Grounding and bonding

Grounding vs bonding

The two words are used loosely on job sites, but Article 250 keeps them apart. Grounding means connecting to the earth, or to something that extends the earth connection, such as a grounding electrode conductor. Bonding means connecting metal parts together so they are electrically continuous and can carry whatever current a fault puts on them.

They solve different problems. Grounding the system holds its voltage steady relative to the earth and limits the voltage that lightning, surges on the utility lines or accidental contact with higher-voltage lines can impose. Bonding equipment creates the effective ground-fault current path of 250.4(A)(5): a low-impedance route back to the source, so that a fault draws enough current to open the breaker or fuse quickly. Every edition says plainly that the earth is not an effective ground-fault current path. If you remember one thing from this page, make it that: bonding clears faults, the earth does not.

System grounding: why the neutral is connected to earth

Most premises wiring is a grounded system. Under 250.20(B), an AC system from 50 to 1000 V that supplies premises wiring must be grounded if it can be grounded so that no ungrounded conductor is more than 150 V to ground, if it is a three-phase four-wire wye system using the neutral as a circuit conductor, or if it is a four-wire delta system using the midpoint of one winding. That covers 120/240 V single-phase homes, 208Y/120 V and 480Y/277 V buildings, and high-leg delta services.

In those systems the point that is grounded, usually the neutral, becomes the grounded conductor. It is connected to the grounding electrodes through the grounding electrode conductor at the service, and the utility grounds it again at its transformer. Ungrounded systems still exist in industry, where continuity of service matters more than clearing the first fault; they need ground detectors (250.21) and are the NEC’s counterpart of the IEC “IT” system described in earthing systems.

The main bonding jumper and the single neutral-to-ground bond

At the service, the main bonding jumper connects the grounded conductor to the equipment grounding conductors and the service enclosure (250.24(B)). In a typical residential panel it is a green screw or a strap that joins the neutral bar to the can. It is the hinge of the whole system: it is where fault current on the equipment grounding conductors crosses over to the neutral and returns to the utility transformer.

The jumper is sized from the service conductors using Table 250.102(C)(1). For a 200 A service on 3/0 AWG copper it is 4 AWG copper. Downstream of the service disconnect, the grounded conductor may not be connected to equipment grounding conductors or to enclosures (250.24(A)(5) and 250.142(B)), apart from narrow exceptions for existing installations and separately derived systems.

Why subpanels separate neutral and ground

A subpanel is fed by a four-wire feeder: two ungrounded conductors, a neutral and an equipment grounding conductor. Its neutral bar sits on insulators, its ground bar is bonded to the enclosure, and the bonding screw supplied with the panel stays out. The reason is current sharing. If neutral and ground were bonded again at the subpanel, the neutral current would have two parallel paths back to the service and would split between them in inverse proportion to their resistance.

Take a 100 ft feeder with 3 AWG copper conductors on a 100 A breaker and the 8 AWG copper equipment grounding conductor Table 250.122 requires for it. With 30 A of unbalanced neutral current and a second bond at the subpanel, 7.2 A would flow on the grounding conductor all the time. In a real building it also spreads onto metal conduit, water pipes and duct, where it shows up as shocks from piping, heating at loose fittings, hum on audio systems and nuisance tripping of ground-fault protection. The NEC calls this objectionable current (250.6).

The same applies to a detached building. A garage or barn fed by a feeder gets its own grounding electrode system (250.32(A)), a feeder with an equipment grounding conductor, and an isolated neutral (250.32(B)(1)). The only exception is a single branch circuit, including a multiwire branch circuit, that carries an equipment grounding conductor. Old three-wire feeders that used the neutral for grounding are tolerated only where they were legal when installed and nothing has since changed.

The fault-current path

When a hot conductor touches a metal case, the breaker trips only if enough current flows. That current needs a complete metal loop: out on the hot, through the fault into the case, back on the equipment grounding conductor to the service, across the main bonding jumper and home to the transformer on the neutral.

