Technical Article

NEC 2023 Basics: Grounding and Bonding Piping Systems and Exposed Structural Metal

August 01, 2023 by Lorenzo Mari

Learn about the rules for bonding piping systems and exposed structural metal.

NEC Section 250.104 requires bonding the building’s exposed structural metal, interior water piping, and other metal piping systems. This requirement differs from the one outlined in Section 250.50, which requires bonding the metal underground water pipe as a component of the grounding electrode system.

Image used courtesy of Pixabay

National Electrical Code Section 250.104 Bonding of Piping Systems and Exposed Structural Metal

Section 250.104(A) Metal Water Piping

250.104(A)(1) General

Bond the metal water piping system to any of the following:

The service equipment enclosure.
The service grounded conductor.
The grounding electrode conductor – if large enough.
One or more grounding electrodes used – the grounding electrode conductor and bonding jumpers must have an acceptable size.

This rule aims to equalize the bonded components’ potential, eliminating dangerous voltage that may appear during ground faults. The rule does not apply to isolated sections of metal water piping integrated into a nonmetallic water piping system. Bonding these isolated pieces might add shock hazards.

Figure 1 shows a bonding jumper between a metal water piping system and a grounding electrode conductor in a service-supplied building. Note that the hot and cold metal water pipes are electrically connected, requiring only one bonding jumper. Connect the bonding jumper either to the hot or cold water pipe.

Figure 1. Metal water piping system bonded to the grounding electrode conductor. Image used courtesy of Lorenzo Mari

The water heater in Figure 1 has mechanical and electrical connections within the equipment. A failure of the electrical insulation can energize the connected piping system.

The bonding jumpers must comply with the following rules:

Install per 250.64(A), (B), and (E).
Keep the points of attachment reachable.

Only the connections of the bonding jumpers must be accessible for a checkup – not the total length.

Size per Table 250.102(C)(1), with two exceptions:

◦ Not required to be larger than size N° 3/0 copper or N° 250 kcmil aluminum or copper-clad aluminum.

◦ As permitted in sections 250.104(A)(2) (buildings of multiple occupancy) and (A)(3) (buildings served by feeders or branch circuits).

Example: Compute the bonding jumper size in Figure 1 if the service conductor’s size is N° 3/0 AWG. Assume copper conductors.

Solution: Enter Table 250.102(C)(1) with conductor size N° 3/0 AWG, copper, and read size N° 4 AWG, copper.

250.104(A)(2) Multiple Occupancy Buildings

This section permits bonding the metal water piping of individual units in a multiple occupancy building to the equipment grounding busbar of the switchgear, switchboard, or panelboard enclosure. Observe that the bonding jumper connects the water pipes to the equipment grounding terminal bar, not the neutral terminal bar. These rules relate to apartment houses, shopping centers, office buildings, etc.

The restrictions of this permission are:

Isolate individual pipes from all other units by nonmetallic water pipes.

One way to accomplish isolation is to have a nonmetallic water distribution system feeding the metal water piping in each unit.

Do not bond to the service equipment enclosure.
Size the bonding jumper per Section 250.102(D) – which means sizing per Table 250.122.

As the input to Table 250.122 is the rating or setting of the automatic protective overcurrent device feeding each unit, this figure settles the minimum bonding jumper size.

Figure 2 shows a typical connection in a multiple occupancy building.

Figure 2. Bonding metal water pipes in a multiple occupancy building. Image used courtesy of Lorenzo Mari

250.104(A)(3) Buildings Supplied by Feeders or Branch Circuits

Bond the metal water piping system of a building supplied by a feeder or branch circuit to any of the following:

The enclosure of the building’s disconnecting means.
The equipment grounding conductor run with the feeder or branch circuit.
One or more of the building’s grounding electrodes.

Size the bonding jumper per Section 250.102(D). The bonding jumper size does not need to be larger than the largest feeder or branch-circuit ungrounded conductor.

Section 250.102(D) sizes the bonding jumpers on the load side of overcurrent devices and refers to Table 250.122.

Section 250.104(B) Other Metal Piping

This section covers metal piping systems not intended to carry water, such as air, process fluids, or liquids.

Bond all interior metal piping susceptible to being energized. This requirement realizes that metal pipes are electrically conductive and may pose a shock hazard.

Note that the NEC highlights gas piping. This fact is important because, since the gas is flammable, we could erroneously conclude that these pipes should not be connected to the electrical system.

Section 250.52(B)(1) prohibits using a metal underground gas piping system as a grounding electrode. As the bonding required in Section 250.104(B) does not make the gas piping part of the grounding electrode system, this requirement does not conflict with Section 250.52(B)(1). Typically, gas utilities provide isolating fittings to prevent the underground gas piping from turning into a grounding electrode.

Examples of other metal piping lines are:

Pneumatic
Waste, drain, and vent
Oxygen, air, and vacuum
Air conditioning

Connect the metal pipes likely to become energized to one of the following:

The equipment grounding conductor of the circuit capable of energizing the piping system. There is no need to use an additional bonding jumper if the equipment grounding conductor – connected to the non–current-carrying metal parts of the appliance – supplies a bonding connection to the metal piping affixed to the device.
The service equipment enclosure.
The service grounded conductor.
The grounding electrode conductor – if large enough.
One or more grounding electrodes used – the grounding electrode conductor and bonding jumpers must have an acceptable size.

