Grounding + Bonding Reference

Why this matters

Grounding + bonding are the #1 source of NEC interpretation confusion + #1 cause of dangerous electrical conditions in older homes. Wrong grounding = shock hazard; wrong bonding = equipment damage. The electrician who understands the distinction + applies correctly protects life. This is the field card.

Grounding vs bonding (the distinction)

Grounding: connection to earth. Provides reference for system voltage; dissipates lightning + transient surges.

Bonding: connecting metallic parts of the electrical system together. Ensures equal potential between metal parts; provides return path for fault current.

Both required: by NEC for residential + commercial; complement each other.

Grounding electrode system (NEC 250.50)

250.50 is not a pick-one list. EVERY electrode on the list that is PRESENT at the building has to be bonded together into a single grounding electrode system; only when none is present do you install one. So a house with a qualifying metal water pipe AND a Ufer uses both, bonded together.

The list: metal underground water pipe (if 10+ ft in earth); metal in-ground support structure / steel-framed building; concrete-encased Ufer ground (20+ ft of 4 AWG OR larger bare copper, or 1/2" rebar, inside the footing); ground ring (20+ ft of 2 AWG bare copper buried 30"+); driven rod (8 ft, 5/8" steel or copper-clad); plate electrode (2 sq ft of surface, buried 30"+). Supplemental-electrode rules at NEC 250.53.

The water pipe is the one that catches people: a metal water pipe electrode ALWAYS has to be supplemented by an additional electrode, because a plumber can cut it out of the ground on any future repair.

Grounding electrode conductor (GEC)

Connects grounding electrode to service equipment. Sizing per NEC 250.66, by the largest ungrounded service conductor (copper service conductor to copper GEC):

• 2 AWG or smaller: 8 AWG GEC
• 1 or 1/0: 6 AWG
• 2/0 or 3/0: 4 AWG
• Over 3/0 through 350 kcmil: 2 AWG
• Over 350 through 600 kcmil: 1/0 AWG
• Over 600 through 1100 kcmil: 2/0 AWG
• Over 1100 kcmil: 3/0 AWG

Read the ladder as continuous: each row picks up where the one above it stopped, so a 3/0 service is 4 AWG and 4/0 steps to 2 AWG. Material: copper OR aluminum, with aluminum a size larger and barred from earth contact or damp masonry contact. Continuous from electrode to service grounding bus. Note the separate rule at 250.66(A) through (C): the GEC to a rod-only electrode need be no larger than 6 AWG copper, to a concrete-encased electrode no larger than 4 AWG copper, and to a ground ring no larger than the ring conductor.

Equipment grounding conductor (EGC)

Green OR bare wire: provides fault current path.

Runs with circuit conductors: same cable / conduit.

Connected to: metal parts of equipment (frames, junction boxes, etc.) + equipment grounding terminal (EGT).

Sized per NEC 250.122: by circuit breaker (typically #14 for 15A, #12 for 20A, #10 for 30A).

Bonding jumper

Connects metal parts together for fault current path.

Service bonding jumper (NEC 250.28): connects neutral bus to grounding bus AT main service equipment only.

System bonding: ensures all metal parts have continuous fault path.

Equipment bonding: per-equipment ground bonding (e.g., CSST bonding per NFPA 54).

Service entrance grounding

Main service panel: neutral + ground bonded together (single point of bonding).

Below the service: neutral + ground SEPARATE (subpanels).

Critical: never bond neutral + ground anywhere except at service entrance.

Bonding screw / strap: identifies bonding location at main panel.

Subpanel grounding

Neutral + ground SEPARATE at subpanel.

4-wire feeder required: 2 hots + neutral + EGC.

Don't bond at subpanel: would create parallel return paths + ground potential differences = current on grounding conductors.

Older 3-wire feeders to a DETACHED structure: earlier code editions permitted the grounded (neutral) conductor to carry fault current out to a separate building where no other metallic path linked the two. That allowance is gone. Existing ones may stay where they are; anything new gets a 4-wire feeder with the EGC run and the neutral isolated at the remote panel (NEC 250.32).

Bonding requirements (NEC 250)

Service entrance: neutral + ground bonded at single point. Water pipe metal: bonded if used as grounding electrode. Gas pipe (CSST): bonded per NFPA 54 + manufacturer. Pool / spa: equipotential bonding grid around perimeter (NEC 680). Communications: bonded to grounding system. Lightning protection: separate but bonded (NFPA 780).

