Does the NEC require you to ground piping systems and structural steel? Be careful how you answer this question.
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All Code references are based on the 2005 NEC. The grounding and bonding requirements in this column apply to solidly grounded systems that operate at not more than 600V, such as 120/240V, 120/208V, and 277/480V.
You've probably seen project requirements that call for the grounding of piping systems and exposed structural steel. Those requirements, although well intended, miss the mark. The stated intention of such requirements is nearly always the removal of dangerous voltage on specific types of metal parts in the event of a ground fault. These metal parts include exposed structural steel members, electrically conductive metal water piping systems, metal sprinkler piping, metal gas piping, and other metal piping systems. But these requirements fail to make that intention a reality. That's because you remove dangerous voltage on metal parts through bonding, not through grounding.
Although you don't need to ground piping systems and structural steel, you do need to bond them. And you need to bond them in a manner that establishes an effective ground-fault current path [250.4(A)(4)].
Metal water piping systems. You don't need to bond isolated sections of metal water piping connected to a nonmetallic water piping system. But where you have metal water piping systems (rather than just isolated sections that are metal), you must accomplish the bonding via one of three sets of requirements. Which set you conform to depends on whether your installation is a building supplied by a service, a building supplied by a feeder, or a multiple occupancy building.
Building supplied by a service. You must bond the metal water piping system to one of the following [250.104(A)(1)] (Fig. 1 above):
Service equipment enclosure
Grounded neutral service conductor
Grounding electrode conductor, if sized per Table 250.66
One of the electrodes of the grounding electrode system
Size the metal water pipe bonding jumper per Table 250.66, based on the largest ungrounded service conductor.
Here's an easy pop quiz for you. What size bonding jumper must you use for the metal water piping system, if the service conductors are 4/0 AWG (Fig. 2)?
The correct answer is 2 AWG, as per Table 250.66.
Where hot and cold water pipes are electrically connected, you need only one bonding jumper — to either the cold- or hot-water pipe. Otherwise, use a single bonding jumper sized per 250.104(A)(1) to bond the hot- and cold-water piping together.
Building or structure supplied by a feeder. You must bond the metal water piping system of a building or structure supplied by a feeder to one of the following:
The equipment grounding terminal of the building disconnect enclosure
The feeder equipment grounding (bonding) conductor
One of the electrodes of the grounding electrode system
Size the metal water piping system-bonding jumper per Table 250.66, based on the feeder circuit conductors that supply the building or structure. You don't need to make this bonding jumper larger than the ungrounded feeder conductors.
Multiple occupancy building. If the metal water piping systems in individual occupancies do not mechanically connect to each other, you can bond the metal water piping system (for each occupancy) to the equipment-grounding terminal of the panelboard [250.104(A)(2)]. Size this bonding jumper per Table 250.122, based on the ampere rating of the occupancy feeder overcurrent protection device.
Other metal piping systems. You must bond metal piping systems (such as gas or air) to an effective ground-fault current path, if they're likely to become energized [250.104(B)]. According to the NFPA's National Fuel Gas Code, the equipment-grounding (bonding) conductor for the circuit that may energize the piping can serve as the bonding means.
Because the equipment-grounding (bonding) conductor for the circuit that may energize the piping can serve as the bonding means, the NEC doesn't require further bonding on the part of the electrical installer. Bonding of all metal piping and metal ducts within the building provides an additional degree of safety, but it isn't an NEC requirement [250.104(B) FPN].
Structural metal. If exposed structural metal that forms a metal building frame is likely to become energized, you must bond it to (Fig. 3) one of these [250.104(C)]:
Service equipment enclosure
Grounded neutral service conductor
Grounding electrode conductor, if sized per Table 250.66
One or more of the electrodes of the grounding electrode system
This rule doesn't require you to bond sheet metal framing members (studs) or the metal skin of a wood frame building, but doing so is a good practice. Size the bonding jumper for the structural metal per Table 250.66, based on the feeder or service conductors that supply the building (or structure). This bonding jumper must be:
Copper where within 18 inches 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)].
Separately derived systems. In the area served by a separately derived system (SDS), you must bond the nearest available point of the metal water piping system to the grounded neutral terminal of the SDS. At the SDS end, you must make this bond at the same location where the grounding electrode conductor and system-bonding jumper terminate [250.104(D)(1)].
Size this metal water piping-bonding jumper per Table 250.66, based on the largest ungrounded conductor of the SDS. You don't need a water pipe bonding jumper, if you:
Use the water pipe as the grounding electrode for the SDS, or
Bond the metal water pipe to the structural metal building frame being used as the grounding electrode for the SDS [250.104(D)(1) Ex 2] (Fig. 4).
Where exposed structural metal forms the building frame, you must bond it to the grounded neutral conductor of each SDS. At the SDS end, you must make this bond at the same location where the grounding electrode conductor and system-bonding jumper terminate [250.32(A)]. Size each bonding jumper per 250.66, based on the largest ungrounded conductor of the SDS. You don't need a structural metal bonding jumper, if you:
Use the metal structural frame as the grounding electrode for the SDS, or
Bond the structural metal frame to metal water piping being used as the grounding electrode for the SDS.
Common grounding electrode conductor. Where you have installed a common grounding electrode conductor for multiple SDSs — as permitted by 250.30(A)(4) — you must bond exposed structural metal and interior metal piping (in the area served by the SDS) to the common grounding electrode conductor [250.104(D)(3)]. But you don't have to install a separate bonding jumper from each derived system to metal water piping (and to structural metal members), if you have bonded the metal water piping and the structural metal members (in the area served by the SDS) to the common grounding electrode conductor.
Lightning protection system. Does the facility have a lightning protection system? If yes, bond this system to the building (or structure) grounding electrode system [250.106]. Do not use the grounding electrode for a lightning protection system as the building (or structure) grounding electrode (250.60). See NFPA 780, Standard for the Installation of Lightning Protection Systems, for additional details on grounding and bonding requirements for lightning protection.
Metal raceways, enclosures, frames, and other metal parts of electrical equipment may require bonding or spacing from the lightning protection conductors (per NFPA 780). Separation from lightning protection conductors is typically 6 feet through air, or 3 feet through dense materials, such as concrete, brick, or wood.
People often talk about grounding metal piping systems and structural steel. The danger with such talk is it can mislead you into thinking you have met the requirements of Art. 250 and several other standards, when you haven't. Bonding deficiencies are notorious for permitting power-quality problems to exist. But the worst part is they leave you with an unsafe facility. For safety and performance, bond.