Weekly digest #171: bonding gotchas

Bonding is not grounding

Bonding ties metal parts together so fault current has a low impedance path back to the source. Grounding ties the system to earth. Two different jobs, two different rule sets, and the inspector will catch you if you mix them up. NEC 250.4(A) lays out the performance requirements for grounded systems, and 250.4(A)(3) through (5) is where bonding lives.

The earth is not a fault current path. Driving more rods will not clear a fault. If the equipment grounding conductor or bonding jumper is missing or undersized, the breaker will not trip and the metal stays energized. That is the gotcha that puts people in the hospital.

Read 250.4 once a quarter. The language is dense but every word matters when you are explaining a redline to a GC.

Service bonding: the main bonding jumper

At the service, the grounded conductor (neutral) and the equipment grounding system are bonded together exactly once. That connection is the main bonding jumper, and it is the only place neutral current is allowed on the metal. NEC 250.24(A)(5) prohibits a neutral to case connection on the load side. Violate that and you get objectionable current on conduits, water pipes, and anything else metallic.

Common field mistakes:

• Leaving the green bonding screw out of a service-rated panel (250.28).
• Installing the green screw in a sub-panel that should have an isolated neutral (250.24(A)(5)).
• Using a separately derived system without a system bonding jumper sized per 250.28(D) and 250.30(A)(1).
• Bonding the neutral at a generator that is not a separately derived system, creating parallel neutral paths.

Generators trip people up the most. If the transfer switch does not switch the neutral, the generator is not separately derived and the neutral stays bonded only at the service. Switch the neutral and you have a separately derived system that needs its own SBJ and grounding electrode per 250.30.

Metal raceway and the four-foot rule

Listed metal raceway is recognized as an equipment grounding conductor under 250.118. That fitness depends on every fitting being made up tight and every coupling being listed for grounding. One loose connector and your EGC path is dead.

NEC 250.86 requires bonding around any reducing washers, oversized knockouts, or concentric/eccentric knockouts on circuits over 250 volts to ground. On services, 250.92(B) requires bonding regardless of voltage, using one of the methods in 250.92(B)(1) through (4). Standard locknuts are not on that list. You need bonding bushings, bonding locknuts, or bonding jumpers around every concentric KO at the service.

If you can fit a screwdriver tip between the locknut and the enclosure ring, you have a concentric knockout. Add a bonding bushing with a jumper back to the neutral bar at the service, or to the EGC bus on the load side over 250V to ground.

Flex is the other trap. FMC and LFMC are only EGCs in limited conditions per 250.118(5) and (6): 6 feet or less in the ground return path, fittings listed for grounding, and overcurrent protection at or below the values in the table. Longer than 6 feet of flex in the ground path, or a vibration-prone connection, and you pull a separate EGC.

Sizing bonding jumpers

Three different rules cover three different situations and they are not interchangeable:

1. Supply-side bonding jumper, sized per 250.102(C) using Table 250.102(C)(1) based on the largest ungrounded service conductor.
2. Equipment bonding jumper on the load side, sized per 250.102(D) using Table 250.122 based on the upstream OCPD.
3. Bonding jumper for parallel raceways, where each raceway gets its own jumper sized to the OCPD, not the full feeder.

The supply-side rule catches people because it scales with conductor size, not breaker size. A 400A service with parallel 3/0 copper conductors needs a #4 supply-side bonding jumper, even though there is no main breaker yet. Run that calc before you order lugs.

Pools, pipes, and other special bonds

Pool bonding under 680.26 is its own world. The equipotential bonding grid ties together the shell, perimeter surfaces within 3 feet, all metal within 5 feet, pumps, ladders, and anything else conductive, with a #8 solid copper minimum. The grid does not connect to the service grounding electrode through a separate run, but the pump motor EGC ties the grid back to the system. Miss a tie point and you get a tingly pool.

Metal water piping gets bonded per 250.104(A) with a conductor sized from Table 250.102(C)(1). Structural metal frames get bonded under 250.104(C) when they are likely to become energized. Gas piping under 250.104(B) is bonded through the EGC of the appliance circuit, no separate run required, but verify with the local AHJ because some jurisdictions amend this.

On a remodel, walk the basement before you quote. If the old bond clamp is on a section of pipe that got replaced with PEX, your bond is floating and you need to land it on the first 5 feet of metal entering the building.

Inspection-day checklist

Before the inspector shows up, walk the service and hit these points. Most red tags on bonding come from the same handful of items.

• Main bonding jumper or green screw installed at service, removed at all sub-panels.
• Bonding bushings on every concentric KO at the service, jumpered back to the neutral bar.
• Grounding electrode conductor unspliced or spliced only with listed irreversible means (250.64(C)).
• Intersystem bonding termination installed and accessible per 250.94.
• Separately derived systems have their SBJ landed and a grounding electrode within the 250.30 distance limits.
• CSST gas piping bonded per the manufacturer instructions, typically a #6 to the service ground.

Document the bond points with photos before you close walls. When the AHJ asks where the SBJ is on a remote transformer, a phone photo beats a return trip.