: Part III for the grounding electrode system and its conductor, Part IV for service enclosures and raceways, Part V for bonding, and Part VI for equipment grounding conductors.

A grounding electrode system is assembled, not chosen. Section 250.50 bonds together every electrode present from 250.52(A)(1) through 250.52(A)(7), and if none exists one is installed from 250.52(A)(4) through 250.52(A)(8). The numbers to hold are 3.0 m (10 ft) of earth contact for water pipe and in-ground support structures, 6.0 m (20 ft) and at least 50 mm (2 in.) of concrete for a concrete-encased electrode with rebar not less than 13 mm (1/2 in.) or copper not smaller than 4 AWG, 6.0 m (20 ft) of 2 AWG copper for a ground ring, 2.44 m (8 ft) for a rod, and 0.186 m2 (2 ft2) for a plate.

Section 250.53 supplies the supplemental electrode and its 25-ohm exception, the 1.8 m (6 ft) spacing, the 2.44 m (8 ft) driving depth, and the 750 mm (30 in.) depths; 250.64 keeps the conductor continuous, protected, and bonded at both ends of a ferrous raceway; 250.66 starts from the largest ungrounded conductor, with 250.66(A) through (C) capping rod, pipe, or plate, concrete-encased, and ground ring connections. On the bonding side, 250.30 assembles the separately derived system rules and 250.104 assigns each piping or structural system its table. Equipment grounding runs through 250.110, 250.112, and 250.118 to Table 250.122, where the device rating governs and upsizing ungrounded conductors forces a proportional increase under 250.122(D).