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Grounding versus Bonding

PART V. BONDING

Sections

250.104 Bonding of Piping Systems and Exposed Structural Metal.

Author’s Comment: To remove dangerous voltage on metal parts from a ground fault, electrically conductive metal water piping systems, metal sprinkler piping, metal gas piping, and other metal piping systems, as well as exposed structural steel members that are likely to become energized, must be bonded to an effective ground-fault current path [250.4(A)(4)].
(A) Metal Water Piping System. Metal water piping systems must be bonded in accordance with (1), (2), or (3).

Author’s Comment: Bonding isn’t required for isolated sections of metal water piping connected to a nonmetallic water piping system.
(1) Building Supplied by a Service. The metal water piping system of a building or structure must be bonded to one of the following: Figure 250–149
  • Service equipment enclosure
  • Grounded neutral service conductor
  • Grounding electrode conductor where the grounding electrode conductor is sized in accordance with Table 250.66
  • One of the electrodes of the grounding electrode system
    The metal water pipe bonding jumper must be sized in accordance with Table 250.66, based on the largest ungrounded service conductor.
Question: What size bonding jumper is required for the metal water piping system if the service conductors are 4/0 AWG? Figure 250–150

(a) 6 AWG     (b) 4 AWG     (c) 2 AWG     (d) 1/0 AWG

Answer: (c) 2 AWG, Table 250.66

Author’s Comment: Where hot and cold water pipes are electrically connected, only one bonding jumper is required, either to the cold or hot water pipe. Otherwise, a single bonding jumper, sized in accordance with 250.104(A)(1), must be used to bond the hot and cold water piping together.
(2) Multiple Occupancy Building. When the metal water piping system in an individual occupancy is metallically isolated from other occupancies, the metal water piping system for that occupancy can be bonded to the equipment grounding terminal of the occupancy’s panelboard. Figure 250–151

The bonding jumper used for this purpose must be sized to the ampere rating of the occupancy’s feeder overcurrent protection device in accordance with Table 250.122.

(3) Building or Structure Supplied by a Feeder. The metal water piping system of a building or structure that is supplied by a feeder must be bonded to:
  • The equipment grounding terminal of the building disconnect enclosure,
  • The feeder equipment grounding (bonding) conductor, or
  • One of the electrodes of the grounding electrode system.
The bonding jumper for the metal water piping system must be sized to the feeder circuit conductors that supply the building or structure in accordance with Table 250.66. The bonding jumper is not required to be larger than the ungrounded feeder conductors.
Author’s Comment: It makes no sense to size the metal water piping bonding jumper in accordance with Table 250.66, based on the feeder circuit conductor size. The bonding jumper should be sized in accordance with Table 250.122, based on the building or structure feeder overcurrent protection device.
(B) Other Metal Piping Systems. Metal piping systems such as gas or air that are likely to become energized must be bonded to an effective ground-fault current path. The equipment grounding (bonding) conductor for the circuit that may energize the piping can serve as the bonding means.

Author’s Comments:
• This exact text is contained in NFPA 54, National Fuel Gas Code.
• Because the equipment grounding (bonding) conductor for the circuit that may energize the piping can serve as the bonding means, no action is required by the electrical installer. Figure 250–152

FPN: Bonding of all metal piping and metal ducts within the building provides an additional degree of safety, but this isn’t an NEC requirement.
(C) Structural Metal. Exposed structural metal that forms a metal building frame that is likely to become energized must be bonded to: Figure 250–153
  • Service equipment enclosure
  • Grounded neutral service conductor
  • Grounding electrode conductor where the grounding electrode conductor is sized in accordance with Table 250.66
  • One of the electrodes of the grounding electrode system
Author’s Comment: This rule doesn’t require the bonding of sheet metal framing members (studs) or the metal skin of a wood frame building, but it would be a good practice.
The bonding jumper for the structural metal must be sized to the feeder or service conductors that supply the building or structure in accordance with Table 250.66, and the bonding jumper must be:
  • Copper where within 18 in. of earth [250.64(A)].
  • Securely fastened and adequately protected if exposed to physical damage [250.64(B)].
  • Installed without a splice or joint, unless spliced by irreversible compression connectors listed for the purpose or by the exothermic welding process [250.64(C)].


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