Weekly digest #81: bonding gotchas

Bonding is not grounding

Bonding connects metal parts together so they stay at the same potential. Grounding connects that system to earth. Mix them up and you get callbacks, or worse, a service that looks fine on a meter but lights up a washing machine the first time somebody leans on it.

NEC 250.90 is the one line that gets quoted and then ignored. Bonding shall be provided where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed. Capacity is the word most inspectors focus on. A #14 jumper wrapped around a locknut does not have capacity for a 200A fault.

This week we are walking the common bonding mistakes that show up on rough inspections and in failed megger tests, with the code cites you will want in your phone.

Service equipment: the main bonding jumper

At the service, the neutral is bonded to the enclosure through the main bonding jumper per NEC 250.24(B) and 250.28. Downstream of the service, those two must be kept separate. Every apprentice learns this, and every journeyman still finds a green screw driven in on a subpanel somewhere.

Check the bonding jumper sizing against Table 250.102(C)(1). For services over 1100 kcmil copper, you are into the 12.5% rule. The quick field check: if the service conductors are larger than 1100 kcmil, the jumper is sized at 12.5% of the phase conductor area, not picked from the column.

• Main bonding jumper: required at one point only, at the service disconnect.
• System bonding jumper: required at separately derived systems per 250.30(A)(1).
• Subpanels fed from the service: neutral isolated, equipment ground bonded to the enclosure, bonding screw removed.
• Second building feeders: follow 250.32, no re-grounding of the neutral unless the 2008-and-earlier exception applies to an existing installation.

Metal raceway and the locknut trap

Standard locknuts are listed for grounding on circuits up to 250V per NEC 250.92(B), but only where the raceway itself is the equipment grounding conductor and the connections are made up wrench tight. Above 250V, or where concentric or eccentric knockouts are present, you need a bonding bushing or bonding locknut and a jumper sized per 250.102(C) or 250.122 depending on whether you are on the line or load side.

The trap: a 200A feeder lands in a panel through a concentric KO. The installer torqued the locknut hard, signed off, and moved on. Under a fault, the thin metal tabs vaporize and the raceway is no longer in the fault current path. Every concentric or eccentric knockout on the line side of service equipment needs a bonding jumper per 250.92(B)(4), full stop.

Carry a handful of bonding bushings and a short spool of #6 on the truck. You will not regret it on the day the inspector notices the concentric KO you forgot about.

CSST, gas piping, and the thing that burns houses down

Corrugated stainless steel tubing has its own bonding rule, and it is the one that catches plumbers, HVAC techs, and occasionally electricians off guard. NEC 250.104(B) covers other metal piping systems likely to become energized. Most CSST manufacturers, and the 2009 and later codes, require a direct bond to the CSST system with a minimum #6 copper, attached to the rigid pipe portion upstream of the CSST.

This is separate from the general 250.104(B) bond. A lightning strike induces current on the gas line, the CSST wall is thin enough to arc through, and you have a fire. The #6 bond to the rigid pipe gives the induced current somewhere to go.

• Bond point: on the rigid black iron, not on the CSST itself.
• Conductor: #6 copper minimum, continuous, to the grounding electrode system.
• Check the CSST manufacturer instructions. Some brands now ship with arc-resistant jackets and have different bonding language.

Pools, spas, and the equipotential grid

Article 680 is where bonding stops being theoretical. NEC 680.26 requires an equipotential bonding grid around permanently installed pools, a #8 solid copper conductor tying together the pool shell reinforcement, perimeter surfaces within 3 feet, metal fittings, and pool equipment.

The perimeter surface bond is the one that gets missed on existing pool service calls. If the deck was poured without a bonded grid and you are adding a new pump, you are technically on the hook to address it. Document what is there, photograph it, and have the conversation with the owner before you energize anything.

On a spa replacement, meter from the bonding lug on the new pump back to any accessible metal within 5 feet of the water. You want near zero ohms. If you see anything above a couple ohms, something in the grid has opened up.

Quick field checklist before you close up

Most bonding failures are visible if you look for them. Before the cover goes back on, walk the installation with this list.

1. One main bonding jumper at the service, nowhere else on the premises wiring system.
2. Neutral and ground separated at every subpanel and remote building panel.
3. Bonding bushings and jumpers at every concentric or eccentric KO on the line side.
4. Metal water piping bonded per 250.104(A), within 5 feet of entry where required.
5. CSST bonded with #6 on the rigid section per manufacturer and 250.104(B).
6. Separately derived systems have a system bonding jumper and a grounding electrode conductor per 250.30.
7. Pool and spa equipotential grid continuous and tested before any concrete or deck goes over it.

None of this is new code. It is the stuff that gets skipped when the job is running behind and the inspector is booked out two weeks. Slow down at the service and at the water, and most of the callbacks go away.