Weekly digest #111: bonding gotchas

Bonding is where inspections go sideways. Not because the code is vague, but because crews mix it up with grounding, skip bushings, or trust paint to carry fault current. This week's digest walks through the bonding traps that keep showing up on punch lists, with article citations you can point at when the AHJ asks.

Bonding vs. grounding: stop using them interchangeably

Grounding connects the system to earth. Bonding connects conductive parts together so fault current has a low-impedance path back to the source. NEC 250.4(A)(3) and 250.4(A)(4) spell this out, and the distinction matters because earth is a lousy fault-current path. If your bonding is weak, the breaker doesn't trip, and the raceway stays energized.

Article 100 gives you the definitions: bonded (bonding) means connected to establish electrical continuity and conductivity. Grounded (grounding) means connected to ground or to a conductive body that extends the ground connection. Two different jobs, one common failure mode when they get conflated on a print.

Field tip: if a journeyman tells you the ground rod will clear a fault, walk them over to 250.4(A)(5). The earth is not an effective ground-fault current path. Period.

Concentric and eccentric knockouts: bushings aren't optional

This is the number-one bonding violation I see on service equipment. NEC 250.92(B) requires bonding at service raceways and equipment, and 250.97 extends that requirement to circuits over 250 volts to ground. Locknuts alone don't cut it when a knockout is concentric or eccentric, because the ring of steel between the connector and the enclosure wall compromises the fault path.

You need one of the methods in 250.92(B)(2) through (B)(4): bonding jumpers around the concentric rings, threaded hubs, or bonding-type locknuts or bushings listed for the purpose. A standard plastic bushing with a grounding lug doesn't automatically satisfy this, check the listing.

• Service raceways: always bond per 250.92(B), regardless of voltage.
• Feeders and branch circuits over 250V to ground with concentric or eccentric knockouts: bond per 250.97.
• Standard knockouts (no rings) on circuits 250V or less to ground: listed fittings and locknuts are generally sufficient per 250.86 and 250.96(A).

Pools, spas, and equipotential bonding

Article 680 is its own animal. The equipotential bonding grid in 680.26 is not a grounding electrode system, and it's not connected to earth as its primary function. It's there to eliminate voltage gradients in the swim area so a swimmer doesn't become the path between two differently-referenced conductive surfaces.

The 8 AWG solid copper minimum in 680.26(B) connects perimeter surfaces, metallic components, pool water via 680.26(C), and fixed metal parts within 5 feet of the inside walls. Rebar, copper grid, or a listed system all work, but the connections need to be listed for direct burial or encased in concrete. Inspectors will look at every single bond point.

Field tip: photograph the grid before the deck is poured. When the GC asks six months later why the bond lug on the ladder anchor is missing, you'll have proof it was done and tied.

Metal water piping and structural steel

NEC 250.104(A) requires bonding of interior metal water piping, and this still trips people up on remodels. If there's any metal water pipe in the building, it gets bonded to the service equipment, grounded conductor at the service, grounding electrode conductor, or one of the grounding electrodes. Sizing comes from Table 250.102(C)(1).

Structural metal, when it qualifies as a grounding electrode under 250.52(A)(2), becomes part of the grounding electrode system. But interior metal building frames that aren't electrodes still need bonding per 250.104(C) when they're likely to become energized. Don't skip this on metal-framed commercial jobs, the inspector won't.

1. Identify whether the metal piping or frame is an electrode or just a likely-to-be-energized conductive part.
2. Size the bonding jumper per Table 250.102(C)(1), based on the largest ungrounded service conductor.
3. Attach with listed fittings, 250.70, and keep the connection accessible where required.

Paint, coatings, and surface prep

NEC 250.12 requires nonconductive coatings like paint, lacquer, and enamel to be removed at threads, contact points, and connection points. Factory paint on a panel interior where a bonding lug lands? Scrape it. Galvanizing on strut where you're grounding equipment? Usually fine, galvanizing is conductive, but check the listing on the clamp.

This is an easy gig-punch item to fix before the inspector shows up. Hit threaded hubs, bonding bushing landings, and any lug-to-enclosure contact with a file or a star washer listed for grounding. Star washers alone don't substitute for 250.12 when the coating is thick.

Separately derived systems and generator bonds

Generators and transformers as separately derived systems get bonded per 250.30(A). The system bonding jumper goes in one place, either at the source or at the first disconnect, not both. Two bonds create parallel paths through the grounded conductor and the equipment grounding conductor, which means objectionable current flows on the EGC during normal operation. That's a 250.6 violation and a nuisance-tripping GFCI nightmare.

When a portable or vehicle-mounted generator is used as a non-separately derived system, 250.34 applies instead, and the bonding jumper stays in the service. Read the transfer switch nameplate carefully, whether the neutral is switched tells you which rule you're under.

• Switched neutral transfer switch: separately derived, bond at the generator or first disconnect per 250.30(A).
• Solid (unswitched) neutral: not separately derived, keep the bond at the service only.
• Never install two system bonding jumpers on the same derived system.