Weekly digest #140: grounding gotchas

Equipment grounding vs. grounded conductor: don't confuse the two

The equipment grounding conductor (EGC) and the grounded (neutral) conductor serve different jobs, and mixing them up past the service disconnect is one of the most common violations on any inspection punch list. NEC 250.24(A)(5) is explicit: once you leave the service equipment, the neutral and the EGC stay separated. No bonding screws in subpanels. No shared terminal bars.

On a remodel where a detached garage subpanel was fed before the 2008 code cycle, you may still find a three-wire feeder with the neutral bonded at the remote panel. That's legacy and no longer compliant. NEC 250.32(B)(1) now requires a four-wire feeder with a separate EGC to any separate structure, and the neutral stays isolated on its own bar.

Quick field check when you open a subpanel:

• Neutral bar floating on insulators, not bonded to the can
• Green bonding screw removed or never installed
• EGC landed on a grounding bar that IS bonded to the enclosure
• No white conductors on the ground bar, no bare or green on the neutral bar

Grounding electrode conductor sizing: read the table, not your memory

NEC Table 250.66 governs the GEC size based on the largest ungrounded service conductor. The shortcut most guys remember is #6 copper for a 200A service to a ground rod, and that's correct, but it stops there. The rod itself only ever needs a #6 regardless of service size per 250.66(A), because the rod's resistance is the limiting factor, not the copper.

Where it gets missed: the GEC to a concrete encased electrode (Ufer) is sized per the table, up to #4 max per 250.66(B). The GEC to a metal water pipe has no such cap, so a 400A service with parallel 3/0 copper feeders needs a #1/0 GEC to the water pipe. That's a chunky conductor and it needs to be continuous or irreversibly spliced per 250.64(C).

If the GEC has to be spliced, use an exothermic weld or a listed irreversible compression connector. A split bolt in a J-box does not cut it, and an inspector will fail it every time.

Bonding around water meters and dielectric unions

NEC 250.68(C)(1) allows the interior metal water pipe to be used as a conductor for bonding, but only within the first five feet of entry into the building. Past that, it's a bonding jumper question, not a grounding electrode question.

The water meter is the usual gotcha. Plumbers pull meters. When they do, the ground path disappears unless you've installed a bonding jumper around it per 250.53(D)(1). Same goes for dielectric unions at the water heater, which break continuity by design. A #6 copper jumper with listed clamps on each side is the fix, and it takes ten minutes.

Things that will fail a bonding inspection on a service upgrade:

1. No jumper around the water meter
2. No jumper around a dielectric union in a bonded section of pipe
3. Clamp listed for copper pipe used on galvanized, or vice versa
4. Bonding conductor buried in a wall with no accessible connection

GFCI vs. grounding: a GFCI is not a ground

NEC 406.4(D)(2) lets you replace a two-prong receptacle with a GFCI on an ungrounded circuit, and label it "No Equipment Ground." This trips up homeowners and some electricians into thinking the GFCI somehow creates a ground. It doesn't. It just opens the circuit fast enough to prevent electrocution on a line-to-ground fault through a person.

What you lose: surge protection for electronics, proper operation of any device that relies on the EGC for noise reference (AV gear, some computers), and the ability to claim the outlet is grounded. The label matters. If you skip the "No Equipment Ground" sticker, the next sparky down the road will assume the EGC is there and land equipment accordingly.

Ground rods: one is rarely enough

NEC 250.53(A)(2) requires a supplemental electrode when a single rod can't show 25 ohms or less to earth. In practice, almost nobody meters the rod. Code gives you an out: drive two rods spaced at least six feet apart and you're done, no measurement needed. Most inspectors expect to see two rods as a matter of course.

Spacing matters. Six feet is the minimum per 250.53(A)(3), but the rule of thumb is to space them at a distance equal to or greater than the rod length. Two eight-foot rods three feet apart are electrically almost one rod because their spheres of influence overlap.

If you're in rocky soil and can't drive a rod the full eight feet, 250.53(G) lets you bury it at a 45 degree angle or lay it in a trench at least 30 inches deep. Document it, photograph it before backfill, and note it on the job ticket.

Grounding at separately derived systems

Generators, transformers, and some UPS systems are separately derived systems (SDS) and they need their own grounding electrode per NEC 250.30(A). The system bonding jumper, which bonds the grounded conductor to the metal enclosure and the EGC, is installed at one and only one location, typically at the source or the first disconnect.

Transfer switches are where this falls apart. A service-rated automatic transfer switch that switches the neutral keeps the generator neutral separate from the service neutral, so the generator is a true SDS with its own bonding jumper and ground electrode. A three-pole switch that doesn't switch the neutral leaves them common, so the generator is NOT an SDS and must NOT have a bonding jumper at the generator, or you create a parallel neutral path and objectionable current on the EGC.

Before energizing, check three things:

• Does the transfer switch switch the neutral? Check the nameplate and the wiring diagram.
• Is there a bonding jumper in the generator? Some come from the factory bonded.
• Does the generator have its own grounding electrode if it's an SDS?