4.4 Bonding of Metal Water Piping, Gas Piping & Structural Steel

4.4 Bonding of Metal Water Piping, Gas Piping & Structural Steel

Quick Answer: Under NEC 250.104(A), interior metal water piping must be bonded using a conductor sized from Table 250.102(C)(1) based on the service-entrance conductors. Crucially, under the 5-foot rule (NEC 250.68(C)(1)), interior water pipe can only be used as a grounding conductor to interconnect electrodes within the first 5 feet of entering the building. Structural building steel forming the frame must also be bonded per Table 250.102(C)(1). Fuel gas piping is generally bonded by the equipment grounding conductor of the appliance it feeds, but Corrugated Stainless Steel Tubing (CSST) requires a dedicated 6 AWG copper bonding jumper. Finally, NEC 250.94 mandates an Intersystem Bonding Termination (IBT) with at least 3 terminals for communications grounding.

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1. Purpose and Theory of Equipotential Bonding (NEC 250.104)

In modern buildings, metallic systems—such as domestic water piping, hydronic heating lines, natural gas pipes, and structural steel beams—crisscross throughout walls, ceilings, and floors. If an electrical conductor's insulation fails and contacts ungrounded metal piping, the entire plumbing network could become energized at line voltage (120V, 277V, or 480V).

                    EQUIPOTENTIAL BONDING PRINCIPLE (NEC 250.104)

     +-------------------------------------------------------------------------+
     | If a bare phase conductor faults against an UNBONDED metal water pipe:  |
     | - The pipe sits at 120V to earth indefinitely.                          |
     | - A person washing hands who touches the faucet and a wet tile floor    |
     |   receives a fatal electrical shock!                                    |
     +-------------------------------------------------------------------------+
                                        |
                    [APPLY MANDATORY NEC 250.104(A) BONDING]
                                        |
                                        v
     +-------------------------------------------------------------------------+
     | With the metal water pipe BONDED to the electrical service enclosure:   |
     | - The fault current instantly flows back through the bonding jumper.    |
     | - Amperage surges into hundreds of amps, tripping the breaker in 10 ms! |
     | - Touch potential between faucet and drain is equalized to NEAR ZERO!   |
     +-------------------------------------------------------------------------+

Equipotential Bonding ensures that all exposed conductive metal surfaces in a building remain at the same electrical potential (voltage) under both normal operation and fault conditions, completely eliminating touch-potential voltage gradients across a person's body.

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2. Bonding Interior Metal Water Piping Systems (NEC 250.104(A))

NEC 250.104(A) governs the bonding of interior metal water piping systems:

"The interior metal water piping system shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or to the one or more grounding electrodes used."

Sizing the Water Pipe Bonding Jumper

The bonding jumper for interior metal water piping is sized in accordance with NEC Table 250.102(C)(1) based on the rating of the largest ungrounded service-entrance conductor (or 12.5% rule for services exceeding 1,100 kcmil copper).

             WATER PIPE BONDING JUMPER & THE CRITICAL 5-FOOT RULE

         Exterior Soil                      Building Interior
    =======================||=================================================
                           ||     (Within 5 ft)     |    (Beyond 5 ft)
      Underground Metal    ||                       |
      Water Pipe (10+ ft)  ||  [Main Water Valve]   |  Interior Domestic Water
    -----------------------||----+-------------+----+-------------------------
     (Grounding Electrode) ||    |             |    |  (Bonded per 250.104(A))
                           ||    +-Jumper Around-+    (CANNOT be used to
                           ||      Water Meter!       interconnect other electrodes!)
                           ||           |
                           ||           v
                           ||     [Point of GEC Connection]
                           ||     (Must be <= 5 ft from entrance!)

The 5-Foot Rule for Water Pipe Electrodes (NEC 250.68(C)(1))

Electricians must never confuse the water pipe grounding electrode conductor connection with the interior water piping bonding jumper:

• NEC 250.68(C)(1) Mandate: The connection of the grounding electrode conductor to a metal underground water pipe electrode must be made within 5 feet (1.52 m) of the point where the pipe enters the building.
• The Reason: Plumbers frequently cut out sections of interior copper pipe to install plastic water filters, water softeners, pressure valves, or PEX branch piping. If an electrician terminated a GEC on copper pipe 20 feet inside the building, a plumber replacing a section with PEX would silently sever the building's grounding system!
• Commercial Exception (NEC 250.68(C)(1) Ex.): In industrial and commercial buildings where conditions of maintenance ensure that only qualified persons service the installation, and the entire length of interior piping is exposed, the connection is permitted more than 5 feet from the entrance point.

Bonding Around Water Meters & Filters (NEC 250.104(A)(1))

Where water meters, pressure regulators, dielectric unions, or inline filtration devices are installed in the water line, a bonding jumper must be installed around the equipment to guarantee continuous electrical conductivity across the joint.

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

General Metal Piping Systems

Under NEC 250.104(B), any metal piping system (such as compressed air, vacuum, or chemical lines) that is likely to become energized must be bonded to the service equipment enclosure, grounded conductor, or grounding electrode system. The bonding jumper is sized per Table 250.122 based on the rating of the circuit that is likely to energize the piping.

