NEC Article 250

Grounding and Bonding Code Requirements

Grounding electrode system options, GEC sizing, main bonding jumper, water and gas pipe bonding, equipotential bonding at pools, and the NEC 250.130(C) rules for retrofitting ungrounded outlets.

By Brian WilliamsPublished April 21, 2026Updated April 22, 2026

Why grounding matters: The grounding system creates a low-impedance fault current path so breakers trip quickly during a ground fault. Without it, a hot wire touching a metal appliance chassis energizes the chassis at 120V and waits for someone to complete the circuit. Grounding saves lives; it is not optional.

The Grounding Electrode System

NEC 250.50 requires all electrodes present at the building to be bonded together to form a single grounding electrode system. Acceptable electrodes per NEC 250.52:

(A)(1) Metal underground water pipe

In direct contact with earth for at least 10 ft. Requires a supplemental electrode per 250.53(D)(2). Increasingly rare as PEX and plastic replace copper water service.

(A)(2) Metal frame of the building

Structural metal in direct contact with earth, or effectively grounded via one of the other electrodes. Rare in residential construction.

(A)(3) Concrete-encased (Ufer)

20 ft of bare copper (4 AWG min) or 20 ft of 1/2-inch rebar encased in at least 2 inches of concrete footing. Best electrode type available. Preferred for new construction.

(A)(4) Ground ring

Minimum 20 ft of 2 AWG bare copper encircling the building, buried at least 30 inches deep. Used for commercial, communication towers, and high-value lightning protection.

(A)(5) Rod and pipe

8-ft minimum length copper-clad steel rod, driven vertically so the entire 8 ft is in earth. Resistance to ground must be 25 ohms or less; otherwise a second rod is required, spaced at least 6 ft from the first.

(A)(6) Plate electrode

Minimum 2 sq ft of metal plate surface, buried at least 30 inches deep. Used where rock layers prevent rod driving.

Ufer vs Ground Rod

New construction

If a concrete footing with rebar exists, NEC 250.50 requires the Ufer to be used as the primary grounding electrode. Most inspectors will fail a new-construction install that uses only rods when a Ufer is available.

Ground rod install rules

• Minimum 8 ft driven into earth (not 8 ft of rod on the surface)
• Copper-clad steel, galvanized steel, or stainless steel; min 5/8-inch diameter
• Drive at an angle up to 45 degrees if rock prevents vertical install
• If rock prevents angled install, bury the rod horizontally at least 30 inches deep
• Second rod required unless 25-ohm resistance test passes; spacing 6 ft minimum
• Approved clamp rated for direct-burial (acorn style on exposed top)

Grounding Electrode Conductor (GEC) Sizing

Per NEC Table 250.66, sized based on the largest ungrounded service conductor:

Service Conductor Size	GEC Copper	GEC Aluminum
2 AWG Cu / 1/0 AWG Al or smaller	8 AWG	6 AWG
1 or 1/0 AWG Cu / 2/0 or 3/0 AWG Al	6 AWG	4 AWG
2/0 or 3/0 AWG Cu / 4/0 or 250 kcmil Al	4 AWG	2 AWG
Over 3/0 thru 350 kcmil Cu / over 250 thru 500 kcmil Al	2 AWG	1/0 AWG
Over 350 thru 600 kcmil Cu / over 500 thru 900 kcmil Al	1/0 AWG	3/0 AWG

Size caps for specific electrode types:

• To a rod electrode only: GEC does not need to exceed 6 AWG copper
• To a Ufer (concrete-encased) only: GEC does not need to exceed 4 AWG copper
• To a ground ring only: GEC does not need to exceed the ring conductor size
• Table 250.66 values apply to water pipe, plate, and metal frame electrodes

Main Bonding Jumper

NEC 250.28 requires the neutral bus and the equipment grounding bus to be bonded together at exactly one point: the service disconnect (usually the main panel). This is the main bonding jumper (MBJ). Subpanels downstream of the service keep neutral and ground isolated.

Where the MBJ goes

Installed at the service disconnect only
Panel-supplied green screw that bonds neutral bus to enclosure; must be installed
Sized per Table 250.102(C)(1) based on the service conductor size
Subpanels MUST NOT have the MBJ; separate neutral and ground bars required
Detached structure feeders: separate ground from neutral at the detached panel per 250.32

Bonding of Piping and Metal Systems

Metallic water piping

NEC 250.104(A)

Interior metal water piping must be bonded. Bonding jumper sized per Table 250.102(C)(1). Connect to the service equipment enclosure, the grounding electrode conductor, or the grounded conductor at the service.

Metallic gas piping

NEC 250.104(B)

Interior metal gas piping that may become energized must be bonded. Bonding jumper sized per Table 250.122 (based on the circuit breaker for the appliance that could energize it).

CSST gas piping

Manufacturer and local

Corrugated stainless steel tubing requires direct bonding with minimum 6 AWG copper per most CSST manufacturers. Bond to the grounding electrode system, not just to the grounded service conductor.

Metallic HVAC ducting

NEC 250.104(B)

Metal ducts that may become energized must be bonded, same rules as gas pipe. Practically, most homes have the duct bonded via the furnace grounding to the branch-circuit EGC.

Swimming pool equipotential

NEC 680.26

8 AWG solid copper bonding grid around pool, bonded to the pool shell, pump, ladders, diving boards, metal decking within 5 ft, and any other fixed metal. Separate from pool equipment grounding.

