Grounding and Bonding Practice Test

Article 100 defines bonding as being connected to establish electrical continuity and conductivity, which is what carries fault current and keeps metal parts at the same potential. Grounding, by contrast, is a connection to earth or to a conductive body that extends the ground connection. The two terms are not interchangeable: bonding is about continuity between parts, grounding is the tie to earth.

NEC Article 100 (definitions of Bonded/Bonding vs Grounded)

250.50 requires that all grounding electrodes described in 250.52(A)(1) through (A)(7) that are present at a building or structure be bonded together to form the grounding electrode system. You do not pick one; every qualifying electrode that exists gets tied in. A metal underground water pipe used as an electrode always requires a supplemental electrode per 250.53(D)(2).

NEC 250.50

A single rod, pipe, or plate electrode that does not achieve 25 ohms or less to earth must be supplemented by one additional electrode. Once the second electrode is added (and the two are spaced at least 6 ft apart per 250.53(A)(3)), no further resistance measurement is required. The code never demands a specific ohm value be reached, only the one supplemental electrode.

NEC 250.53(A)(2), Exception

A rod, pipe, or plate electrode must be installed so that at least 8 ft of length is in contact with the soil (250.53(G)). Rods are not less than 8 ft long (250.52(A)(5)). Where rock bottom is hit, the rod may be driven at up to a 45-degree angle or buried in a trench at least 30 in deep.

NEC 250.52(A)(5), 250.53(G)

A metal underground water pipe is a qualifying electrode only where 10 ft or more of it is in direct contact with the earth. Because interior plumbing is increasingly plastic and the pipe can be replaced, a water pipe electrode must always be supplemented by another electrode (250.53(D)(2)). Interior metal water piping more than 5 ft from the point of entrance cannot serve as a conductor to interconnect electrodes.

NEC 250.52(A)(1)

250.122(B) requires the wire-type equipment grounding conductor to be increased in proportion to the circular-mil increase of the ungrounded conductors. The 20 A base equipment grounding conductor is 12 AWG (6,530 cmil). The ungrounded conductors go from 12 AWG (6,530 cmil) to 8 AWG (16,510 cmil), a 2.53x area increase. 6,530 x 2.53 = 16,510 cmil, which rounds to 8 AWG. Upsizing the phase conductors for voltage drop drags the grounding conductor up with them.

NEC 250.122(B); Table 250.122 (20 A row); Chapter 9 Table 8 (circular mils)

Table 250.122 lists 3 AWG copper for a 400 A overcurrent device. 250.122(F) requires a full-size equipment grounding conductor, sized on the circuit overcurrent device, in each parallel raceway, not the table size divided among the raceways. So each of the two raceways carries its own 3 AWG copper equipment grounding conductor. The parallel arrangement does not reduce the per-raceway size.

NEC 250.122(F); Table 250.122 (400 A row)

250.94 requires an intersystem bonding termination: an accessible external means, with at least three terminals, for bonding communications, CATV, satellite, and similar systems to the building grounding electrode system. It gives low-voltage installers a listed point to land their bonds instead of clamping onto the service raceway or a rod, keeping all systems at a common potential.

NEC 250.94

The main bonding jumper is the connection at the service between the grounded (neutral) conductor and the service disconnect enclosure / equipment grounding conductor. It is the single point where the grounded conductor is tied to equipment ground, giving line-to-ground fault current a low-impedance path back to the source. A system bonding jumper does the same job for a separately derived system (250.30); an equipment bonding jumper bonds enclosures/raceways together.

NEC 250.24(B), 250.28

Past the service disconnect the neutral and equipment grounding conductor must stay separate: 250.142(B) prohibits using the grounded (neutral) conductor to ground equipment on the load side. Bonding the neutral to enclosures downstream would put normal neutral current on metal parts and equipment grounding conductors in parallel with the neutral. The neutral-to-ground bond happens only at the service (or at a separately derived system).

