10.3 Bonding of Interior Metal Piping, Structural Steel & Other Systems (NEC 250.104)

10.3 Bonding of Interior Metal Piping, Structural Steel & Other Systems (NEC 250.104)

Quick Answer: Under NEC Section 250.104(A), interior metal water piping systems must be bonded to the service equipment, grounded conductor, or grounding electrode conductor using a bonding jumper sized per NEC Table 250.102(C)(1) based on the largest ungrounded service-entrance conductor. Exposed structural building steel must likewise be bonded per NEC 250.104(C) using Table 250.102(C)(1). In contrast, other metal piping—including fuel gas piping (NEC 250.104(B))—is sized per NEC Table 250.122 based on the circuit likely to energize it, and the branch-circuit equipment grounding conductor supplying gas appliances (furnace, water heater) is explicitly permitted to serve as the bonding means.

Inside any modern residential, commercial, or industrial structure, non-electrical metallic piping and structural frameworks weave continuously past electrical branch circuits, feeders, lighting fixtures, and appliances. If a motorized appliance experiences an internal ground fault, or if an overhead electrical conduit chafes against building steel or a copper water pipe, the entire metallic system could be elevated to 120, 208, 277, or 480 volts to ground. Without equipotential bonding, anyone touching a water faucet, natural gas valve, or building column could be electrocuted. NEC Section 250.104 establishes the definitive standards for bonding these non-electrical conductive building elements.

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The Concept of Equipotential Bonding

Equipotential bonding is the deliberate physical joining of all normally non-current-carrying metallic systems within a facility to ensure that, under normal conditions and during lightning surges or electrical ground faults, zero difference of potential (voltage) exists between any two exposed conductive surfaces.

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Bonding Interior Metal Water Piping: NEC 250.104(A)

Candidates must distinguish between the metal water pipe grounding electrode (NEC 250.52(A)(1)) and the interior metal water piping bonding jumper (NEC 250.104(A)):

• The grounding electrode is the 10+ feet of metal pipe in direct contact with the earth outside the building, connecting the system to earth.
• The interior bonding jumper interconnects the interior plumbing network so that a fault anywhere in the building plumbing trips an overcurrent device rather than energizing plumbing fixtures.

Sizing and Termination Rules (NEC 250.104(A)(1))

• Sizing Table: The bonding jumper for interior metal water piping must be sized from NEC Table 250.102(C)(1) based on the rating of the largest ungrounded service-entrance conductor (or parallel set of conductors).
• Attachment Points: The bonding jumper may be connected to:
  1. The service equipment enclosure.
  2. The grounded service conductor (neutral busbar).
  3. The grounding electrode conductor (GEC), where of sufficient size.
  4. One or more grounding electrodes of the grounding electrode system.
• Accessibility: All bonding jumper connection points must be accessible per NEC 250.104(A)(1).
• Water Meters & Dielectric Unions: Where water meters, dielectric unions, water softeners, or pressure-reducing valves create an electrical interruption in the piping, a bonding jumper shunt must be installed across the equipment to preserve electrical continuity throughout the building.

Multi-Occupancy Buildings (NEC 250.104(A)(2))

In commercial multi-tenant buildings or apartment complexes where individual tenant spaces are plumbed with plastic (PEX/PVC) main risers that feed isolated metallic branch piping within each tenant space, running a dedicated bonding jumper from every apartment back to the main service switchboard is impractical.

• Code Exception: NEC 250.104(A)(2) permits the interior metal piping within an individual tenant occupancy to be bonded to the panelboard equipment grounding terminal serving that occupancy, using a bonding conductor sized per NEC Table 250.122 based on the rating of the panelboard overcurrent protective device.

Multiple Buildings Supplied by Feeder (NEC 250.104(A)(3))

Where an interior metal water piping system exists in a separate building supplied by a feeder or branch circuit (e.g., a detached garage or farm workshop), the piping is bonded to the equipment grounding terminal of the building disconnecting means with a jumper sized under 250.102(D), which means Table 250.122 based on the rating of the feeder or branch-circuit OCPD. Table 250.102(C)(1) is not used here, because the building is not supplied by service conductors.

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

NEC Section 250.104(B) governs all other interior metal piping systems that are "likely to become energized," including compressed air piping, pneumatic lines, medical gas piping, and fuel gas (natural gas and propane) piping.

