Weekly digest #50: grounding gotchas

Bonding vs grounding: stop mixing them up

Grounding connects the system to earth. Bonding ties metallic parts together so they sit at the same potential. NEC 250.4(A)(1) covers system grounding; 250.4(A)(3) and (A)(4) cover bonding of equipment and other conductive material. Mix these up on paper and the inspector will catch it. Mix them up in the field and you get shock hazards or nuisance trips.

The equipment grounding conductor (EGC) is a bonding conductor, not an earthing conductor. It carries fault current back to the source so the breaker opens. Earth does not clear faults. Low-impedance path to the source does.

If you are relying on a ground rod to trip a breaker, you have already failed. Rods stabilize voltage and handle lightning, nothing more.

The neutral-to-ground bond lives in exactly one place

Per NEC 250.24(A)(5) and 250.142, the grounded conductor (neutral) bonds to the equipment grounding system at the service disconnect, and only there. Downstream subpanels must keep neutrals and grounds isolated. This is the single most common violation on residential remodels.

When a subpanel arrives from the factory, the bonding screw or strap is usually installed. Pull it. If the neutral bar and ground bar are tied together in a subpanel, parallel neutral current rides the EGC and every metallic pathway between panels. That heats conduit, energizes enclosures, and shows up as tingle voltage on well casings and water lines.

• Service equipment: neutral bonded to ground, main bonding jumper in place.
• Subpanel in same building: four-wire feeder, neutral floating, separate ground bar bonded to enclosure.
• Separate structure: follow 250.32, which since the 2008 NEC requires an EGC run with the feeder and the neutral kept isolated.

GEC sizing trips people up

The grounding electrode conductor is sized from Table 250.66, based on the largest ungrounded service conductor. The equipment grounding conductor is sized from Table 250.122, based on the overcurrent device ahead of it. Different tables, different inputs, different results.

If you upsize ungrounded conductors for voltage drop, 250.122(B) requires you to upsize the EGC proportionally. A lot of guys forget this on long runs. Run 1/0 copper to a 60A feeder for voltage drop and your #10 EGC is now undersized.

Upsize the hots, upsize the ground. Calculate the ratio by circular mils, not by ampacity.

Ground rods: one is almost never enough

NEC 250.53(A)(2) is the rule that gets skipped. A single rod electrode must be supplemented by an additional electrode unless you can prove the single rod has 25 ohms or less resistance to earth. Nobody measures it. So drive the second rod, space it at least six feet from the first, and move on.

Better yet, use what is already in the ground. If there is a concrete-encased electrode (Ufer) per 250.52(A)(3), it has to be used. A 20-foot piece of #4 bare copper or half-inch rebar in the footing outperforms any rod you can drive, and it is already there. Inspectors are writing this up more often as the 2020 and 2023 cycles tightened the language.

• 250.52(A)(1): metal underground water pipe, 10 feet minimum contact, must be supplemented.
• 250.52(A)(2): metal frame of the building if effectively grounded.
• 250.52(A)(3): concrete-encased electrode, required if present in new construction.
• 250.52(A)(4): ground ring, #2 bare copper minimum, 20 feet minimum.
• 250.52(A)(5)(6)(7)(8): rods, plates, other listed electrodes.

EGC continuity through EMT and flex

EMT is a listed equipment grounding conductor per 250.118(4). Same for rigid and IMC. But the fittings matter. Setscrew connectors in wet locations corrode, threadless couplings loosen, and suddenly your fault path is gone. If you are running EMT in a car wash, a food plant, or outdoors, pull a green wire. The code allows it, your inspector will not argue, and you will sleep better.

Flexible metal conduit (FMC) is only a valid EGC under 250.118(5): total length in the ground-return path six feet or less, circuit 20A or less, fittings listed for grounding. Liquidtight flex (LFMC) has its own limits in 250.118(6). For anything feeding a motor over 20A, pull the EGC. Do not rely on the flex.

• Motor whips: always pull an EGC, regardless of flex length.
• Wet or corrosive locations: pull an EGC even in rigid or EMT.
• Isolated ground receptacles per 250.146(D): pull two EGCs, one for the box, one insulated to the receptacle.

Separately derived systems and the bonding jumper

Transformers, generators with a transfer switch that switches the neutral, and some UPS systems are separately derived. NEC 250.30 requires a system bonding jumper between the derived neutral and the metal frame, and a grounding electrode conductor to a nearby electrode. Put the bonding jumper in one place, the source or the first disconnect, never both.

Two bonding jumpers on a generator is the classic mistake. Factory jumper inside the gen, plus a switched-neutral transfer switch, plus a service bond upstream, gives you parallel neutral paths and a generator frame that can float above ground during a fault. Pull the factory jumper on the generator when the transfer switch switches the neutral, or leave it and use a solid neutral transfer switch. Pick one topology and commit.