8.3 Bonding of Raceways, Enclosures & Piping Systems

Bonding of Raceways, Enclosures & Piping Systems

Bonding is the critical engineering practice that ties non-current-carrying metallic systems together to create an electrically continuous equipotential plane and an effective ground-fault current path. Without comprehensive bonding, metallic conduits, electrical enclosures, interior plumbing pipes, structural building steel, and gas lines can become energized at lethal line voltages during electrical insulation failures or lightning events. Because service raceways carry unprotected utility fault currents, and circuits operating over 250 volts to ground readily sustain destructive plasma arcs, the National Electrical Code enforces exacting bonding mandates under NEC 250.92, 250.97, and 250.104. Candidates preparing for the Kentucky Journeyman Electrician examination must master these rules, distinguishing clearly between service bonding, branch-circuit knockout bonding, and interior piping bonding.

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1. Service Raceway & Enclosure Bonding (NEC 250.92)

Service-entrance conductors lack customer-side overcurrent protection. If an ungrounded service conductor faults to a metallic service raceway or meter cabinet, the only device that can interrupt the fault is the utility company's primary distribution fuse or transformer breaker. This utility fuse may require seconds or minutes to clear a secondary fault, allowing fault currents of $10{,}000\text{ A}$ to $50{,}000\text{ A}$ or more to surge through the raceway.

+-----------------------------------------------------------------------------------+
|                    SERVICE RACEWAY BONDING (NEC 250.92)                           |
|                                                                                   |
|  PROHIBITED FOR SERVICE BONDING:                                                  |
|  [ Standard Locknut ] ===[ Enclosure Wall ]===[ Standard Plastic Bushing ]        |
|  DANGER: Enclosure paint, loose fit, and thin threads create high resistance.    |
|          Massive utility fault current vaporizes the locknut, causing ARC FIRE!  |
|                                                                                   |
|  APPROVED SERVICE BONDING METHODS (NEC 250.92(B)):                                |
|  Method 1: Threaded bosses or hubs on enclosure made wrench-tight.                |
|  Method 2: Threadless connectors (compression) listed and made wrench-tight.      |
|  Method 3: Grounding / Bonding Bushing with bonding jumper lug + Table 250.102(C) |
|            copper jumper connected directly to service grounded neutral bus.      |
|  Method 4: Listed bonding locknuts (with hardened bonding set-screws).            |
+-----------------------------------------------------------------------------------+

The Absolute Prohibition of Standard Locknuts

Under NEC 250.92(B), standard locknuts and standard non-grounding bushings shall not be relied upon for bonding service raceways and enclosures. Standard locknuts bite through paint only lightly and lack the cross-sectional mass to survive high-magnitude utility fault currents without blowing apart.

Permitted Service Bonding Methods (NEC 250.92(B))

NEC 250.92(B) mandates that electrical continuity at service equipment, meter sockets, and service raceways shall be ensured by one of the following four approved methods:

1. Threaded Couplings & Bosses: Raceways threaded into threaded entries or threaded hubs (such as Myers hubs on outdoor meter cabinets) made wrench-tight.
2. Threadless Fittings Made Wrench-Tight: Threadless couplings and connectors made up tight for metal raceways and metal-clad cables, such as EMT compression or set-screw fittings.
3. Bonding Bushings with Bonding Jumpers: A listed bonding bushing (grounding bushing) equipped with a lay-in lug and a bonding jumper wire connected directly to the service grounded neutral terminal bar.
4. Other Listed Devices: Listed bonding-type locknuts (locknuts equipped with a hardened steel set-screw designed to penetrate enclosure paint and steel) or bonding wedges.

Sizing Service Bonding Jumpers

Bonding jumpers installed on service raceways are Supply-Side Bonding Jumpers (SSBJ). They must be sized from NEC Table 250.102(C)(1) based on the size of the ungrounded service-entrance conductors routed through that raceway.

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2. Bonding Around Knockouts for Circuits Over 250V to Ground (NEC 250.97)

In standard residential and light-commercial systems ($120/240\text{ V}$ single-phase or $208\text{Y}/120\text{ V}$ 3-phase), the voltage to ground is $120\text{ V}$, which falls under the 250V threshold. However, in commercial and industrial facilities utilizing 480Y/277V systems, the voltage to ground is 277 volts.

