Weekly digest #170: grounding gotchas

Why grounding still bites us

Grounding looks simple on paper. Bond everything, drive a rod, call it done. The field tells a different story. Most failed inspections and shock complaints trace back to the same handful of mistakes, and they hide in plain sight because the lights still come on.

This digest pulls together the grounding errors we keep seeing on service calls, panel swaps, and remodel work. None of this is theoretical. Every one of these has cost somebody a callback, a red tag, or a hospital trip.

The neutral-ground bond, and where it does not belong

The system bonding jumper belongs at one place only: the service disconnect, per NEC 250.24(A) and 250.28. After that point, neutrals and grounds run separate. Sounds obvious. Then you open a 30 year old subpanel in a detached garage and find the bonding screw still tight, neutrals and grounds sharing bars, and a feeder with no equipment grounding conductor.

That setup energizes every metal box downstream through the neutral. Pull the bond. Add a separate EGC if the feeder does not have one. If the feeder is direct buried without an EGC and was installed under older rules, 250.32(B)(1) Exception now requires correction during any significant alteration.

• Service equipment: neutral bonded to ground, one location.
• Subpanels in the same building: isolated neutral, separate ground bar, no bonding screw.
• Detached structures fed after 2008: feeder must include an EGC, treat like a subpanel.
• Separately derived systems (generators, transformers): bond at the source or first disconnect, not both.

Ground rods are not the ground fault path

This one trips up apprentices and homeowners constantly. The ground rod handles lightning and stray voltage to earth. It does not clear a fault. The earth is too resistive to trip a breaker, and 25 ohms of rod resistance is plenty to keep a faulted conductor sitting at line voltage while you touch it.

NEC 250.4(A)(5) is explicit: the earth shall not be used as the sole equipment grounding conductor or fault current path. The low impedance path back to the source is the EGC, bonded at the service. The rod is supplemental.

Tip from a service tech in Arizona: "If a customer says their generator backfeed worked fine without a transfer switch because they drove a rod, walk away. The rod is doing nothing for fault clearing. The neutral is still tied to the utility."

Bonding metal water and gas piping

NEC 250.104(A) requires the metal water piping system to be bonded with a conductor sized per Table 250.102(C)(1). Gas piping, including CSST, gets bonded under 250.104(B) and the manufacturer's listing. CSST without a dedicated bond has caused fires from nearby lightning strikes, and most CSST manufacturers now require a #6 AWG direct bond to the service ground.

Two places people mess this up. First, bonding to a short stub of copper that transitions to PEX three feet later. The bond has to be on the metal portion that is likely to become energized, and PEX breaks the path. Second, running the bond to a subpanel ground bar instead of the service. Bring it to the service equipment or the grounding electrode conductor.

1. Locate the metal water service entry, within 5 feet of entry into the building.
2. Verify continuity past the meter (jumper if the meter is dielectric or removable).
3. Size the bonding conductor per the service entrance conductors, not the panel rating.
4. Land the other end on the GEC or service neutral, not a subpanel.

EGC sizing on long runs and parallel feeders

Table 250.122 sizes the EGC by the upstream overcurrent device, not the conductor ampacity. A 100 amp feeder still needs a #8 copper EGC even if you upsized the ungrounded conductors to #1 for voltage drop. But here is the catch in 250.122(B): if you upsize the phase conductors for voltage drop, you must upsize the EGC proportionally by circular mil area.

On parallel feeders run in separate raceways, each raceway needs a full sized EGC per 250.122(F). Not half. Not split. Each one. People still run a single EGC in one of two parallel pipes and call it good. It is not. A fault in the pipe without the EGC has to travel through the conduit, fittings, and a neutral that may or may not be bonded right.

Ground fault testing before you close the panel

Energizing a panel and watching the breakers hold is not a ground fault test. Megger the EGC to ground if the run is long. Loop impedance test the receptacle ends with a tester that actually measures fault loop impedance, not just the indicator lights on a $15 plug-in tester. Those plug-in testers cannot detect a bootleg ground, where someone jumpered neutral to ground at the receptacle.

The bootleg ground is the silent killer of remodels. Outlet reads correct on a three-light tester, GFCI even trips, but the equipment ground is carrying neutral current and any open neutral upstream puts line voltage on the chassis of whatever is plugged in.

From a master in Ohio: "I find one bootleg ground per ten houses I touch. Always check with a tester that drops a real load between hot and ground. If the breaker trips, the ground is solid. If the GFCI trips and the breaker doesn't, you've got a bootleg."

Quick field checklist

Before you button up, run through this. It takes five minutes and saves callbacks.

• One neutral-ground bond, at the service. Verified.
• Subpanels: bonding screw out, ground bar added, neutrals isolated.
• EGC sized per 250.122, upsized if phase conductors were upsized.
• Water and gas bonds present, on metal, sized correctly, landed at the service.
• Ground rod or rods, two if you cannot prove 25 ohms or less, per 250.53(A)(2).
• Loop impedance or load test at the furthest receptacle, not just a three-light tester.

Grounding is not the glamorous part of the trade. It is the part that decides whether someone goes home tonight. Treat every bond like it has to clear a fault, because eventually one of them will.