Grounding + Bonding Fundamentals (NEC Article 250)
Why this matters
NEC Article 250 is the longest + most-misunderstood article in the National Electrical Code. The electrician who understands grounding + bonding from first principles passes every inspection, troubleshoots stray voltage problems competently, + makes the home truly safe. The electrician who applies grounding "rules" without understanding causes the dangerous misinstalls that send current through the wrong path + cost lives. This is the foundational knowledge that separates the apprentice from the journeyman.
Three different concepts (often confused)
Grounding - connecting electrical system to earth (ground rods, water pipe, etc.). Purpose: stabilize voltage during faults + lightning. The earth itself is a poor conductor; grounding doesn't typically carry significant fault current.
Bonding - connecting all metal parts of the electrical system together so they share the same voltage. Purpose: provide low-impedance fault current path back to the source. This is what actually trips breakers + saves lives during faults.
Equipment grounding conductor (EGC) - the green/bare wire in your romex; technically a bonding conductor that carries fault current. Provides path from equipment back to service panel + transformer.
The confusing nomenclature: the EGC is called a "ground" but functionally it's the bonding path. The "ground rod" is the actual grounding electrode.
The grounding electrode system (NEC 250.50 - 250.70)
Use ALL primary electrodes present: concrete-encased electrode (Ufer) - rebar in foundation footing, best if present; metal underground water pipe (at least 10 ft contact with earth); ground rod (8 ft × 5/8" copper or stainless - two minimum unless single rod tests < 25 Ω); ground ring (20 ft of #2 AWG copper buried 30" deep around building). All primary electrodes must be bonded with #4 or #6 AWG copper to a common point + that point bonded to service neutral. Multiple electrodes = redundancy + lower resistance (single rod often 25 - 200 Ω; two rods 6 ft apart drop significantly).
The bonding system (NEC 250.90 - 250.110)
At the service panel, EGC + grounding electrode conductor + neutral all bond together via main bonding jumper (green screw or jumper wire). This is the SINGLE POINT where neutral + ground connect. Beyond the service, neutral + ground are SEPARATE in subpanels + branch circuits - bonding them downstream creates parallel paths + dangerous stray current. Metallic raceways must be bonded across non-conductive joints via bonding bushing or jumper. Service-side: meter base, service conduit, neutral bus, ground bar all bonded.
Subpanel grounding (the #1 misinstall)
In a subpanel: neutral + ground busses are SEPARATE, bonding jumper REMOVED, 4-wire feed from main (2 hots + neutral + EGC), EGC lands on ground bus, neutrals on neutral bus, grounds on ground bus. NEVER combine in a sub. Why it matters: combining N + G creates parallel paths - normal load current returns through both EGC AND neutral, putting voltage on all bonded metal parts (panels, conduit, appliance cases). Identifying the failure: measure neutral-to-ground voltage > 0 V on a loaded circuit; > 2V indicates the failure.
Equipment grounding (NEC 250.118 - 250.122)
Allowed EGCs back to service: copper wire (green/bare per Table 250.122), aluminum wire (rare residential), RMC, IMC, EMT, Type AC (BX), Type MC, Type MI. NOT permitted as sole EGC: FMC + LFMC (need accompanying ground OR bonding bushing); NM (Romex) must have its dedicated ground wire (Romex includes one).
EGC sizing per NEC 250.122: 15A → #14, 20A → #12, 30A → #10, 60A → #10, 100A → #8, 200A → #6, 400A → #3 (all copper AWG).
Common grounding mistakes
Single ground rod without resistance test (code requires < 25 Ω or two rods minimum). Ground rod + water pipe not bonded to common point + service neutral. Subpanel with bonded neutral (most dangerous misinstall). Undersized EGC doesn't carry fault current fast enough. No bonding across non-conductive conduit joints. Connection corrosion at bare wire + galvanized clamp + moisture - use anti-oxidant compound. Gas line is NOT a grounding electrode (gas piping bonded TO the system, but not the grounding electrode). Painted clamp on rebar - scrape contact point first.
Service-side bonding (meter base + main disconnect)
Service neutral bonded to meter base + service disconnect + grounding electrode conductor. EGC bonded TO neutral at service disconnect via main bonding jumper. Concentric/eccentric knockouts: install bonding bushing in service-side knockouts without concentric ground continuity. Reducing washers create non-conductive gap; need bonding bushing or jumper.
Pool / spa / outdoor bonding (NEC 680 + 250)
See Pool + Spa Electrical NEC 680 - equipotential bonding is the stringent version required where water + electricity create electrocution risk. Outdoor lighting + signs + receptacles + boxes bonded via metal box or EGC. Lightning protection grounding bonded to service grounding electrode system per NFPA 780.
Diagnostics
Stray voltage on metal parts:
- Measure voltage between metal part + a known-ground reference (water pipe, earth)
- If > 1 V, problem exists
- Trace back to identify ground/neutral combine OR bad bonding
Open ground at outlet:
- Plug-in tester (3-light) - lights indicate condition
- "Open ground" = no continuity from outlet ground pin to panel
- Repair: pull new wire OR add ground from outlet to nearest grounded box
High impedance ground:
- Earth resistance tester (Megger) on grounding electrode system
-
25 Ω resistance - add electrodes
- Typical residential: 5 - 15 Ω goal
Ground loop / parallel path:
- Clamp-on ammeter around EGC
- Should read ~0 A in normal operation
- Reading > a few mA indicates parallel current path (N + G bonded somewhere downstream)
- Trace + correct
Service contract opportunity
Annual ground / bond verification:
- Continuity test from service to grounding electrodes
- Bond check at sub-panels, equipment, water/gas lines
- Earth resistance test if appropriate
- Documentation report for customer
Premium safety service for high-value homes, server rooms, medical environments, pool/spa.
NEVER assume the grounding electrode is "good enough" without testing. Old homes with single ground rods often have 50 - 200 Ω resistance - barely better than nothing. A house struck by lightning OR with a service-side fault depends on that grounding to dump current to earth fast enough to trip the upstream breaker. The 20-minute earth resistance test (Megger) on every electrical service upgrade catches the silent killer + lets you sell the additional ground rod that prevents the lightning damage. The professional difference: not just "doing the work to code" but doing the work to actually protect the customer.
References
- NEC 2023 Article 250 (Grounding + Bonding)
- NEC Article 100 (Definitions - review "Grounded," "Grounding Conductor," "Bonded")
- IEEE Std 142 (Recommended Practice for Grounding)
- Manuall internal: Pool + Spa Electrical NEC 680, Service Panel Upgrade, Annual Electrical Inspection