Traditional Rigid Steel / Black Iron Fuel Gas Piping

• Code Allowance: For standard rigid black iron or steel gas piping, NEC 250.104(B) states that the equipment grounding conductor for the circuit that supplies the gas-utilizing equipment (e.g., the 14 AWG or 12 AWG copper equipment ground feeding a furnace or water heater) is permitted to serve as the bonding means.
• No additional dedicated bonding jumper back to the main electrical panel is required by the NEC for standard rigid threaded steel gas pipes.

Corrugated Stainless Steel Tubing (CSST) Special Requirements

Corrugated Stainless Steel Tubing (CSST) is a flexible gas piping system widely used in residential and commercial construction. However, CSST has extremely thin stainless steel walls (often only 0.010 inches / 0.25 mm thick).

                 CSST LIGHTNING ARREST & BONDING MECHANISM

        Nearby Lightning Strike (Ground Potential Rise / Induced Charge)
                                      |
                                      v
                 +-----------------------------------------+
                 |     Thin-Wall CSST Gas Pipe (0.010")    |
                 +-----------------------------------------+
                                      | (Arcing puncture hazard!)
                                      v
                 +-----------------------------------------+
                 | Adjacent Grounded Metal (Duct / Wire)   |
                 +-----------------------------------------+

    CSST SOLUTION: DIRECT BONDING JUMPER (MINIMUM 6 AWG COPPER)
    - Clamped to brass fitting / rigid pipe ahead of CSST run.
    - Bonded directly to service ground bus or GEC.
    - Equalizes potential and diverts surge currents safely around thin CSST!

• The Lightning Hazard: When a nearby lightning strike occurs, massive electrostatic charges are induced on metal structures. The voltage difference between the CSST and adjacent grounded metal objects (such as HVAC ductwork, building steel, or electrical wiring) can cause an electric arc to jump across the air gap. This arc instantly burns a pinhole through the thin CSST wall, igniting pressurized natural gas or propane and creating structural fires.
• Mandatory CSST Bonding: In accordance with CSST manufacturer installation instructions, NFPA 54 (National Fuel Gas Code), and 527 CMR (Massachusetts Electrical Code amendments), traditional yellow-jacketed CSST must be bonded with a minimum 6 AWG copper bonding jumper connected directly to the rigid pipe or CSST brass termination fitting ahead of the CSST run and routed to the electrical service enclosure, GEC, or grounding electrode.

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4. Bonding Structural Metal Building Frames (NEC 250.104(C))

Where exposed structural metal that forms a building frame is not effectively grounded, it must be bonded to establish an equipotential envelope:

• Sizing: Sized in accordance with NEC Table 250.102(C)(1) based on the largest ungrounded service-entrance conductor.
• Point of Connection: The bonding jumper may terminate at:
  1. The service equipment enclosure;
  2. The grounded conductor at the service;
  3. The grounding electrode conductor (where of sufficient size); or
  4. One or more grounding electrodes of the grounding electrode system.
• Exposed Structural Steel: This rule applies to exposed metal framing members that form the building structure. Steel trusses, structural I-beams, and columns interconnected by bolted or welded connections create an equipotential cage that protects occupants and sensitive building automation systems.

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5. Piping & Structural Bonding Summary Reference Table

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6. Intersystem Bonding Termination (IBT) (NEC 250.94)

Historically, cable television (CATV), telephone, and satellite dish installers would drive their own separate ground rods or attach grounding clamps to random metal parts, such as loose sheet metal siding, water spigots, or flexible conduit. During a lightning strike, different ground rods would rise to drastically different voltages, blowing out televisions, modems, and consumer electronics.

To solve this cross-system danger, NEC 250.94 mandates an Intersystem Bonding Termination (IBT):

                    INTERSYSTEM BONDING TERMINATION (IBT) DEVICE

       +-------------------------------------------------------------------+
       |               INTERSYSTEM BONDING TERMINATION BLOCK               |
       |               (Mounted externally near service equipment)         |
       |                                                                   |
       |  [Terminal 1] ====== CATV / Broadband Coaxial Ground Block        |
       |                                                                   |
       |  [Terminal 2] ====== Telephone / Network Interface Device (NID)   |
       |                                                                   |
       |  [Terminal 3] ====== Satellite Dish / Antenna Discharge Unit      |
       +-------------------------------------------------------------------+
                                         |
                         Minimum 6 AWG Copper Conductor
                                         |
                                         v
                       [Service Disconnect Enclosure or GEC]

Key Requirements of NEC 250.94(A):

1. Accessibility: Must be installed external to enclosures and readily accessible at the service equipment or metering equipment enclosure.
2. Capacity: Must have the capacity for connection of not less than three (3) intersystem bonding conductors.
3. Listing: Must be listed as grounding and bonding equipment.
4. Connection / Sizing: Must be connected to the service equipment enclosure, meter socket, or GEC using a conductor not smaller than 6 AWG copper.
5. Impact: Guarantees that communications, television, telephone, and electrical systems all share the exact same ground reference point, eliminating destructive ground loop voltages.