9.4 Bonding: Services, Metal Piping, Structural Metal and Other Systems

Bonding at the Service — 250.92

Service conductors are unprotected. A fault between a service conductor and a metal raceway must be carried by that raceway back to the source without any overcurrent device to clear it quickly, so the Code demands a more robust connection than an ordinary locknut.

250.92(A) Bonding of Equipment for Services. The non-current-carrying metal parts of equipment indicated in (A)(1) and (A)(2) shall be bonded together:

1. All raceways, cable trays, cablebus framework, auxiliary gutters, or service cable armor or sheath that enclose, contain, or support service conductors, except as permitted in 250.80;
2. All enclosures containing service conductors, including meter fittings, boxes, or the like, interposed in the service raceway or armor.

250.92(B) Method of Bonding at the Service. Electrical continuity at service equipment, service raceways, and service conductor enclosures shall be ensured by one of the following methods:

1. Bonding equipment to the grounded service conductor in a manner provided in 250.8;
2. Connections utilizing threaded couplings or threaded hubs on enclosures, if made up wrenchtight;
3. Threadless couplings and connectors if made up tight for metal raceways and metal-clad cables;
4. Other listed devices, such as bonding-type locknuts, bushings, or bushings with bonding jumpers.

Standard locknuts or bushings shall not be the only means for the bonding required by this section. That sentence is the enforceable takeaway: a plain locknut on each side of a knockout is not sufficient bonding for a service raceway.

Method (B)(1) is the one most often used in practice: a bonding bushing with a jumper to the grounded service conductor, or a bonding jumper around the meter socket.

Circuits Over 250 Volts to Ground — 250.97

For circuits of over 250 volts to ground, the electrical continuity of metal raceways and cables with metal sheaths that contain any conductor other than service conductors shall be ensured by one or more of the methods specified for services in 250.92(B), except for (B)(1).

Exception: Where oversized, concentric, or eccentric knockouts are not encountered, or where a box or enclosure with concentric or eccentric knockouts is listed to provide a reliable bonding connection, the following methods shall be permitted: threadless couplings and connectors for cables with metal sheaths; two locknuts, on rigid metal conduit or intermediate metal conduit, one inside and one outside of boxes and cabinets; fittings with shoulders that seat firmly against the box or cabinet, with one locknut on the inside; listed fittings.

So on a 480Y/277 V system — 277 volts to ground, which is over 250 — a concentric knockout in a panelboard requires a bonding-type fitting or bonding jumper unless the enclosure is listed to provide a reliable bonding connection through the knockout. On a 208Y/120 V system, 120 volts to ground, ordinary locknuts are fine.

The Intersystem Bonding Termination — 250.94

Communications systems shall be bonded together at the building served in accordance with 250.94(A) or (B).

(A) Intersystem Bonding Termination Device. An intersystem bonding termination (IBT) for connecting intersystem bonding conductors shall be provided external to enclosures at the service equipment or metering equipment enclosure and at the disconnecting means for any additional buildings or structures. The intersystem bonding termination shall comply with the following:

1. Be accessible for connection and inspection;
2. Consist of a set of terminals with the capacity for connection of not less than three intersystem bonding conductors;
3. Not interfere with opening the enclosure for a service, building or structure disconnecting means, or metering equipment;
4. At the service equipment, be securely mounted and electrically connected to an enclosure for the service equipment, to the meter enclosure, or to an exposed nonflexible metallic service raceway, or be mounted at one of these enclosures and be connected to the enclosure or to the grounding electrode conductor with a minimum 6 AWG copper conductor;
5. At the disconnecting means for a building or structure, be securely mounted and electrically connected to the metallic enclosure or grounding electrode conductor with a minimum 6 AWG copper conductor;
6. The terminals shall be listed as grounding and bonding equipment.

The purpose is to keep telephone, CATV, satellite, and network protectors at the same potential as the power system. Before the IBT existed, installers grounded communications systems to a separate rod or a cold-water pipe, producing potential differences during surges that destroyed equipment and injured people.

250.94(B) permits, as an alternative, connection to an aluminum or copper busbar not less than 6 mm thick x 50 mm wide (1/4 in. x 2 in.) and of sufficient length to accommodate at least three terminations, securely fastened and accessible.