On a 120 V circuit with 100 ft of 12 AWG copper, the hot and the equipment grounding conductor together are about 0.386 Ω at 75 °C, so the fault current could reach 311 A before transformer and service impedance are counted: about 16 times the rating of a 20 A breaker, well into its instantaneous trip. Now remove the equipment grounding conductor and rely on a ground rod at the appliance. Even if everything else were a perfect conductor, a 25 Ω rod would limit the fault to 4.8 A. The breaker never trips and the case sits close to 120 V. That is the arithmetic behind the rule that the earth is never the fault path and behind the warning against “isolated” ground rods for equipment.

Separately derived systems

A transformer with no direct connection between primary and secondary, such as a 480 V delta to 208Y/120 V unit, creates a new system: a separately derived system (250.30). It needs its own neutral-to-ground bond, the system bonding jumper, installed in one place only, at the transformer or at the first disconnect. A supply-side bonding jumper connects the transformer enclosure to that first disconnect, and a grounding electrode conductor connects the system to the building’s grounding electrode system. All the subpanel rules above then apply downstream.

Generators can go either way. If the transfer switch switches the neutral, the generator is a separately derived system and is bonded like one. If the neutral passes straight through, the generator is not separately derived (250.35(B)); the premises keeps its single bond at the service, and a generator that brings its own neutral-to-frame bond would create the second bond this page warns against.

The grounding electrode system

Every electrode that is present at a building must be bonded together into one grounding electrode system (250.50). The electrodes recognised by 250.52(A) are:

• Metal underground water pipe in contact with the earth for at least 10 ft. It always needs a supplemental electrode (250.53(D)(2)), and continuity may not depend on water meters or filters.
• Metal in-ground support structures, such as piles, in contact with the earth for at least 10 ft.
• Concrete-encased electrode: at least 20 ft of rebar of at least ½ in diameter, or of bare copper not smaller than 4 AWG, encased in at least 2 in of concrete in a footing or foundation in direct contact with the earth. The 2026 edition adds a note that concrete separated from the soil by insulation or vapor barrier is not in direct contact.
• Ground ring: at least 20 ft of bare copper not smaller than 2 AWG encircling the building, at least 30 in deep.
• Rod and pipe electrodes at least 8 ft long: rods at least ⅝ in diameter unless listed, pipe at least trade size ¾.
• Plate electrodes exposing at least 2 ft² to the soil, at least 30 in deep, and other listed electrodes or local underground metal systems.

Metal underground gas piping and aluminum are never electrodes (250.52(B)). A rod must have at least 8 ft in contact with the soil; where rock prevents driving it straight down, it may be driven at an angle of up to 45° or buried in a trench at least 30 in deep. The 2026 edition rewrites 250.53(A)(4) as a list and makes clear that the rod must first be driven vertically, with the angle and trench options reserved for rock.

The 25 Ω rule. A single rod, pipe or plate has to be supplemented by another electrode (250.53(A)(2)) unless it has a resistance to earth of 25 Ω or less, which only a fall-of-potential or clamp-on test can show. Electrodes of this kind must be at least 6 ft apart (250.53(A)(3)), and the informational note points out that spacing them at twice the rod length, 16 ft for standard rods, makes the pair work better. The bonding jumper that is the only connection to a supplemental rod need not exceed 6 AWG copper. None of this is about tripping breakers: as the fault-path example shows, no practical electrode passes enough current for that. Electrodes are there for lightning, surges and voltage stability.

GEC vs EGC sizing, and why they differ

The grounding electrode conductor (GEC) connects the service to the electrodes. It is sized from the largest ungrounded service conductor, or the equivalent area of a parallel set, using Table 250.66, because what it may have to carry depends on the size of the supply, not on any one breaker. Its size is capped: the part that runs only to rods, pipes or plates need not be larger than 6 AWG copper, and the part that runs only to a concrete-encased electrode need not be larger than 4 AWG copper (250.66(A) and (B)), because the earth connection of those electrodes, not the wire, limits the current.

