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: do NOT look for elevated neutral-to-ground voltage at the subpanel. A bonded subpanel reads LOW there, because the two bars are tied together, so that test points the wrong way. The test that finds it is a clamp-on ammeter around the feeder EGC with load running: current on the EGC means load current is coming home on the grounding path, which means neutral and ground are tied somewhere downstream of the service. Then find and remove the extra bond. Elevated neutral-to-ground voltage is a different fault, a loose or high-impedance neutral.
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/ground resistance tester on the grounding electrode system (fall-of-potential or clamp-on ground tester; this is a different instrument from an insulation megohmmeter even though both are sold under the Megger name)
-
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 ohms resistance - barely better than nothing for the job the electrode actually has.
Be precise about what that job is, because getting it backwards is how people build the wrong fix. The electrode does NOT clear faults and does NOT trip breakers. At 25 ohms, 120 V through earth moves under 5 amps, nowhere near enough to open a 20 A breaker; that is why the earth path is not a fault path. A ground fault is cleared by the bonded metallic path back to the source and the main bonding jumper at the service, not by the rod. What the electrode DOES is limit voltage imposed by lightning and line surges and stabilize voltage to earth, and that is the work a 200 ohm rod fails at.
So test it, and sell the second rod on the honest reason: surge and lightning performance and code completeness. If the complaint is a breaker that will not trip or a frame that is energized, the electrode is not your problem and adding rods will not fix it. Look at the bonding and the EGC. The professional difference is not just "doing the work to code" but knowing which system does which job.
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