Weekly digest #200: grounding gotchas

Bonding vs grounding: get the words right

Grounding connects to earth. Bonding connects metal parts together to keep them at the same potential. Two different jobs, two different conductors, and inspectors will fail you for sloppy terminology on a panel schedule.

NEC 250.4(A)(1) covers the grounding electrode. NEC 250.4(A)(3) and (A)(4) cover bonding of equipment and other conductive material. The earth itself is never a fault current path, ever. If you're relying on dirt to clear a fault, you've already lost.

The equipment grounding conductor (EGC) is the fault path. The grounding electrode conductor (GEC) is the dirt connection. Mix them up on paper and the AHJ assumes you mixed them up in the box.

Service grounding: where most rejections happen

NEC 250.24(A) requires the grounded service conductor to be bonded to the enclosure at the service disconnect, and only there. Run that neutral-to-ground bond downstream and you've created parallel paths through every metallic raceway and water line in the building.

The classic field mistake: leaving the bonding screw or strap installed in a sub-panel that used to be a main. When you convert a service or add a feeder, pull the bond. NEC 408.40 and 250.142(B) are explicit about this.

• Service disconnect: neutral and ground bonded together (one location).
• All downstream panels: isolated neutral bar, separate EGC bar.
• Detached structures: see 250.32, single feeder with EGC, no re-bond.
• Separately derived systems: bond at the source or first disconnect per 250.30.

Grounding electrode systems: more than one rod

NEC 250.50 requires all electrodes present at a building to be bonded together into a grounding electrode system. That means metal water pipe (within 5 feet of entry, per 250.52(A)(1)), structural steel, concrete-encased electrode (Ufer), and ground rings all get tied in.

If you drive rods, NEC 250.53(A)(2) requires either a documented 25 ohm reading on a single rod or a second rod spaced at least 6 feet away. Most electricians just drive two rods and call it done because measuring resistance with a clamp meter or fall-of-potential test takes time most jobs don't budget for.

Drive the second rod even when the first one tests under 25 ohms. Soil dries out, conditions change, and the second rod costs less than a callback or a re-inspection.

The Ufer (concrete-encased electrode, 250.52(A)(3)) is the best electrode you'll ever install and the cheapest if you catch it before the slab pour. Twenty feet of #4 bare copper or 1/2 inch rebar in the footing, period.

EGC sizing and parallel runs

NEC 250.122 sizes the EGC based on the overcurrent device protecting the circuit, not the ungrounded conductor ampacity. A 200A breaker feeding #3/0 copper still requires a #6 copper EGC, full stop.

Where it gets people: parallel feeders. NEC 250.122(F) requires a full-size EGC in each parallel raceway. If your 600A service runs in three parallel conduits, each conduit gets an EGC sized for 600A (that's #1 copper), not three EGCs sized for 200A each. Inspectors catch this one constantly on commercial pulls.

1. Determine the OCPD rating ahead of the conductors.
2. Pull Table 250.122 for the minimum EGC size.
3. If conductors are upsized for voltage drop, upsize the EGC proportionally per 250.122(B).
4. For parallel sets: full-size EGC in every raceway.

GFCI, GFPE, and the grounding interaction nobody explains

GFCIs work by sensing imbalance between the hot and neutral, not by needing a ground. That's why a two-wire receptacle replacement under 406.4(D)(2)(b) is allowed without an EGC. The "No Equipment Ground" sticker is required, however, and skipping it is a write-up.

GFPE per 230.95 (1000A and over services) is a different animal. It trips on ground fault current returning through the EGC or the earth, and it's miscoordinated with downstream branch breakers more often than not. When a 1200A main trips on a single phase-to-ground in a kitchen, that's GFPE doing exactly what it was designed to do.

If a building has GFPE on the main and you're troubleshooting nuisance trips, check for a downstream neutral-to-ground bond first. It's the cause 80% of the time.

Quick field reference

Keep these numbers in your head. They cover most of what an inspector will ask about on a residential or light commercial rough.

• 250.66: GEC sizing from service conductor size.
• 250.122: EGC sizing from OCPD rating.
• 250.52: Permitted grounding electrodes.
• 250.53(A)(2): Two-rod rule (or 25 ohm test).
• 250.64(C): GEC must be continuous or irreversibly spliced.
• 250.104: Bonding of metal water and gas piping.
• 250.118: Types of equipment grounding conductors.

When in doubt, bond it. An extra bonding jumper on a metal water line or a CSST gas system has never failed an inspection. A missing one has shut down plenty of jobs.