GND-2 — Service Grounding and Bonding

Exam checkpoints

Locate the service disconnect and connection point

Under 2017 NEC 230.70(A), the service disconnecting means must be readily accessible and located either outside the building or structure or inside nearest the point of entrance of the service conductors. It cannot be installed in a bathroom. The local authority determines how much service-conductor length is "nearest" under the construction conditions. The disconnecting means must be suitable for use as service equipment, and service overcurrent protection is located in relation to it as required by 230.91.

For a grounded ac service, 250.24(C) requires the grounded service conductor to be routed with the ungrounded conductors to each service disconnecting means and connected to the grounded-conductor terminal or bus there. Section 250.24(A)(1) requires the grounding electrode conductor connection to be made at an accessible point from the load end of the service drop or lateral up to and including the terminal or bus where the grounded service conductor connects at the service disconnect. The grounding electrode connection is not moved to an ordinary downstream panel merely because that panel is convenient.

This arrangement brings three functions together without making them identical. The grounded service conductor carries normal line-to-neutral load current and provides the source-return segment for service-supplied ground faults. The grounding electrode conductor connects the grounded system to the grounding electrode system. The bonding components connect service metal to the grounded conductor so a fault on supply-side metal can return to the source.

Make the required service bond

Section 250.24(B) requires an unspliced main bonding jumper to connect the grounded conductor to each service-disconnecting-means enclosure in accordance with 250.28. This is the intentional service bonding point: it connects the enclosure and equipment-grounding paths to the grounded service conductor. A removable green screw, strap, wire, or bus can serve when identified and installed for that equipment. Leaving the neutral bus isolated at the service disconnect breaks the intended service fault-current path.

Section 250.92 separately requires bonding of service raceways, cable armor, fittings, meter and service enclosures, and other metal parts on the supply side. Ordinary contact through standard locknuts is not always sufficient, particularly where concentric or eccentric knockouts impair continuity. Listed bonding-type fittings, threaded connections, bonding jumpers, or other methods in 250.92(B) provide the required path. Follow utility rules for sealed metering equipment, but do not confuse utility access requirements with the NEC bonding objective.

Why is supply-side bonding special? A normal feeder EGC begins on the load side of the service disconnect; it does not run beside the utility service conductors to provide a return path. If an ungrounded service conductor faults to a metal raceway, bonded service metal must carry fault current to the grounded service conductor through the main or supply-side bonding path and back to the source. A ground rod and soil cannot perform that job.

Size each component under its own rule

The grounding electrode conductor is sized under 250.66 from the largest ungrounded service-entrance conductor or equivalent area for parallel conductors, subject to electrode-specific limits in 250.66(A) through (C). For example, the sole connection to a rod, pipe, or plate electrode is not required to be larger than 6 AWG copper under 250.66(A), even when the service calculation from the main table would produce a larger conductor. That limit does not resize the main bonding jumper.

The main bonding jumper is sized under 250.28(D), using 250.102(C)(1). For large conductors above the table threshold, use at least 12.5 percent of the area of the largest ungrounded service conductor or the equivalent area for parallel conductors. Example: assume two parallel 600 kcmil copper service conductors per phase and a copper main bonding jumper. Equivalent area is 2 × 600 = 1,200 kcmil. Because that exceeds 1,100 kcmil copper, the minimum calculated jumper area is 1,200 × 0.125 = 150 kcmil copper. This calculation applies to the stated copper, parallel-conductor, main-bonding-jumper conditions; do not use it for every service conductor or for an aluminum jumper without the applicable rule.

A table lookup or percentage calculation does not complete the installation. Check parallel-conductor rules, material, terminal ratings, conductor area, enclosure provisions, and whether a supply-side bonding jumper rather than the main bonding jumper is being sized. The GEC and bonding jumper may connect near the same service equipment, but one connects to electrodes and the other completes the metal-to-source fault loop.

Isolate the neutral downstream

Section 250.24(A)(5) generally prohibits connecting the grounded conductor to normally non-current-carrying metal, EGCs, or equipment on the load side of the service disconnecting means, except where another identified rule permits it. In an ordinary feeder-supplied downstream panel, the neutral bus is insulated from the enclosure and the equipment-grounding bus; the EGC bus is bonded to the enclosure. Remove or omit the main bonding screw or strap there as the equipment instructions require.

If the downstream neutral is bonded again, normal neutral current has parallel return routes: part can flow on the feeder EGC, metal raceway, building steel, cable armor, and other bonded systems. Assume a 36 A neutral load and, only for illustration, an intended neutral path of 0.03 ohm in parallel with an accidental metal path of 0.06 ohm. Current divides inversely with resistance: the 0.03 ohm path carries 24 A and the 0.06 ohm metal path carries 12 A. The metal is then carrying continuous normal current, not just fault current. Actual installations require full impedance analysis, but the example exposes the hazard.

Do not memorize "neutral and ground connect only once" without qualifications. Multiple service disconnect enclosures can require bonding as 250.24(B) specifies, and separately derived systems have their own bonding rules. The exam-safe statement is: bond at the service or system bonding point required or permitted by the applicable rule; after that point, keep grounded conductors separated from enclosures and equipment-grounding paths unless a specific exception applies.

A final service check should trace both currents. Normal neutral current returns only on the grounded conductor. A service-enclosure fault returns over bonded metal, the main or supply-side bonding connection, and the grounded service conductor to the source. The electrode system limits voltage to earth but is not the clearing path. Confirm disconnect location, service-equipment marking, grounded-conductor termination, GEC connection, main bonding jumper, supply-side bonding, downstream isolation, and torque and listing instructions.