Bonding Metal Water Piping — 250.104(A)

250.104(A)(1) General. Metal water piping systems installed in or attached to a building or structure shall be bonded to any of the following: the service equipment enclosure; the grounded conductor at the service; the grounding electrode conductor where of sufficient size; or to one or more grounding electrodes used. The bonding jumper shall be sized in accordance with Table 250.102(C)(1) based on the size of the service-entrance conductors and shall be installed in accordance with 250.64(A), (B), and (E). The points of attachment of the bonding jumper shall be accessible.

250.104(A)(2) Buildings of Multiple Occupancy and (A)(3) Multiple Buildings or Structures Supplied by a Feeder or Branch Circuit contain the parallel rules for those situations, with the feeder-supplied case sized per Table 250.122 based on the rating of the overcurrent device.

Distinguish two very different roles for a water pipe:

A building can have both: the underground portion serves as an electrode and the interior piping is bonded.

Bonding Other Metal Piping — 250.104(B)

Where installed in or attached to a building or structure, a metal piping system, including gas piping, that is likely to become energized shall be bonded to any of the following: the equipment grounding conductor for the circuit that is likely to energize the piping; the service equipment enclosure; the grounded conductor at the service; the grounding electrode conductor, if of sufficient size; or to one or more grounding electrodes used.

The bonding conductor(s) or jumper(s) shall be sized in accordance with Table 250.122, using the rating of the circuit that is likely to energize the piping. The equipment grounding conductor for the circuit that is likely to energize the piping shall be permitted to serve as the bonding means.

So the gas line serving a 15-ampere-circuit furnace is bonded by the furnace circuit's own 14 AWG copper equipment grounding conductor — no separate jumper is required. This is the most common misunderstanding in the field: people install a dedicated 4 AWG bond to the gas meter, which is not required and is sized by the wrong table.

Structural Metal — 250.104(C)

Exposed structural metal that is interconnected to form a metal building frame and is not intentionally grounded or bonded and is likely to become energized shall be bonded to the service equipment enclosure, the grounded conductor at the service, the disconnecting means for buildings or structures supplied by a feeder or branch circuit, the grounding electrode conductor where of sufficient size, or to one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with Table 250.102(C)(1) and installed in accordance with 250.64(A), (B), and (E). The points of attachment shall be accessible unless installed in compliance with 250.68(A) Exception No. 2.

Separately Derived Systems — 250.104(D)

Where a separately derived system is installed, metal water piping in the area served, structural metal, and other bonding requirements are addressed by 250.104(D)(1) through (D)(3), generally requiring the metal water piping system in the area served by the separately derived system to be bonded to the grounded conductor at the source, with the jumper sized per Table 250.102(C)(1) based on the derived ungrounded conductors.

Other Bonding Rules Worth Tabbing

• 250.96(A) Bonding Other Enclosures. Metal raceways, cable trays, cable armor, cable sheath, enclosures, frames, fittings, and other metal non-current-carrying parts that serve as equipment grounding conductors shall be effectively bonded where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed on them. Any nonconductive paint, enamel, or similar coating shall be removed at threads, contact points, and contact surfaces or be connected by means of fittings designed so as to make such removal unnecessary.
• 250.96(B) Isolated Grounding Circuits. Where required for the reduction of electrical noise, an equipment enclosure supplied by a branch circuit shall be permitted to be isolated from a raceway by one or more listed nonmetallic raceway fittings, provided the metal raceway is supplemented by an internal insulated equipment grounding conductor.
• 250.98 Bonding Loosely Jointed Metal Raceways. Expansion fittings and telescoping sections of metal raceways shall be made electrically continuous by equipment bonding jumpers or other means.
• 250.100 Bonding in Hazardous (Classified) Locations. Regardless of the voltage, electrical continuity shall be ensured by the bonding methods specified in 250.92(B)(2) through (B)(4).
• 250.102(E) Installation. Equipment bonding jumpers shall be permitted to be installed inside or outside of a raceway or enclosure. Where installed on the outside, the length of the equipment bonding jumper shall not exceed 6 feet (1.8 m) and shall be routed with the raceway or enclosure.