10.4 Bonding Methods, Equipment Requiring Grounding & Receptacle Grounding

Why bonding methods matter

Every metal raceway, box, fitting, and enclosure in the ground-fault return path must stay electrically continuous for the life of the installation. DLI's knowledge areas include bonding, equipment grounding, and grounding of receptacles and equipment. Exam questions often describe a fitting or knockout and ask whether the bonding is adequate.

Bonding service equipment (250.92)

Parts that must be bonded: all raceways, cable trays, cablebus framework, auxiliary gutters, and cable armor or sheaths that enclose service conductors, and all enclosures containing service conductors, including meter fittings and boxes.

Methods (250.92(B)): bonding must be done by one or more of these:

1. bonding equipment to the grounded service conductor;
2. threaded couplings or threaded hubs on enclosures, made up wrenchtight;
3. threadless couplings and connectors made up tight for metal raceways and metal-clad cables; or
4. other listed devices, such as bonding-type locknuts, bonding bushings, or wedges, or bushings with bonding jumpers.

Bonding jumpers must be used around impaired connections such as reducing washers or oversized, concentric, or eccentric knockouts. Standard locknuts or bushings may not be the only means of bonding; they may provide the mechanical connection only.

Bonding other enclosures (250.96 to 250.98)

• 250.96(A): metal raceways, cable trays, cable armor, enclosures, frames, fittings, and other metal non-current-carrying parts that serve as EGCs must be bonded where necessary to ensure continuity and the capacity to carry any fault current. Nonconductive coatings such as paint must be removed at threads and contact surfaces, or fittings must be designed to make removal unnecessary (250.12).
• 250.96(B) Isolated grounding circuits: to reduce electromagnetic noise, an equipment enclosure supplied by a branch circuit may be isolated from the metal raceway by a listed nonmetallic raceway fitting at the point of attachment, if the enclosure is grounded by an internal insulated equipment grounding conductor.
• 250.97 Over 250 V: for circuits over 250 V to ground (such as 277 V lighting and 480Y/277 V motor circuits), raceway and cable continuity must be ensured by the 250.92(B) methods. Where oversized, concentric, or eccentric knockouts are not encountered, or the box is listed to provide reliable bonding through such knockouts, these are also permitted: threadless couplings and connectors for metal-sheathed cables; two locknuts, one inside and one outside, on RMC or IMC; fittings with shoulders that seat firmly against the box (such as EMT connectors) with one locknut inside; and listed fittings.
• 250.98 Loosely jointed raceways: expansion fittings and telescoping sections of metal raceways must be made electrically continuous with equipment bonding jumpers or other means.
• 250.106 Lightning protection: where a lightning protection system is installed, its grounding electrodes must be bonded to the building grounding electrode system.

Bonding conductors and jumpers (250.102)

Equipment that must be grounded

Fastened in place or permanently wired (250.110)

Exposed, normally non-current-carrying metal parts of fixed equipment likely to become energized must be connected to an EGC where any of these apply:

• within 8 ft vertically or 5 ft horizontally of ground or grounded metal objects and subject to contact by persons;
• located 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 EGC (metal-clad, metal-sheathed, metal raceway, or a wiring method with an EGC); or
• operating with any terminal at over 150 V to ground, with limited exceptions.

Section 250.112 also lists equipment that must always be grounded, such as switchboard frames, motor frames, motor controller enclosures, elevator and crane equipment, and signs.

Cord-and-plug-connected equipment (250.114)

In residential occupancies, these must have an EGC: refrigerators, freezers, and air conditioners; clothes washers and dryers, dishwashers, ranges, and kitchen waste disposers; information technology equipment; sump pumps and electrical aquarium equipment; hand-held, stationary, and light industrial motor-operated tools; and motor-operated appliances such as hedge clippers, lawn mowers, snow blowers, and wet scrubbers. Listed tools and appliances protected by a system of double insulation are exempt where marked.

How fixed equipment is connected (250.134 and 250.136)

Equipment fastened in place or connected by a permanent wiring method is connected to an EGC of one of the 250.118 types, such as a qualifying metal raceway, qualifying cable armor, or a wire EGC. A wire-type EGC must be contained within the same raceway or cable, or otherwise run with the circuit conductors, which keeps the impedance of the fault path low. Equipment secured to and in electrical contact with a metal rack or structure that is provided for its support and connected to an EGC is considered connected to an EGC (250.136). The structural metal frame of a building may not be used as the required EGC for ac equipment.

Connection methods (250.8)

EGCs and bonding jumpers must be connected by listed pressure connectors, terminal bars, pressure connectors listed as grounding and bonding equipment, exothermic welding, machine screws that engage at least two threads or are secured with a nut, thread-forming machine screws that engage at least two threads in the enclosure, connections that are part of a listed assembly, or other listed means. Sheet-metal screws may not be used, and connections may not depend solely on solder.

Boxes and receptacles

EGCs in boxes (250.148)

Where circuit conductors are spliced or terminated in a box, all EGCs associated with those circuits must be connected within the box or to the box with devices suitable for the use. In a metal box, the connection is made with a grounding screw used for no other purpose, equipment listed for grounding, or a listed grounding device. The arrangement must be such that removing a receptacle or luminaire does not interrupt EGC continuity to other outlets.

Bonding the receptacle to the box (250.146)

An equipment bonding jumper must connect the receptacle grounding terminal to a grounded metal box, unless one of these applies:

Replacing a nongrounding receptacle (250.130(C))

When a grounding-type receptacle replaces an old two-wire receptacle, or a branch circuit is extended, the new EGC may be connected to any accessible point on the grounding electrode system or grounding electrode conductor, the equipment grounding terminal bar in the enclosure where the branch circuit originates, an EGC of another branch circuit that originates from that same enclosure, or, for grounded systems, the grounded service conductor within the service equipment enclosure.

Worked example: a 277 V lighting circuit

A 277 V lighting circuit in EMT enters a junction box through an existing concentric knockout with some rings removed.

1. The circuit is over 250 V to ground, so 250.97 applies.
2. A concentric knockout is an impaired connection, and the box is not listed for bonding through it.
3. A standard EMT connector with one locknut is not enough. Bridge the knockout with a bonding bushing and bonding jumper, or install a wire-type EGC sized from Table 250.122 in the raceway and bond the box to it.