The equipment grounding conductor (EGC) runs with each circuit. Its job is to carry fault current long enough for the breaker or fuse to open, so it is sized from the rating of that device using Table 250.122, and it never has to be larger than the circuit conductors (250.122(A)). Some examples:

So a 200 A service with 3/0 AWG copper conductors needs a 4 AWG copper GEC, while a 200 A feeder leaving it needs only a 6 AWG copper EGC. On large services the difference grows: with 4 parallel sets of 500 kcmil copper (2,000 kcmil in total) the GEC tops out at 3/0 AWG, while the main bonding jumper, which must carry the full fault current inside the service equipment, follows the 12.5% rule and comes to 250 kcmil.

Upsizing for voltage drop. When you increase the ungrounded conductors beyond the minimum size, for example to limit voltage drop, the EGC has to grow in proportion to the circular-mil area (250.122(B) through the 2023 edition, 250.122(D) in 2026). A 20 A circuit upsized from 12 AWG to 10 AWG needs an EGC of at least 6,530 × 10,380 / 6,530 = 10,380 cmil, which is 10 AWG. Since the 2020 edition, increases made only for ambient-temperature correction or for more than three current-carrying conductors do not trigger the rule, and a qualified person may size the EGC to provide an effective fault path instead. The ground wire size calculator does the proportional arithmetic, and the wire size calculator flags when an upsize triggers it.

Bonding metal piping and structural steel

Metal that is not part of the electrical system can still become energized by a fault, so 250.104 brings it into the bonding network:

• Metal water piping (250.104(A)) is bonded to the service enclosure, the grounded conductor at the service, the GEC where it is large enough, or the electrodes. The jumper is sized from the service conductors with Table 250.102(C)(1), the same table as the main bonding jumper: 4 AWG copper for a 3/0 AWG copper service.
• Other metal piping likely to become energized, including gas piping (250.104(B)), is bonded back to the service with a jumper sized from Table 250.122 for the rating of the circuit likely to energize it; that circuit’s equipment grounding conductor is allowed to serve as the bond. The fuel gas code and manufacturers’ instructions may call for a separate bonding jumper on corrugated stainless steel tubing. Gas piping is never an electrode.
• Exposed structural steel that is interconnected into a frame and likely to become energized (250.104(C)) is bonded to the service equipment, the GEC or the electrodes, again sized from Table 250.102(C)(1).

Bonding these systems does not make them grounding electrodes, and it does not replace equipment grounding conductors. It keeps everything a person can touch at the same potential, so that a fault anywhere produces current rather than a dangerous voltage between two pieces of metal.

Section numbers by edition

The grounding tables barely change between editions, but section letters do. The references you are most likely to cite:

• The 2020 edition changed the aluminum EGC for 5000 A and 6000 A from 1200 to 1250 kcmil, so that it matches a standard size, and added the ambient and adjustment carve-out and the qualified-person exception to 250.122(B).
• The 2026 edition re-letters 250.122 and adds a cap: the EGC in each raceway of a parallel run need not be larger than the largest ungrounded conductor in that raceway. Table 250.122 values are unchanged (250.122(D) is the new home of the proportional rule).
• Grounding for systems over 1000 V moved from Part X of Article 250 to the new Article 270 in 2026.
• 250.53(A)(4), on driving rods, became a list in 2026; 250.52(A)(3) gained the note on insulation and vapor barriers under footings.

Common mistakes

• Bonding neutral and ground in a subpanel, or leaving the panel’s bonding screw installed.
• Landing neutrals and grounds on the same bar in a subpanel.
• Treating a ground rod as fault protection. An “isolated” rod at a pole light or a well pump with no equipment grounding conductor leaves the metal live during a fault.
• Sizing the EGC from the wire rather than from the overcurrent device, or forgetting to upsize it when the wire is upsized.
• One rod and done, without a test showing 25 Ω or less, or a second rod closer than 6 ft.
• Relying on the water pipe alone. It must be supplemented, and plastic repairs can break its continuity.
• Skipping the concrete-encased electrode in new construction, where the footing steel is the best electrode you will get.
• A three-wire feeder to a new detached building. Using the neutral for grounding is allowed only for existing installations that were legal when they were built.