Weekly digest #141: bonding gotchas

Why bonding trips people up

Bonding failures rarely show up on a megger. They show up as tingle voltage on a pool ladder, a hot water pipe, or a burned-up service neutral after a utility fault. The code treats bonding and grounding as separate concepts for a reason, and the field guys who keep getting callbacks are usually blurring the two.

Bonding is about keeping metal parts at the same potential so fault current has a low-impedance path back to the source. Grounding is about tying that system to earth. NEC 250.4(A) spells out the performance requirements for grounded systems, and 250.4(B) covers ungrounded. Read both back to back and the distinction clicks.

The service bonding jumper everyone forgets

On a typical 200A residential service, the main bonding jumper (MBJ) is the green screw in the panel. Easy. What gets missed is the supply-side bonding jumper when you run a meter-main combo with a separate disconnect, or when the raceway between meter and service disconnect is PVC with a metal fitting on one end. 250.92(B) requires bonding on the supply side that does not rely on standard locknuts and bushings.

Acceptable methods under 250.92(B): bonding bushings with jumpers, threaded hubs, threaded couplings, or listed bonding locknuts. A standard locknut plus a plastic bushing is not a bonding means on the line side, even if it looks tight.

• Line side of service: bonding-rated fittings only (250.92(B))
• Load side: standard locknuts and bushings are fine (250.97 exceptions apply for circuits over 250V to ground)
• Reducing washers are never a bonding means on concentric/eccentric knockouts above 250V to ground

If you can back off a locknut with your fingers after torquing, you do not have a bond. Use a bonding bushing with a jumper to the neutral or ground bar and move on.

Separately derived systems and the bonding jumper location

Transformers trip up even experienced guys. A separately derived system (SDS) needs a system bonding jumper (SBJ) installed at exactly one point, either at the transformer or at the first disconnect. Not both. 250.30(A)(1) is clear about this, and putting jumpers at both locations creates parallel neutral paths that carry normal load current on the equipment grounding conductor.

If you walk up to a dry-type transformer and see the XO bonded to the case and a separate SBJ in the downstream panel, pull one. Which one depends on where the grounding electrode conductor lands. The GEC and SBJ must be at the same point.

1. Identify the SDS (transformer secondary, generator with switched neutral, etc.)
2. Pick the bonding point: source or first disconnect
3. Install the SBJ at that point, sized per 250.102(C) based on the largest ungrounded conductor
4. Land the GEC at the same point, sized per 250.66
5. Verify no second jumper exists downstream

Pools, spas, and equipotential bonding

Article 680 has its own bonding rules and they are stricter than the rest of the code. The equipotential bonding grid under 680.26 is not optional, and it is not the same as equipment grounding. You are creating a zone where a swimmer cannot become the path between two different potentials.

A #8 solid copper conductor ties together the pool shell reinforcing steel, metal perimeter surfaces within 3 feet of the pool wall, metal fittings, pump motors, and any other metal within 5 feet of the inside pool wall. The bond does not have to extend back to the service. 680.26(B)(7) handles the water bond separately, minimum 9 square inches of conductive surface area in contact with the water.

Inspectors will ask to see the bonding before the deck pours. If you did not take photos of the #8 tied to the steel and the perimeter ring, you will be chipping concrete. Every time.

Metal water piping: bonded, not grounded

250.104(A) requires metal water piping systems to be bonded. The bonding jumper is sized per 250.102(C)(1), based on the largest ungrounded service conductor, and lands on the water pipe within 5 feet of entry to the building. If there is a dielectric union or plastic section between the entry and your connection point, you just bonded half a pipe.

This is bonding, not grounding. The water pipe has not been an acceptable grounding electrode by itself since the 1987 code cycle, and interior metal piping cannot be used as a grounding electrode conductor path per 250.52(A)(1). If the water service is plastic, you still bond any interior metal piping that is likely to become energized.

• Bond within 5 feet of pipe entry (250.104(A)(1))
• Size jumper per 250.102(C)(1)
• Gas piping: bond per 250.104(B), typically through the EGC of the circuit likely to energize it
• Structural metal: bond if it is interconnected to form a building frame (250.104(C))

Quick field checks before you close up

Before the cover goes back on, run through a short mental list. Most bonding callbacks come from skipping one of these.

1. MBJ installed and torqued, green screw visible or wire jumper landed
2. Supply-side raceways have bonding bushings or threaded connections
3. Only one neutral-to-ground bond in the entire system, at the service or SDS source
4. Subpanels: neutral isolated, ground bar bonded to enclosure
5. Water, gas, and structural steel bonded where required
6. Any pool, spa, or fountain equipotential grid documented with photos

Bonding is cheap to do right on the front end and expensive to fix after drywall. When in doubt, add the jumper. 250.6 covers objectionable current if you end up with parallel paths, but missing a bond entirely is the failure mode that gets people hurt.