Weekly digest #51: bonding gotchas

Bonding is not grounding

Bonding connects metal parts together so they stay at the same potential. Grounding connects the system to earth. Mixing up the two leads to missed code violations and hazardous installs. NEC 100 defines them separately for a reason.

The bonding jumper carries fault current back to the source. The grounding electrode conductor drains static and stabilizes voltage to earth. If you run one and skip the other, the breaker never trips on a ground fault and the enclosure stays energized.

Field inspectors see this weekly: a ground rod driven at a subpanel with no equipment grounding conductor back to the service. That rod is not a fault path. Copper back to the main bonding jumper is.

The main bonding jumper trap

At the service disconnect, the grounded (neutral) conductor gets bonded to the enclosure via the main bonding jumper per NEC 250.24(B) and 250.28. One location only. Past the service, neutral and ground stay separate.

The common mistake is leaving the bonding screw or strap installed in a downstream subpanel. Current then splits between the neutral and the EGC, energizing conduit, boxes, and appliance frames. You will read voltage on the panel can with a meter referenced to a known ground.

Before you energize a subpanel, pull the bonding screw and verify it is in your pocket, not in the panel. Takes ten seconds, saves a callback.

Detached structures fed from a feeder now require a separate EGC and no neutral-to-ground bond at the building disconnect, per the 2008 and later cycles of NEC 250.32(B). If you are working on an older farm or outbuilding, expect to find the old three-wire feeder and plan the upgrade.

Metal water pipe and structural steel

NEC 250.104(A) requires the interior metal water piping system to be bonded. The bond lands on the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor, or one of the grounding electrodes. Size the jumper per Table 250.102(C)(1) based on the largest ungrounded service conductor.

Structural metal that is interconnected and forms a building frame gets bonded under 250.104(C). Same sizing table. If a plumber swaps a section of copper for PEX after you finish, your bond path may be broken. Note it on the as-built and walk the run before the final.

• Bond within 5 feet of where the water pipe enters the building, per 250.52(A)(1), if the pipe qualifies as an electrode.
• Separate gas piping bond under 250.104(B), typically through the EGC of the circuit that can energize the pipe.
• CSST requires an additional bond per manufacturer instructions, often a #6 solid from the CSST to the GEC system.

Pools, spas, and equipotential bonding

Pools are where bonding mistakes get expensive and dangerous. NEC 680.26 requires an equipotential bonding grid around permanently installed pools. The grid ties the shell, perimeter surfaces within 3 feet, metal fittings, pool equipment motors, and any metal within 5 feet horizontally and 12 feet above.

Use solid #8 AWG copper minimum. Connect to structural reinforcing steel or a copper conductor grid at 12 inch spacing. The bond is not grounded to a rod, it equalizes potential so a swimmer cannot become the path between two metal parts at different voltages.

If the pool was poured before you got the call, you can install a perimeter copper ring buried 4 to 6 inches deep, 18 to 24 inches from the pool wall, and tie every metal part to it.

Reducing washers, concentric knockouts, and listed fittings

NEC 250.97 addresses bonding for circuits over 250 volts to ground. At every concentric or eccentric knockout, you need a bonding bushing, bonding locknut, or listed fitting that maintains the fault path. Standard locknuts do not qualify when rings remain.

On a 480Y/277V service or feeder, a missed bonding bushing at a panel knockout can leave enough impedance in the fault path that the breaker holds instead of tripping. Use a bonding bushing with a jumper to the neutral bar at the service, or to the EGC downstream.

• Check every concentric knockout, not just the one with the conduit in it.
• Reducing washers on services over 250V to ground need a bonding jumper around them.
• Listed raintight fittings with integral grounding can satisfy the requirement, verify the listing.

Separately derived systems

Transformers, generators with a switched neutral, and large UPS units often create separately derived systems. NEC 250.30 governs the bonding. You need a system bonding jumper at the transformer or at the first disconnect, sized per 250.102(C), plus a supply-side bonding jumper and a grounding electrode conductor to a local electrode.

The mistake here is treating the transformer secondary like a subpanel and leaving the bond out entirely, or double-bonding at both the transformer and the downstream panel. Pick one location, document it, and make sure the EGC from that point forward stays separate from the neutral.

For generators, check whether the transfer switch switches the neutral. If yes, the generator is a separately derived system and needs its own bond and electrode. If no, the service bond handles it and you do not bond at the generator.