NEC Article 250 Explained: Complete Grounding & Bonding Guide
Grounding is one of the most critical safety aspects of electrical work. This guide breaks down NEC Article 250 into practical knowledge for everyday electrical installations.
In This Guide
Why Grounding Matters
Proper grounding serves three critical functions in electrical systems:
- Personnel safety - Provides a path for fault current to trip overcurrent devices, preventing electrocution
- Equipment protection - Limits voltage on equipment enclosures during faults
- System stability - Establishes a reference point for the electrical system and helps with surge protection
Without proper grounding, a fault in equipment could energize metal enclosures at full voltage, creating a deadly shock hazard that may not trip the breaker.
Grounding vs Bonding: Know the Difference
The NEC uses specific terminology that's important to understand:
Grounding
Connecting to the earth (ground rods, water pipes, concrete-encased electrodes). Establishes earth reference.
Bonding
Connecting metal parts together to ensure electrical continuity. Creates fault current path back to source.
Grounded Conductor
The neutral - a current-carrying conductor that's intentionally grounded at the service.
Grounding Conductor
The equipment ground (EGC) - NOT intended to carry current except during faults.
Key Point: Bonding (not grounding to earth) is what actually clears faults. Earth grounding alone has too much resistance to trip breakers. The equipment grounding conductor provides the low-impedance path needed.
Grounding Electrode System (250.50)
NEC 250.50 requires that all grounding electrodes present at a building be bonded together to form the grounding electrode system. Required electrodes include:
Required Electrodes (if present)
Metal Underground Water Pipe (250.52(A)(1))
Must have 10 feet or more in direct contact with earth. Must be supplemented by an additional electrode.
Metal Frame of Building (250.52(A)(2))
If effectively grounded (10 feet of structural metal in earth, or bonded to concrete-encased electrode).
Concrete-Encased Electrode "Ufer Ground" (250.52(A)(3))
20+ feet of 4 AWG or larger bare copper, or 1/2" rebar, encased in 2+ inches of concrete in contact with earth.
Ground Ring (250.52(A)(4))
2 AWG or larger bare copper, minimum 20 feet, encircling building in direct contact with earth at 2.5 feet depth.
Made Electrodes (250.52(A)(5))
If none of the above electrodes exist, you must install made electrodes:
- Ground rods - 5/8" diameter, 8 feet long minimum, driven vertically
- Pipe electrodes - 3/4" trade size, 8 feet long, galvanized or metal-coated
- Plate electrodes - 2 sq ft surface area, 1/4" thick (steel) or 0.06" thick (nonferrous)
The 25-Ohm Rule (250.53(A)(2))
A single ground rod must have resistance to ground of 25 ohms or less. If it exceeds 25 ohms, a second rod is required (spaced at least 6 feet apart). With two rods, no further testing is required regardless of resistance.
Grounding Electrode Conductor (GEC) Sizing
The GEC connects the grounding electrode system to the service equipment. Size per NEC Table 250.66:
| Service Conductor Size (Copper) | GEC Size (Copper) | GEC Size (Aluminum) |
|---|---|---|
| 2 AWG or smaller | 8 AWG | 6 AWG |
| 1 AWG or 1/0 AWG | 6 AWG | 4 AWG |
| 2/0 or 3/0 AWG | 4 AWG | 2 AWG |
| Over 3/0 through 350 kcmil | 2 AWG | 1/0 AWG |
| Over 350 through 600 kcmil | 1/0 AWG | 3/0 AWG |
| Over 600 through 1100 kcmil | 2/0 AWG | 4/0 AWG |
Note: GEC to ground rods/pipes never needs to be larger than 6 AWG copper. GEC to concrete-encased electrode never needs to be larger than 4 AWG copper.
Equipment Grounding Conductors (EGC)
The EGC provides the fault return path from equipment back to the source. Size per NEC Table 250.122 based on the overcurrent device protecting the circuit:
| Overcurrent Device (Amps) | Copper EGC | Aluminum EGC |
|---|---|---|
| 15 | 14 AWG | 12 AWG |
| 20 | 12 AWG | 10 AWG |
| 30 | 10 AWG | 8 AWG |
| 40 | 10 AWG | 8 AWG |
| 60 | 10 AWG | 8 AWG |
| 100 | 8 AWG | 6 AWG |
| 200 | 6 AWG | 4 AWG |
Bonding Requirements
Main Bonding Jumper (250.28)
At the service, the grounded conductor (neutral) must be bonded to the equipment grounding conductor and enclosure. This is the ONLY place where neutral and ground should be connected in most systems.
Bonding of Metal Piping (250.104)
- Metal water piping - Must be bonded to service equipment, grounding electrode conductor, or grounding electrode
- Other metal piping - Gas piping, etc. must be bonded if likely to become energized
- Structural steel - Must be bonded if likely to become energized
Bonding Around Concentric/Eccentric Knockouts
Standard locknuts are NOT sufficient for bonding on the line side of service equipment. Use bonding bushings or bonding locknuts with bonding jumpers per 250.92(B).
Common Grounding Mistakes
Neutral-Ground Bond in Sub-Panel
The neutral and ground should only be bonded at the main service. In sub-panels, keep them separate. Bonding at both creates parallel paths and can cause neutral current on equipment grounds.
Bootleg Ground
Connecting ground to neutral at a receptacle instead of running an actual ground wire. This is dangerous and won't trip GFCI devices properly.
Undersized GEC or EGC
Not following Tables 250.66 and 250.122. Undersized grounding conductors may not clear faults properly.
Single Ground Rod Without Testing
Installing one ground rod without testing for 25 ohms or less resistance. If you can't test, install two rods.
Not Bonding Water Heater
Forgetting to bond metal water piping at water heater where dielectric unions interrupt the grounding path.
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