Ground rod install

Drive 8 ft × 5/8" copper-clad steel rod: full depth below grade.

Connect to GEC with acorn clamp OR exothermic weld.

Two rods required (NEC 250.53(A)(2)): if single rod doesn't reach 25 ohms OR less resistance. Most jurisdictions: install 2 rods 6+ ft apart automatically; don't test.

Don't bury connection point unless specifically rated below grade.

Ground resistance testing

Earth resistance tester: clamp-on OR fall-of-potential.

The 25-ohm figure is not a performance target in the NEC. It is the single threshold at 250.53(A)(2): a single rod, pipe, or plate electrode that does NOT measure 25 ohms or less has to be supplemented by a second electrode. There is no code requirement to hit a number once the second rod is in. Some industries and OEM specs do set their own targets (5 ohms is common for sensitive equipment); those come from the spec, not from the NEC.

Soil resistivity affects: dry sand vs wet clay = 100x difference.

Multiple ground rods: lowers total resistance.

Most residential: 2 rods give sufficient resistance without testing.

Common grounding mistakes

Bonding neutral + ground at subpanel (parallel paths; current on grounding system; arc + heat). Skipping ground rod when no underground water pipe. Connecting GEC + EGC at fixture (parallel path). Missing CSST bonding (lightning risk). Improper pool / spa bonding (shock + electrocution). Skipping ground continuity test after install.

Pool + spa, generator, CSST, comms bonding

Pool/spa (NEC 680): equipotential bonding grid; all metal within 5 ft bonded with #8 copper. Reduces voltage gradient near pool. Required: deck reinforcement, ladders, diving board mounts, ground around pool, motor. Service issue: bonding corroded; voltage measured between pool + deck.

Generator: portable per OEM (some don't bond N+G OR don't require separate ground); standby bonded per NEC + manufacturer with transfer switch.

CSST (NFPA 54): yellow CSST jacket REQUIRES bonding to grounding system. 6 AWG min conductor + bonding clamp. Un-bonded = lightning arcs through pipe = pinhole leak + fire. Critical; never skip.

Communications (NEC 800): coax, ethernet, phone, fiber bonded to electrical grounding at entry point with ground block + 6 AWG to grounding bus. Common issue: cable installer skipped bonding; lightning surge enters home.

Tools + materials: acorn clamps; lugs sized for conductor + bus; bonding jumpers; bonding clamps (CSST, water pipe); exothermic weld (Cadweld; permanent rod connection); ground bar in panel.

Ground fault path

Fault occurs: hot wire shorts to metal frame of equipment.

Path: hot → fault → metal frame → EGC → grounding bus → neutral bus (at service) → utility transformer neutral → utility transformer → fault hot.

Current flows: until breaker trips OR fuse blows.

No ground path: fault current can't return; breaker doesn't trip; energized frame = shock hazard.

GFCI vs grounding

GFCI (Ground Fault Circuit Interrupter): detects current imbalance between hot + neutral; trips on small imbalance (4 - 6 mA).

Not grounding: GFCI doesn't replace grounding; complements it.

3-prong outlet without ground: code-allowed if labeled "No Equipment Ground" + GFCI-protected (NEC 406.4).

Knob + tube wiring (legacy)

Ungrounded wiring: pre-1940; no ground wire.

Modernization options:

• Rewire (best)
• Install GFCI throughout (interim; allows 3-prong appliances)
• 2-prong outlets remain (code-allowed but limited)

Truck stock

Acorn clamps (5/8" + 1/2"), bonding lugs, bonding clamps (water pipe, CSST), 6 AWG green bonding wire, 8 AWG bare copper, ground rods, multimeter (continuity test), megger (insulation resistance testing on motor / cable).

References

• NEC Article 250 (Grounding + Bonding)
• NEC 250.50 - 70 (grounding electrode system)
• NEC 250.32 (separate building feeder + grounding)
• NEC 680 (Swimming Pools, Fountains, + Similar Installations)
• NEC 800 (Communications Circuits)
• NFPA 54 (CSST bonding)
• IEEE Std 142 (Green Book - Grounding)
• Manuall internal: Breaker + Panel Reference, Wiring Color Codes