NEC 250.142(B)

Under the current rule an equipment grounding conductor is run with the feeder to the separate structure, and the grounded (neutral) conductor is kept isolated from the equipment grounding conductor and enclosure there. The neutral-to-ground bond stays at the service only. The separate structure still needs its own grounding electrode (250.32(A)). The earth is never an acceptable equipment fault-current return path (250.4(A)(5)).

NEC 250.32(B)(1)

A separately derived system gets a system bonding jumper at one point, either at the source or at the first disconnecting means, tying the grounded conductor to the equipment grounding conductor and supply-side bonding jumper. It is the separately-derived-system equivalent of the service main bonding jumper and establishes the ground reference for the new system. Making that bond at more than one point would parallel neutral current onto the grounding path.

NEC 250.30(A)(1)

The system bonding jumper is sized from Table 250.102(C)(1) on the largest ungrounded (phase) conductor. For copper phase conductors over 350 through 600 kcmil the table gives 1/0 AWG copper. Below 1100 kcmil the 250.102(C)(1) sizes match Table 250.66; above that the table switches to the 12.5% rule. No dedicated ETB bonding-jumper calculator exists yet, so this value is hand-derived from the same 250.66 table data the site audits.

NEC 250.30(A)(1), Table 250.102(C)(1)

A supply-side bonding jumper is sized from Table 250.102(C)(1) on the largest ungrounded conductor. For aluminum conductors over 500 through 900 kcmil the table gives 3/0 AWG aluminum. The supply-side bonding jumper bonds service raceways and enclosures on the line side of the overcurrent device, where standard fittings alone are not accepted (250.92). Below 1100 kcmil copper / 1750 kcmil aluminum these sizes match Table 250.66.

NEC 250.102(C), Table 250.102(C)(1)

250.104(A)(1) sizes the bonding jumper for the metal water piping system from Table 250.66, using the service ungrounded conductor as the input, the same lookup the grounding electrode conductor tool performs. For 700 kcmil copper (over 600 through 1100 kcmil) Table 250.66 gives 2/0 AWG copper. Metal water piping likely to become energized must be bonded so it cannot carry fault current at a dangerous potential.

NEC 250.104(A)(1); Table 250.66

Table 250.66 is entered on the equivalent area of the paralleled conductors: two 3/0 copper = 2 x 167,800 = 335,600 cmil. That falls in the 'over 3/0 through 350 kcmil' copper row, which calls for a 2 AWG copper grounding electrode conductor. A single 3/0 conductor would only need 4 AWG, so paralleling the service conductors increases the electrode conductor. The 250.66(A)/(B) electrode caps could still reduce it for a rod, pipe, plate, or concrete-encased connection.

NEC Table 250.66 (parallel equivalent area)

250.118 lists the wiring methods and conductors permitted as equipment grounding conductors, including a wire, rigid metal conduit, intermediate metal conduit, and electrical metallic tubing (EMT). The load-side neutral is prohibited from that role by 250.142(B), a water pipe is a grounding electrode (not an equipment grounding conductor), and the earth is never an acceptable equipment fault-current path (250.4(A)(5)).

NEC 250.118

An equipment grounding conductor is bare, or if insulated is green or green with one or more yellow stripes. White or gray is reserved for the grounded (neutral) conductor (200.6). Larger conductors (over 6 AWG) and some multiconductor cable conductors may be re-identified green at terminations under 250.119(A) and (B).

NEC 250.119

On the line side of the service overcurrent device there is no branch or feeder breaker to clear a fault, so the available fault current and duration are higher. 250.92(B) therefore requires specified bonding methods, bonding jumpers, threaded connections, or listed bonding fittings around concentric or eccentric knockouts, rather than relying on standard locknuts and bushings. At the service, the grounded conductor is actually permitted for bonding (250.92(B)(1)).

NEC 250.92(A), 250.92(B)

250.104(C) requires exposed interconnected structural metal likely to become energized to be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor (if sized large enough), or one or more grounding electrodes. The bonding jumper is sized from Table 250.102(C)(1). This keeps the building steel from becoming an energized surface during a fault. It is a bonding requirement, not a way to use the steel as an electrode.

NEC 250.104(C)