The Problem with Concentric and Eccentric Knockouts

Electrical boxes and panel cabinets are stamped at the factory with concentric rings (rings centered on the same point) or eccentric rings (rings with offset centers) to accommodate multiple conduit trade sizes (e.g., knockouts punched for $1/2"$, $3/4"$, and $1"$). The thin metal bridges holding these rings together have very small cross-sectional areas and high electrical resistance. When a $277\text{ V}$ ground fault occurs, the fault current can burn open these fragile bridges, breaking the ground return path and establishing a persistent, dangerous electrical arc.

+-----------------------------------------------------------------------------------+
|                KNOCKOUT BONDING MATRIX FOR CIRCUITS OVER 250V TO GROUND           |
+-----------------------------------------------------------------------------------+
|  CIRCUIT VOLTAGE TO GROUND:                                                       |
|  \le 250V to Ground (e.g., 120V, 208V, 240V):                                     |
|  - Standard locknuts and bushings permitted across concentric/eccentric rings.    |
|  - Enclosure paint must be removed or scratched clean to ensure metal contact.    |
|                                                                                   |
|  > 250V to Ground (e.g., 277V, 480V):                                             |
|  - GENERAL MANDATE (NEC 250.97): Must use listed bonding bushings with jumpers,   |
|    threadless fittings made wrench-tight, or listed threaded hubs.                |
|  - EXCEPTION: Where concentric or eccentric knockouts are tested, listed, and     |
|    certified as providing an effective grounding path (tested per UL standards),  |
|    standard locknuts are permitted if one locknut is inside and one is outside!   |
+-----------------------------------------------------------------------------------+

Approved Methods Under NEC 250.97:

For circuits operating at over 250 volts to ground, where raceways terminate in enclosures with concentric or eccentric knockouts, electrical continuity must be ensured by one of the following:

1. Threaded entries or listed threadless fittings made wrench-tight.
2. Bonding bushings with bonding jumpers sized per NEC Table 250.122 based on the rating of the upstream circuit overcurrent protective device.
3. Exception (no oversized, concentric, or eccentric knockouts, or a box listed to provide a reliable bonding connection): Then these methods are also permitted: threadless couplings and connectors for metal-sheathed cables; two locknuts, one inside and one outside the box, on RMC or IMC; fittings with shoulders that seat firmly against the box, such as EMT connectors and cable connectors, with one locknut inside; and listed fittings.

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3. Bonding of Metal Piping Systems (NEC 250.104)

Interior metal piping systems inside a building are prime conductive paths. If an ungrounded conductor accidentally chafes against a copper water pipe or a steel gas line, the entire plumbing infrastructure can become energized, creating lethal touch potentials at every sink, shower, and water heater in the facility.

Interior Metal Water Piping Bonding (NEC 250.104(A))

NEC 250.104(A)(1) mandates that the interior metal water piping system shall be bonded to:

• The service equipment enclosure, OR
• The grounded conductor at the service, OR
• The grounding electrode conductor where of sufficient size, OR
• One or more grounding electrodes of the grounding electrode system.

Key Water Piping Sizing & Installation Rules:

1. Sizing Table: The water piping bonding jumper must be sized in accordance with NEC Table 250.102(C)(1) based on the circular mil area of the largest ungrounded service-entrance conductors (or secondary conductors of a separately derived system).
   • Exam Alert: It is NOT sized from Table 250.122 or Table 250.66! Table 250.102(C)(1) governs.
2. Accessibility (250.104(A)(1)): The bonding jumper attachment point must remain accessible.
3. Water Meter Jumpers (NEC 250.68(B)): Water meters, pressure-reducing valves, and water filters are frequently removed for utility maintenance or replaced with plastic units. A bonding jumper must be installed around any water meter or dielectric union to ensure permanent, uninterrupted electrical continuity.
4. Multiple Occupancies (250.104(A)(2)): In multi-tenant buildings where the interior water piping in individual units is metallically isolated from other units by plastic piping, the metal piping in each individual occupancy is permitted to be bonded to the panelboard equipment grounding terminal using Table 250.122 based on the branch circuit supplying the unit.