Figure 3 shows two bonded metal pipes.

Figure 3. Bonding other metal pipes. Image used courtesy of Lorenzo Mari

Additional rules:

Size the bonding conductors or jumpers and the equipment grounding conductors per Table 250.122 on the circuit rating that may energize the pipes.

Keep the points of attachment of the bonding jumpers reachable.

The Informational Note N° 1 recommends bonding all piping and metal air ducts within the location to furnish added safety.

Section 250.104(C) Structural Metal

Bond all exposed structural metal interconnected to form a building frame, not deliberately grounded or bonded, and prone to become energized.

Bond to any of the following:

The service equipment enclosure.
The grounded conductor at the service.
The disconnecting means of buildings or structures supplied with feeders or branch circuits.
The grounding electrode conductor, if sized per Table 250.102(C)(1).
One or more grounding electrodes used, with the grounding electrode conductor or bonding jumper sized per Table 250.102(C)(1).

The bonding conductors or jumpers must comply with the following rules:

Install per 250.64(A), (B), and (E).
Keep the points of attachment reachable – unless installed per 250.68(A), Exception N° 2.

Exothermic or irreversible compression connections to fireproofed structural metal are not required to be accessible. See Figure 4.

Figure 4. Bonding a fireproofed structural metal to the grounded conductor at the service. Image used courtesy of Lorenzo Mari

Size per Table 250.102(C)(1) – not required to be larger than size N° 3/0 copper or N° 250 kcmil aluminum or copper-clad aluminum.

Section 250.104(D) Separately Derived Systems

Bond all metal water piping and structural metal members interconnected to form a building frame to the separately derived system. Follow the rules of sections 250.104(D)(1) through (3).

250.104(D)(1) Metal Water Piping Systems

Bond all metal water piping systems in the region served by a separately derived system to the grounded conductor.

The bonding jumper must comply with the following rules:

Attach to the same place the grounding electrode conductor is connected.
Size per Table 250.102(C)(1) – not required to be larger than size N° 3/0 copper or N° 250 kcmil aluminum or copper-clad aluminum.

Figure 5 shows metal water pipes bonded to a transformer’s secondary neutral (grounded conductor). Note that the grounding electrode conductor is connected at the same point.

Figure 5. Metal water pipes connected to a transformer’s secondary neutral. Image used courtesy of Lorenzo Mari

Exception N° 1. There is no need for a separate bond when the metal water piping system works as a grounding electrode or grounding electrode conductor.

Exception N° 2. There is no need for a separate bond when using the metal in-ground support structure as a grounding electrode, or the metal structural building frame is the grounding electrode conductor bonded to the metal water piping system. See Figure 6.

Figure 6. Metal water piping system bonded to the metal frame of the building. Image used courtesy of Lorenzo Mari

250.104(D)(2) Structural Metal

Bond all exposed structural metal interconnected to form a building frame to the grounded conductor of the separately derived system.

The bonding conductors or jumpers must comply with the following rules:

Attach to the same place the grounding electrode conductor is connected.
Size per Table 250.102(C)(1) – not required to be larger than size N° 3/0 copper or N° 250 kcmil aluminum or copper-clad aluminum.

Exception N° 1. There is no need for a separate bond when using the metal in-ground support structure as a grounding electrode, or the metal structural building frame is the grounding electrode conductor for the separately derived system.

Exception N° 2. There is no need for a separate bond when the water piping system is used as the grounding electrode or grounding electrode conductor and is bonded to the metal structural building frame.

250.104(D)(3) Common Grounding Electrode Conductor

Section 250.30(A)(6) permits installing a common grounding electrode conductor for

multiple separately derived systems. In such a case, connect a bonding jumper from the common grounding electrode conductor to the interior metal water piping and exposed structural steel in the area supplied by each separately derived system.

Figure 7 shows exposed metal structural members and metal piping systems bonded to a common grounding electrode conductor.

Figure 7. Bonding to a common grounding electrode conductor. Image used courtesy of Lorenzo Mari

Bonding Exposed Structural Metal and Piping System Takeaways

Section 250.104 aims at grounding metal structures and pipes that may become hazardous if energized by an electrical circuit.
The phrase “likely to become energized” means that the device may exhibit an electric potential if the insulation of a circuit fails.
This section applies to metal water piping systems installed or attached to a building or structure. It does not pertain to nonmetallic piping.
These rules also apply to metal water pipes not exceeding 3 m below ground – they are not grounding electrodes.
It is permitted to bond isolated metal water piping to the switchgear, panelboard, or switchboard enclosure in each unit of multiple occupancy buildings.
Bond to the grounding electrode system the exposed building steel likely to become energized.
The bonding requirements for separately derived systems are divided into three parts – metal water piping, structural metal, and common grounding electrode conductor.

To catch up on Lorenzo Mari’s series on National Electrical Code 2023 Basics: Grounding and Bonding, follow these links: