Weekly digest #21: bonding gotchas

Why Bonding Trips Up Experienced Hands

Bonding is where seasoned electricians get bitten, not grounding. The code language around bonding jumpers, supply-side versus load-side, and separately derived systems reads clean on paper and gets muddy in the field. Miss a bonding bushing on a concentric knockout and the inspector flags it. Miss it on a fault and somebody gets hurt.

This week pulls together the bonding issues that keep showing up on callbacks and failed inspections. Service equipment, pools, CSST, metal water piping, and the separately derived system you forgot was separately derived. Article 250 Part V is the one to keep bookmarked.

Service Bonding: The Main Bonding Jumper

The main bonding jumper (MBJ) is the single connection between the grounded conductor and the equipment grounding system at the service. NEC 250.24(B) requires it. NEC 250.28 sizes it. Get this wrong and every fault on the load side has nowhere to go.

Factory-supplied green screws get left in the bag more often than anyone wants to admit. Loose panel covers, swapped-out neutrals, and retrofits are where the MBJ gets lost. On a service change, verify the MBJ is installed before energizing. Don't trust that the last hand did it.

• NEC 250.24(B): MBJ required at service disconnect.
• NEC 250.28(D): size per Table 250.102(C)(1), minimum 12.5% of the largest ungrounded conductor when over 1100 kcmil.
• NEC 250.24(A)(5): no load-side neutral-to-ground bonds downstream of the service.

Separately Derived Systems: The One Everyone Misses

A generator with a transfer switch that switches the neutral, a step-down transformer, or a UPS output can all create separately derived systems. NEC 250.30 applies. That means a system bonding jumper at the source or the first disconnect, plus a grounding electrode connection.

The common failure is treating a transformer secondary like it's still part of the primary system. It isn't. You need a new bonding jumper at the secondary, a new grounding electrode conductor, and usually a new grounding electrode. Skip any of those and you've got a floating neutral waiting to bite.

Field tip: if the transfer switch has a 3-pole version and a 4-pole version, that decision drives everything. 4-pole (switched neutral) means the genset is a separately derived system and needs its own N-G bond. 3-pole (solid neutral) means it isn't, and the N-G bond at the genset must be removed.

Metal Water Piping and Structural Steel

NEC 250.104(A) requires metal water piping in a building to be bonded. The bonding jumper sizes per Table 250.102(C)(1), and it has to be accessible. NEC 250.104(C) covers structural metal, same sizing rules.

The gotcha: dielectric unions. A plumber drops one in during a repair and now your bonded copper run ends at a piece of PEX. The bond upstream of the union is fine, the metal fixtures downstream are floating. On remodels, walk the piping before you sign off. If there's a water heater replacement, check the connections above and below it.

• Bonding point must be accessible per NEC 250.68(A).
• Bond to the street side, not past the meter, unless there's a meter bonding jumper.
• CSST (corrugated stainless steel gas tubing) has its own bonding requirement under NEC 250.104(B) and the manufacturer's instructions. 6 AWG copper minimum is typical, but check the listing.

Pools, Spas, and the Equipotential Bonding Grid

NEC 680.26 is its own world. The equipotential bonding grid ties together the pool shell, deck, perimeter surfaces, metal fittings, pumps, lights, and anything else that could develop a voltage gradient. The point is not fault clearing, it's keeping everything near the pool at the same potential so nobody gets shocked stepping out of the water.

An 8 AWG solid copper conductor is the minimum. The bond runs to all metal parts within 5 feet of the pool wall, plus the deck reinforcing steel or a copper grid if the deck isn't reinforced. Miss a ladder anchor or a light niche and the grid isn't a grid anymore.

Field tip: photograph the bonding grid during the rough before the deck pours. Inspectors want to see it, and if there's a callback two years later, you'll want the picture too.

Concentric Knockouts and Reducing Washers

NEC 250.97 requires bonding around concentric or eccentric knockouts on service equipment and any enclosure containing over 250 volts to ground. Reducing washers don't count as a ground path. Neither do concentric rings that haven't been fully removed.

The fix is a bonding bushing with a jumper, a bonding locknut, or a listed bonding wedge. On 480Y/277V panels, this is not optional. Fault current needs metal-to-metal continuity, and a knockout ring with punch marks still attached does not provide it.

1. Inspect every concentric knockout on service gear and high-leg panels.
2. Add a bonding bushing with jumper to the ground bar, sized per NEC 250.102(C).
3. Don't rely on set screws or compression fittings to bond across a concentric ring.

Quick Field Checks Before You Leave the Job

Bonding problems show up on the meter before they show up on a fault. A handful of checks before you pack up catches most of it.

• Neutral-to-ground voltage at the furthest receptacle. Should be under 2V on a lightly loaded circuit. Higher means a loose neutral or a bonding issue.
• Continuity from the MBJ to the grounding electrode, and from the ground bar to a known-good bond point.
• Visual on every bonding bushing and jumper. If you can't see it, assume it isn't there.
• For separately derived systems, verify there's exactly one N-G bond, not zero and not two.

Bonding is cheap to do right and expensive to fix. The code sections are scattered across 250, 680, and the product listings, but the principle is the same everywhere: every piece of metal that could carry fault current needs a low-impedance path back to the source.