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)

Required electrodes (at least one): metal underground water pipe (if 10+ ft in earth); steel-framed building (effectively grounded); concrete-encased Ufer ground (20+ ft of 4 AWG OR larger bare copper inside foundation); ground ring (20+ ft of 2 AWG buried 30"+); driven rod (8 ft × 5/8" copper-clad steel); plate electrode (2 sq ft buried 30"+). Multiple electrodes typically required where supplemental needed (NEC 250.53).

Grounding electrode conductor (GEC)

Connects grounding electrode to service equipment. Sizing per NEC 250.66: < #2 service = #8 GEC; #1/1/0 service = #6 GEC; #2/0-#3/0 service = #4 GEC; > #3/0 = #2 GEC; 250 - 600 kcmil = #2 OR #1/0 GEC. Material: copper OR aluminum. Continuous from electrode to service grounding bus.

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 subpanel feeds: in newer detached structures specifically; modern require 4-wire.

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 Ω 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.

Target: ≤ 25 Ω per NEC (less is better; some industries require ≤ 5 Ω).

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