+-----------------------------------------------------------------------------------+
|                    INTERIOR WATER PIPING BONDING (NEC 250.104(A))                 |
|                                                                                   |
|  Service Equipment Enclosure                                                      |
|        |                                                                          |
|        +===================[ Bonding Jumper per Table 250.102(C)(1) ]             |
|                                           |                                       |
|                                           v                                       |
|  Interior Metal Water Pipe ========================================> Sinks, Tubs  |
|        |                                                                          |
|        +------[ Water Meter / Filter ]------+                                     |
|        |                                    |                                     |
|        +===[ Water Meter Bonding Jumper ]===+ (NEC 250.68(B))                     |
+-----------------------------------------------------------------------------------+

Other Metal Piping Systems & Fuel Gas Piping (NEC 250.104(B))

Under NEC 250.104(B), other metal piping systems, including fuel gas piping, that are likely to become energized must be bonded to the service equipment enclosure, grounded conductor, or grounding electrode system.

The Furnace / Appliance EGC Exception (NEC 250.104(B))

How is standard rigid black iron or galvanized steel gas piping bonded? In standard installations:

Code Rule: The equipment grounding conductor (EGC) for the electrical branch circuit that supplies the appliance or equipment that is likely to energize the piping is permitted to serve as the bonding means.

Example: An electric-ignited gas furnace or gas water heater is supplied by a 120V, 15A or 20A branch circuit containing a copper EGC. That branch circuit EGC terminates on the metal frame of the furnace, which is mechanically and electrically threaded to the gas pipe. Under NEC 250.104(B), this connection satisfies the bonding requirement for standard rigid steel gas piping! No separate external bonding jumper to the service panel is required.

Corrugated Stainless Steel Tubing (CSST) Gas Piping

Unlike thick-walled rigid steel pipe, Corrugated Stainless Steel Tubing (CSST) has very thin metal walls. During nearby lightning strikes, high-voltage electromagnetic surges induce arcing between the CSST and adjacent grounded metal objects (such as copper water lines, electrical conduit, or HVAC ductwork). This arcing instantly burns pinholes through the thin stainless steel wall, allowing pressurized natural gas or propane to escape and cause catastrophic house explosions.

+-----------------------------------------------------------------------------------+
|                         CSST GAS PIPING BONDING MANDATES                          |
+-----------------------------------------------------------------------------------+
|  HAZARD: Indirect lightning creates arc pinholes in thin stainless steel wall.    |
|                                                                                   |
|  BONDING REQUIREMENT (Per NFPA 54 National Fuel Gas Code & Manufacturer Specs):    |
|  - Direct bonding clamp attached to CSST brass manifold or rigid pipe fitting.    |
|  - Bonding Conductor: Minimum 6 AWG copper conductor.                             |
|  - Termination: Connected directly to the electrical Grounding Electrode System   |
|    or service equipment ground bus.                                               |
|  - Conductor Length: Kept as short as practical (typically max 75 ft).            |
+-----------------------------------------------------------------------------------+

• Standard CSST: Requires a dedicated bonding jumper of not less than 6 AWG copper connected between the CSST brass manifold (or adjacent rigid pipe) and the electrical service grounding electrode system.
• Arc-Resistant CSST (Black Jacket): Modern arc-resistant CSST with a conductive black jacket is tested to dissipate lightning energy and, where approved by the AHJ and manufacturer instructions, is permitted to be bonded via the appliance branch circuit EGC.

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4. Bonding Exposed Structural Metal (NEC 250.104(C))

Under NEC 250.104(C), exposed structural metal that forms a building frame and is not intentionally grounded must be bonded to:

1. The service equipment enclosure,
2. The grounded conductor at the service,
3. The grounding electrode conductor (where sized per Table 250.102(C)(1)), OR
4. One or more grounding electrodes of the grounding electrode system.

Sizing Structural Metal Bonding Jumpers

Structural building steel bonding jumpers must be sized in accordance with NEC Table 250.102(C)(1) based on the circular mil area of the largest ungrounded service-entrance conductors. If the service conductors exceed 1100 kcmil copper or 1750 kcmil aluminum, the 12.5% rule applies.

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5. Summary Table: NEC Article 250 Bonding Sizing Rules