3.6 Bonding of Metal Piping, Building Steel & Separately Derived Systems (0202)

Why Bond Piping and Steel at All

A metal water line, a gas line, and a structural steel column are all long conductors running through the building. If any of them can be energized by a fault — a nicked conductor in a stud bay touching a copper line, a failed water heater element, a loose connection in a rooftop unit landing on a steel beam — then that metal becomes a shock hazard everywhere it goes, and it will stay energized until something clears the fault. Bonding gives that metal a low-impedance connection back to the source, so a fault on it draws enough current to open an overcurrent device instead of sitting at 277 volts waiting for someone to touch it.

The three rules of 250.104 are frequently confused because they use different sizing tables. Learn them as a set:

The gas-piping case is the one people get wrong. A gas line is bonded under 250.104(B), and the sizing comes from Table 250.122 for the circuit likely to energize it — usually the 15 or 20 A circuit feeding the furnace or water heater, which means a 14 or 12 AWG bonding conductor. The equipment grounding conductor of that appliance circuit is normally deemed to satisfy the requirement, so no separate wire is needed. Sizing a gas bond from the service conductors is a common over-engineering error.

Note also that 250.104(A) bonds the water piping system, not merely the pipe used as an electrode: the requirement applies whether or not the water pipe qualifies as a grounding electrode, and it applies to hot and cold piping alike.

Structural Metal — What Counts

250.104(C) applies to exposed structural metal that is interconnected to form a metal building frame and is likely to become energized. Two words carry the weight: interconnected (a single isolated lintel is not a building frame) and exposed (metal encased in concrete or otherwise not exposed is outside the rule). Note that this is a bonding requirement and is different from the metal in-ground support structure electrode of 250.52(A)(2), which is a grounding-electrode rule about metal in direct contact with earth for 10 ft or more.

Separately Derived Systems — 250.30

A separately derived system is a wiring system whose power is derived from a source with no direct electrical connection to the supply conductors of another system — most commonly the secondary of a dry-type transformer, and sometimes a generator with a four-pole transfer switch. Because the source is new, the system needs its own grounding and bonding architecture:

1. System bonding jumper (250.30(A)(1)) — an unspliced connection between the grounded conductor and the supply-side bonding jumper / equipment grounding conductors of the derived system, made at one single point only: either at the source (inside the transformer enclosure) or at the first system disconnecting means or overcurrent device. It is sized from Table 250.102(C)(1) based on the derived phase conductors.
2. Supply-side bonding jumper (250.30(A)(2)) — bonds the metal enclosures and raceways between the source and the first disconnect; also sized from Table 250.102(C)(1).
3. Grounding electrode conductor (250.30(A)(5)) — connects the grounded conductor of the derived system to a grounding electrode, at the same point where the system bonding jumper is connected. It is sized from Table 250.66 based on the derived phase conductors.
4. Grounding electrode (250.30(A)(4)) — must be as near as practicable to, and preferably in the same area as, the system bonding jumper connection. The Code gives a preference order: metal water pipe within 5 ft of entrance, or the metal in-ground support structure, or any of the other 250.52(A) electrodes.

The single-point rule, restated

Across an entire building there is exactly one neutral-to-ground bond per system:

• At the service, the main bonding jumper (250.28).
• At each separately derived system, the system bonding jumper (250.30).
• Nowhere else. Every downstream panelboard keeps the neutral bar isolated from the enclosure and from the equipment grounding bar.

A second bond anywhere downstream puts the equipment grounding conductor and the metal raceway system in parallel with the neutral, so normal load current — not just fault current — flows on the building steel, the conduit, and the water piping. That is objectionable current under 250.6, and on a commercial job it shows up as unexplained current on a clamp meter around a ground conductor. When you find it, the fix is to remove the bonding screw or strap at the downstream panel, not to add more bonding.

Inspection sequence for a transformer. Confirm the system bonding jumper exists and is at exactly one point; confirm the GEC lands at the same point; confirm the electrode is nearby and is one of the 250.52(A) types; confirm the secondary neutral is isolated at every downstream panel; and confirm the primary equipment grounding conductor is sized from Table 250.122 for the primary overcurrent device.