Article 250 does two different jobs with two different words. Grounding connects the system to earth: it stabilizes voltage to ground, limits surges from lightning and utility faults, and gives static and surge a path. Bonding connects metal parts together so they are at the same potential and so a fault has a low-impedance path back to the source that will trip the breaker.

Earth does not clear faults. Soil resistance is far too high (a 25 Ω rod on a 120 V circuit passes under 5 A). What clears a fault is the effective ground-fault current path (250.4(A)(5)): equipment grounding conductors, bonded raceways and the main bonding jumper, all leading back to the transformer neutral. Every grounding mistake traces back to confusing these two jobs: an isolated "clean ground" rod with no bond, or a raceway with a missing bonding jumper.

The conductors have names and you will be asked for them: GEC (grounding electrode conductor, service to electrode), EGC (equipment grounding conductor, runs with circuit conductors), MBJ (main bonding jumper at the service), SBJ (system bonding jumper at a separately derived system) and EBJ (equipment bonding jumper around a raceway or between enclosures).

All electrodes present at a building are bonded together into one system: metal underground water pipe (first 10 ft and more), building steel, concrete-encased electrode (Ufer: 20 ft of 4 AWG copper or 1/2 in rebar in the footing), ground ring, and rod, pipe or plate electrodes. Where only a water pipe exists it must be supplemented.

Ground rods (250.52(A)(5)) are copper-bonded steel (UL 467, 10 mil copper minimum for 30-year life), 5/8 in by 8 ft, driven full depth; if rock prevents it, 45° or a 30 in trench. A single rod must read 25 Ω or less or be supplemented by one more rod at least 6 ft away (250.53(A)(2)), and most installers simply drive two and skip the test. Sectional rods with threaded couplers go deeper where the soil improves with depth.

Connections to electrodes are listed for the purpose and, if buried or in concrete, listed for direct burial (250.70): acorn and pipe clamps (bronze, one conductor per clamp unless listed for more), ground rod compression connectors, and exothermic welds. The GEC must be continuous or spliced only with irreversible compression or exothermic connections (250.64(C)).

The GEC to a rod electrode never needs to be larger than 6 AWG copper (250.66(A)); to a Ufer, 4 AWG; to a ground ring, the ring conductor size. Protect 6 AWG and smaller from damage with conduit or armor, and bond ferrous conduit at both ends.

Two tables do nearly all grounding sizing. Table 250.66 sizes the grounding electrode conductor from the largest ungrounded service conductor; Table 250.122 sizes the equipment grounding conductor from the overcurrent device ahead of the circuit.

When ungrounded conductors are upsized for voltage drop, the EGC is upsized proportionally by circular-mil area (250.122(B)). Parallel feeders get a full-size EGC in each raceway. The main bonding jumper follows 250.28(D): Table 250.66, and 12.5 percent of the service conductor area above 1100 kcmil.

Bonding jumpers for raceway and enclosure (250.102) size the same as the EGC for the circuit inside; supply-side bonding jumpers follow 250.66.

The Code accepts four kinds of grounding connection and each has a place.

• Irreversible compression (UL 467, listed for direct burial): a copper connector crimped with a hydraulic tool and the matched die (Figure 8 "C" shape for parallel or cross, Figure 6 for tap, C-tap for parallel taps, ground rod and busbar terminals). Accepted anywhere an exothermic weld is, including GEC splices (250.64(C)) and buried or concrete-encased connections (250.68(C)). No gas, no mold, no weather window, inspectable die emboss. This is the data center, telecom and utility standard today.
• Exothermic welding: a molecular bond with no interface to corrode, the historical standard for substations and ground grids. Needs molds, weld metal, dry conductors and training; cannot be made in the rain or on energized gear.
• Listed mechanical clamps (UL 467): acorn and pipe clamps for rod, pipe and rebar, and lay-in lugs on enclosures, correct for accessible, above-grade connections. Buried clamps must be listed for direct burial.
• Split bolts and set-screw lugs: fine for accessible bonding jumpers and equipment bonding above grade; not for GEC splices and not buried.

What is never a grounding connection: solder (250.8), sheet-metal screws, drywall screws, or a conductor wrapped around a bolt. 250.8 lists the permitted methods and listed pressure connectors, clamps, exothermic welding and machine screws with at least two threads engaged are the whole list.

Metal matters: bronze and copper on copper, dual-rated where aluminum is involved, and tinned connectors in corrosive or coastal soil.

Metal raceway is an equipment grounding conductor only when its joints are electrically continuous and it is bonded at the ends. The Code tightens the rules where the fault current is highest:

• Services (250.92(B)): standard locknuts do not count. Use bonding-type locknuts, grounding bushings with a bonding jumper, or threaded hubs on every raceway entering service equipment.
• Over 250 V to ground (250.97): the same at knockouts that are concentric or eccentric (the punched rings that may not have been removed cleanly), unless the enclosure is listed for the purpose.
• Grounding bushings are insulated-throat bushings with a lay-in lug; the jumper from the lug to the ground bar sizes by 250.122 (or 250.66 on the supply side).
• Flexible raceways (FMC, LFMC) are not an EGC above 6 ft or where flexibility is needed; run a wire EGC.

Enclosures, cabinets and equipment frames get a ground bar bonded to the can and an EGC from each circuit; the bar is where testing and additions happen. Paint under a bonding connection must be removed (250.12).

Data centers and telecom rooms follow ANSI/TIA-607 (J-STD-607) in addition to the NEC: a primary bonding busbar (PBB, formerly MGB) at the service, secondary bonding busbars (SBB) in each telecom room, a telecommunications bonding conductor to the service ground, and a mesh bonding network or rack grounding busbar bonding every cabinet, ladder rack and raised-floor pedestal. Connections to busbars are two-hole irreversible compression lugs with NEMA spacing; every busbar is listed, insulated from its mount and labeled.

Cell sites and towers (Motorola R56 as the common standard) use a buried ground ring around the tower and the shelter, exothermic or irreversible compression connections, tower leg grounds, a master ground bar at the cable entry and surge protection on every line entering the shelter. Soil resistance targets are 5 Ω, not 25.

Solar arrays bond module frames and racking to the EGC with listed lugs or clips (690.43), size the EGC by 250.122, and bond any array electrode to the building system (690.47). See the solar course.

Utility substations are IEEE 80 territory: a grid designed for step and touch voltage under fault, not just resistance, with 4/0 and larger bare copper, exothermic or irreversible compression grid connections, and hot-line and tank ground clamps for equipment. Transmission and distribution hardware (hot-line clamps, arrester terminals, tank ground clamps) is part of that bill of materials.

Three tests cover the grounding system. Fall-of-potential (3-point) measures electrode resistance to remote earth with two auxiliary probes; the 62 percent method gives the number the drawings ask for. Clamp-on ground testers read a rod or down-conductor without disconnecting it, as long as there is a parallel path through the system. Soil resistivity (4-point Wenner) is the design input for ground grids and rings and tells you whether more rods, deeper rods or chemical rods will help.

Bonding continuity is checked with a low-resistance ohmmeter (DLRO, micro-ohm) between the bar and each bonded item; under 0.1 Ω is the usual acceptance. Record every reading with location, date, instrument and weather, photograph buried connections before backfill, and hand the package over with the as-built. Inspectors and commissioning agents ask for the photos first.