NEC Article 250 — Grounding & Bonding
Article 250 covers grounding of electrical systems, equipment grounding, bonding of metal piping and structural steel, and the sizing of grounding and bonding conductors. Correct grounding protects people and equipment; incorrect grounding is a leading cause of failed inspections and re-work.
Grounding Electrode System — 250.50
All electrodes present at the building or structure shall be bonded together to form a grounding electrode system. When none of the listed electrodes are present, one or more of the manufactured electrodes below must be installed.
| Electrode Type | Requirements / Notes |
|---|---|
| Metal underground water pipe | Must be in contact with earth ≥ 10 ft. Must supplement with additional electrode (250.53(D)(2)). |
| Metal in-ground support structure (building/structure frame) | Effectively grounded structural steel in direct contact with earth or encased in concrete in contact with earth. |
| Concrete-encased electrode (Ufer ground) | ≥ 20 ft of bare or zinc-coated steel rebar, ½" minimum, or ≥ 20 ft of AWG 4 copper encased in concrete. |
| Ground ring | Bare copper conductor, AWG 2 minimum, at least 20 ft, buried ≥ 30 in below grade, encircling building. |
| Rod and pipe electrodes | 8 ft minimum length, ⅝" diameter solid/pipe. If single rod < 25 Ω, supplement required. |
| Plate electrode | Each plate must expose ≥ 2 sq ft of surface to exterior soil. Iron/steel ≥ 0.25" thick; nonferrous ≥ 0.060" thick. |
| Other listed electrodes | Chemical, cell-type, and listed manufactured electrodes per manufacturer and listing. |
Grounding Electrode Conductor Sizing — Table 250.66
Minimum GEC size based on the largest ungrounded service-entrance conductor or equivalent area for parallel conductors. AWG unless otherwise noted.
| Largest Ungrounded SE Conductor | Copper GEC | Aluminum GEC |
|---|---|---|
| AWG 2 or smaller | 8 | 6 |
| AWG 1 or 1/0 | 6 | 4 |
| AWG 2/0 or 3/0 | 4 | 2 |
| AWG Over 3/0 through 350 kcmil | 2 | 1/0 |
| AWG Over 350 through 600 kcmil | 1/0 | 3/0 |
| AWG Over 600 through 1100 kcmil | 2/0 | 4/0 |
| AWG Over 1100 kcmil | 3/0 | 250 kcmil |
Equipment Grounding Conductor Sizing — Table 250.122
Minimum EGC size based on the rating of the overcurrent protective device ahead of the circuit. Where conductors are larger than required for ampacity, EGC must be increased proportionally (250.122(B)). AWG unless noted.
| OCPD Rating (A) | Copper EGC | Aluminum EGC |
|---|---|---|
| 15 | 14 | 12 |
| 20 | 12 | 10 |
| 30 | 10 | 8 |
| 40 | 10 | 8 |
| 60 | 10 | 8 |
| 100 | 8 | 6 |
| 200 | 6 | 4 |
| 300 | 4 | 2 |
| 400 | 3 | 1 |
| 500 | 2 | 1/0 |
| 600 | 1 | 2/0 |
| 800 | 1/0 | 3/0 |
| 1000 | 2/0 | 4/0 |
| 1200 | 3/0 | 250 kcmil |
| 1600 | 4/0 | 350 kcmil |
| 2000 | 250 kcmil | 400 kcmil |
Key Bonding Requirements
- Interior metal water piping (250.104(A)): Must be bonded to the grounding electrode system or grounded service equipment enclosure using a conductor sized per Table 250.122.
- Gas piping (250.104(B)): Exposed metal gas piping shall be bonded to the grounding electrode system. Sizing: minimum 10 AWG copper.
- Structural steel (250.104(C)): Exposed structural metal forming a building frame shall be bonded where not otherwise connected to the grounding electrode system.
- Service equipment bonding (250.92): Non-current-carrying metal parts of service equipment enclosures shall be effectively bonded — requires bonding bushing or bonding locknut where concentric/eccentric knockouts are used.
- Main bonding jumper (250.28): Required connection between neutral conductor and equipment grounding conductor at the service disconnecting means. Must not be re-installed at sub-panels (use separate neutral and ground bars).
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