Sizing and Permitted Means

• Sizing Standard: Unlike water piping and structural steel (which are sized per Table 250.102(C)(1)), other metal piping is sized strictly in accordance with NEC Table 250.122, using the rating of the overcurrent device protecting the electrical circuit that is likely to energize the piping.
• Appliance Circuit EGC Permission: Under NEC 250.104(B), the equipment grounding conductor (EGC) of the electrical circuit that supplies an appliance connected to the piping system is explicitly permitted to serve as the bonding means.
  • Example: A residential gas furnace is fed by a 120-volt, 15-ampere dedicated branch circuit containing a 14 AWG copper equipment grounding conductor terminated to the furnace metal chassis. Because the gas manifold bolts directly to the furnace chassis, the 14 AWG branch-circuit EGC completely satisfies the NEC bonding requirement for the entire fuel gas piping system. No separate bonding wire back to the panelboard is required by the NEC!

Corrugated Stainless Steel Tubing (CSST) Gas Piping

While standard threaded black iron gas pipe requires no special jumper beyond the appliance branch circuit, Corrugated Stainless Steel Tubing (CSST) presents a major fire hazard during indirect lightning strikes:

• CSST has thin stainless steel walls (often only 0.010 inch thick). An indirect lightning strike creates high transient ground potential spikes, causing electrical arcing between the thin CSST wall and nearby grounded building steel or electrical conduits. This arcing punctures pinholes in the CSST, igniting pressurized natural gas.
• NFPA 54 (National Fuel Gas Code) & Manufacturer Rules: The fuel gas codes and CSST manufacturers mandate a dedicated direct bonding jumper:
  • Minimum 6 AWG copper conductor (or sized per manufacturer instructions).
  • Clamped to a listed brass CSST fitting or rigid pipe nipple ahead of the first CSST run.
  • Bonded directly to the electrical service grounding electrode system, grounding electrode conductor, or service neutral.
• Arc-Resistant CSST: CSST listed with an arc-resistant jacket under ANSI LC 1/CSA 6.26 is generally bonded through the normal effective ground-fault current path rather than a separate 6 AWG jumper. The fuel gas code adopted in Minnesota and the manufacturer's installation instructions decide which method applies; the NEC points to them in an Informational Note.

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Sizing Bonding Jumpers: NEC Table 250.102(C)(1)

NEC Table 250.102(C)(1) is the universal table used for sizing Main Bonding Jumpers (MBJ), System Bonding Jumpers (SBJ), Supply-Side Bonding Jumpers (SSBJ), Interior Water Piping Jumpers, and Structural Metal Bonding Jumpers.

The 12.5% Rule for Large Services (Table 250.102(C)(1) Note 1)

Where the ungrounded phase conductors exceed 1100 kcmil copper or 1750 kcmil aluminum, the table runs out of standard sizes. Under Note 1:

The bonding jumper shall have an area not less than 12.5 percent of the circular mil area of the largest ungrounded phase conductor (or total area of parallel conductors per phase).

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Bonding Exposed Structural Metal: NEC 250.104(C)

Under NEC Section 250.104(C), exposed structural metal that forms a building frame and is not intentionally grounded or bonded through the grounding electrode system must be bonded to:

1. The service equipment enclosure.
2. The grounded conductor at the service.
3. The grounding electrode conductor where of sufficient size.
4. One or more grounding electrodes of the grounding electrode system.

• Sized in accordance with Table 250.102(C)(1) based on the size of the service-entrance phase conductors.
• In industrial and steel-framed high-rise construction, bonding structural steel is crucial because steel columns frequently support high-voltage lighting, HVAC conduit hangers, and cable trays.

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Intersystem Bonding Termination: NEC 250.94

Telephone, cable television (CATV), community antenna systems, and broadband fiber/coaxial drops entering a building must be bonded to the electrical grounding system to prevent lightning strikes from entering through external communication wires.

Mandatory IBT Specifications (NEC 250.94(A))

An Intersystem Bonding Termination (IBT) must be provided external to enclosures at the service equipment or metering equipment, and at the disconnecting means of a building supplied by a feeder or branch circuit:

1. Location: Accessible for connection and inspection, mounted on or near the service equipment enclosure, meter enclosure, or exposed nonflexible metal service raceway, without interfering with opening any enclosure.
2. Capacity: A set of terminals able to connect not less than three intersystem bonding conductors, and listed as grounding and bonding equipment.
3. Connection: Connected directly to the service equipment enclosure, grounded service conductor, or grounding electrode conductor with a minimum 6 AWG copper conductor.
4. No Isolated Electrodes: Where a communications or CATV installer drives a separate electrode, it must be bonded to the power grounding electrode system with at least a 6 AWG copper conductor. An unbonded rod creates dangerous potential differences between televisions or modems and household outlets during lightning events.