Weekly digest #201: bonding gotchas

Bonding vs grounding: keep them straight

Grounding connects a system to earth. Bonding ties metal parts together so they share a potential and clear faults fast. Mix them up and you build hazards that pass a quick look but fail under fault current.

NEC 250.4(A)(3) and 250.4(A)(4) spell out the performance requirements: bonding must establish an effective ground-fault current path, low impedance, capable of carrying the maximum fault likely to be imposed. A green wire alone does not guarantee that path. Loose locknuts, painted enclosures, and oversized concentric knockouts all sabotage continuity.

Before you leave a job, ask whether every metal part likely to become energized is tied back to the source via a path the breaker can actually see. If the answer is fuzzy, the bond is fuzzy.

Service equipment: the main bonding jumper

The neutral and equipment grounding conductors connect at exactly one point in a service: the main bonding jumper, per NEC 250.24(A) and 250.28. Miss it and ground faults return through the earth, which is a terrible conductor. Duplicate it downstream and neutral current rides the equipment grounding conductors and metal raceways, which is how you get tingles, shocks, and stray-voltage complaints in barns and pools.

On separately derived systems, the system bonding jumper does the same job, sized per Table 250.102(C)(1). Same idea, different label. Pick one location, bond there, and lift neutral-to-ground bonds everywhere else.

Field tip: when troubleshooting nuisance GFCI trips on a sub-panel, the first thing to check is whether the neutral bar is bonded to the can. If it is, lift it. That single screw fixes more "phantom" trips than any other repair.

Concentric and eccentric knockouts

NEC 250.97 is the article that bites people. For circuits over 250 volts to ground, standard locknuts and bushings are not sufficient where concentric or eccentric knockouts remain in the enclosure wall. You need a bonding jumper, bonding bushing, bonding locknut, or a knockout listed for grounding.

The reason is simple: those rings are stamped, not welded. Fault current has to jump thin metal bridges before it reaches the enclosure. Impedance climbs, breakers stall, and the fault rides the raceway hot.

• 480Y/277V panels: bond every raceway with concentric KOs, both ends.
• 208Y/120V: standard locknuts are acceptable if no rings remain, but verify every KO actually punched clean.
• Reducing washers: never count on them for bonding. Add a jumper.
• Listed bonding wedges or bushings with a lay-in lug solve it cleanly.

Metal water and structural steel

NEC 250.104(A) requires metal water piping in or attached to a building to be bonded. The bond size comes from Table 250.102(C)(1), based on the largest ungrounded service conductor. Connection point is the service equipment, the grounded conductor at the service, the grounding electrode conductor, or the grounding electrodes themselves.

Structural metal that is interconnected to form a building frame and is exposed gets bonded under 250.104(C). Same sizing table. People skip this on remodels because the frame is buried in finishes, but if any portion is likely to become energized, it needs a bond. Document where you landed it. The next electrician will thank you.

Other metal piping systems, gas, compressed air, medical gas, fall under 250.104(B). The equipment grounding conductor of the circuit likely to energize the piping is permitted to serve as the bonding means. CSST gas tubing has its own manufacturer requirements that go beyond the NEC, usually a #6 to the gas line ahead of the first fitting. Read the label.

Pools, spas, and equipotential planes

Article 680 is where bonding gets unforgiving. NEC 680.26 requires an equipotential bonding grid around permanently installed pools: perimeter surfaces within 3 feet of the inside walls, structural steel, metal forming shells, ladders, diving boards, and all metal within 5 feet of the inside walls. Solid #8 copper, connected with listed clamps.

The grid does not connect to ground in the usual sense. The point is to keep everything in the swimmer's reach at the same potential, so there is no voltage gradient across a wet body. Pool inspectors will ohm out the bond grid. Have your continuity below 1 ohm and you pass.

Field tip: bond the rebar before the concrete pour. Trying to retrofit an equipotential grid after the deck is finished means saw cuts, drilled-in lugs, and a homeowner who is not happy. Walk the site before the concrete truck shows up.

Quick checks before you energize

None of this matters if the bonds are not actually continuous. A torque wrench and a low-resistance ohmmeter answer the question. Visual checks miss painted surfaces, loose set screws, and the locknut that someone ran finger-tight.

1. Verify the main bonding jumper is installed and only at the service or SDS.
2. Check every concentric KO on systems over 250V to ground for a listed bonding means.
3. Ohm the equipment grounding path from the farthest device back to the panel. Under 1 ohm is the target on branch circuits.
4. Confirm metal water, gas, and structural steel bonds are sized to Table 250.102(C)(1) and landed where the code allows.
5. Torque every bonding lug to the manufacturer's spec. Loose lugs are the most common failure point.

Bonding is not glamorous and nobody calls you back to thank you for it. But it is the single system that decides whether a ground fault opens a breaker or kills somebody. Get it right, document it, move on.