Retrofitting Grounding in Old Homes

NEC 250.130(C) covers the situation where an ungrounded two-wire circuit needs a grounded outlet. Four legal methods:

1. Run a new grounding conductor back to the service

The permanent fix. A new green or bare copper wire from the receptacle box back to the grounding bus in the service equipment. Works well during drywall repair or full rewires.

2. Tap into a nearby grounded circuit

Bond to the equipment grounding conductor of any accessible grounded branch circuit in the same panel. Common during kitchen remodels where new and old circuits share a wall.

3. Bond directly to the grounding electrode system

A grounding conductor run from the box to any point on the grounding electrode system (GEC, ground rod, Ufer, or metallic water pipe within 5 ft of entry). Size per 250.122 based on the circuit breaker.

4. Install a GFCI and label it

Per NEC 406.4(D)(2), replace the two-prong receptacle with a GFCI-protected three-prong receptacle, or protect the circuit with a GFCI breaker. Label both the GFCI receptacle and all downstream receptacles "No Equipment Ground." Does not provide a fault path but protects people.

Never bootleg a ground

A jumper from neutral to the ground screw of a receptacle (bootleg ground) violates NEC 250.142 and creates a chassis shock hazard. Any load that shares the neutral can energize the equipment ground and anything bonded to it. Outlet testers flag bootleg grounds as a correct ground, which is why the only reliable test is an actual grounding electrode check or a voltage measurement between neutral and ground.

Frequently Asked Questions

Can I just add a ground rod to my existing system?

Yes, and it is often required. NEC 250.53(A)(2) requires a supplemental electrode (typically a second ground rod) when the single rod resistance to ground exceeds 25 ohms, or, in practice, any time a single rod is used without testing. Most electricians drive two 8-ft rods spaced 6 ft apart by default and skip the resistance test. Adding a second rod to an existing install is a common retrofit during a panel upgrade.

Is a Ufer ground better than a rod?

Yes, substantially. A Ufer (concrete-encased electrode per NEC 250.52(A)(3)) offers 1 to 5 ohms of resistance to earth in most soils, compared to 25 to 200 ohms for a single rod. Concrete wicks moisture and stays conductive year-round. NEC 250.50 requires using the concrete-encased electrode if it is present (new construction with rebar in the footing), which is why new homes typically have Ufer plus rod. Ufer cannot be retrofitted without destroying the foundation, so existing homes rely on rods or water-pipe electrodes.

Must the gas pipe be bonded?

Yes, the interior metallic gas piping must be bonded to the grounding system per NEC 250.104(B). An equipment bonding jumper sized per Table 250.122 (based on the largest circuit breaker in the panel) connects the gas pipe to the grounding bus or to an accessible grounding point. CSST (corrugated stainless steel tubing) gas lines require direct bonding with a minimum 6 AWG copper conductor per most CSST manufacturer instructions and many local amendments. Unbonded CSST has been linked to lightning-strike fires.

What is equipotential bonding at a pool?

NEC 680.26 requires a bonding grid around in-ground pools to eliminate voltage differences between metal objects a swimmer might touch. The grid consists of 8 AWG solid copper connecting the pool shell (or perimeter), the pool pump, ladders, diving boards, metal decking within 5 ft, and any other metallic parts. The goal is equipotential: everything at the same voltage, so current cannot flow through a swimmer. Missing or broken equipotential bonding has caused pool electrocutions.

My house has no grounding system. What do I do?

Older homes (pre-1960s) often have two-wire ungrounded branch circuits. Retrofit options per NEC 250.130(C): (1) run a new grounding conductor back to the service equipment, (2) bond to a nearby grounded circuit's equipment grounding conductor, (3) bond to the grounding electrode system directly, or (4) replace the receptacle with a GFCI and label "No Equipment Ground." Option 4 is the cheapest but does not provide a true ground path; it only trips on fault current. A full rewire is the permanent fix.

Is GFCI a substitute for grounding?

No, but it is an acceptable safety alternative in specific retrofit cases. NEC 406.4(D) allows a GFCI to replace an ungrounded two-prong receptacle without installing a grounding conductor, with the receptacle labeled "No Equipment Ground." GFCI protects people from shock by detecting current imbalance; it does not provide a fault-current path for surge or equipment-ground bonding. Modern appliances with three-prong plugs often work but lose their equipment grounding path.

Can I retrofit a grounded outlet on an ungrounded circuit?

Only via one of the four NEC 250.130(C) methods above. Installing a three-prong receptacle with the ground pin simply disconnected (or jumpered to neutral) is dangerous and illegal. A jumper from neutral to ground at the receptacle (aka "bootleg ground") is a code violation per NEC 250.142 and can energize the chassis of appliances under a neutral fault. If you see a three-prong receptacle on old cloth-sheathed wiring without a ground wire, assume it is bootlegged until tested.

Why did my ground resistance test fail?

Common causes: (1) dry or sandy soil (ground rods work best in moist loam; deserts need Ufer or chemical rods), (2) rod not fully driven (must be 8 ft in earth, not 8 ft long), (3) rock layer preventing full depth, (4) corroded clamp at the rod top, (5) broken or cut GEC between the rod and the service. Solutions: drive a second rod 6+ ft away, replace the clamp, or use a chemical ground rod (CADWELD type) in rocky/dry soil. The 25-ohm test is only required if you have a single rod; two rods are accepted without testing.