Grounding and bonding

Updated November 8, 2026 · Written for the 2023 NEC

All rules below follow the 2023 NEC.

Grounding vs bonding

Grounding is the connection to earth. Section 250.4(A)(1) says it limits voltage from lightning, line surges or contact with higher-voltage lines, and stabilizes the voltage to earth during normal operation. The grounding electrode conductor (GEC) and the electrodes do this job.

Bonding connects metal parts such as enclosures, raceways and equipment frames into one low-impedance path back to the source, called the effective ground-fault current path (250.4(A)(3) and (A)(5)). It clears faults by letting enough current flow to open the overcurrent device. Equipment grounding conductors (EGCs) and bonding jumpers do this job.

The earth is never an effective ground-fault current path (250.4(A)(5)). If a 120-volt conductor faults to an enclosure connected only to a 25-ohm rod, the current is about 120 ÷ 25 = 4.8 A. That will not trip a 20 A breaker, and the enclosure stays energized.

Which table sizes what

GEC sizing with Table 250.66

The GEC is sized from the service conductor size, not the service rating in amperes. Find the largest ungrounded service-entrance conductor (or add up the area of parallel sets), find that row in Table 250.66, and read the GEC size. Use the aluminum column when the service conductors are aluminum.

Two caps apply no matter how large the service is. The portion of a GEC that connects only to rod, pipe or plate electrodes need not be larger than 6 AWG copper (250.66(A)). The portion that connects only to a concrete-encased electrode need not be larger than 4 AWG copper (250.66(B)).

EGC sizing with Table 250.122

The EGC is sized from the rating of the overcurrent device protecting the circuit. If the rating falls between two rows of Table 250.122, use the next larger row. See overcurrent protection for how that device rating is chosen.

• If the ungrounded conductors are made larger than needed for ampacity, such as for voltage drop, a wire-type EGC must be increased in proportion to circular-mil area (250.122(B)).
• Where conductors run in parallel raceways, each raceway's EGC is sized for the full device rating (250.122(F)).

Main and system bonding jumpers

The main bonding jumper connects the neutral to the EGCs and the service enclosure (250.28). It may be a wire, bus or screw, and a screw must have a visible green finish. It is sized from Table 250.102(C)(1).

The neutral-to-ground connection is made at the service and nowhere downstream (250.24). In a subpanel, neutrals land on an isolated neutral bar and EGCs land on a grounding bar bonded to the cabinet. A second bond puts neutral current on raceways and enclosures, which 250.6 calls objectionable current.

A separately derived system, such as a transformer secondary, gets a system bonding jumper instead (250.30(A)(1)). It is normally installed at one point between the source and the first disconnecting means or overcurrent device, and it is also sized from Table 250.102(C)(1).

Grounding electrodes

Every electrode type in 250.52(A) that is present at a building must be bonded together into one grounding electrode system (250.50). The ones tested most:

• Metal underground water pipe: at least 10 ft in contact with earth (250.52(A)(1)), and always supplemented by another electrode (250.53(D)(2)).
• Concrete-encased electrode: at least 20 ft of 1/2 in or larger rebar, or bare copper not smaller than 4 AWG, in at least 2 in of concrete in contact with earth (250.52(A)(3)).
• Ground ring: at least 20 ft of bare copper not smaller than 2 AWG, buried at least 30 in deep (250.52(A)(4), 250.53(F)).
• Rod and pipe electrodes: at least 8 ft long (250.52(A)(5)). A single rod needs a supplemental electrode unless it measures 25 ohms or less (250.53(A)(2)), and rods must be at least 6 ft apart.

Metal underground gas piping and aluminum are not permitted as electrodes (250.52(B)).

Worked examples

Example 1: GEC and main bonding jumper. A service has two parallel sets of 250 kcmil copper per phase. The GEC runs to the metal water pipe, with a separate conductor to two ground rods.

1. Equivalent area: 2 × 250 kcmil = 500 kcmil copper.
2. Table 250.66 places 500 kcmil copper in the over 350 through 600 kcmil row: 1/0 AWG copper GEC to the water pipe.
3. The conductor serving only the rods need not exceed 6 AWG copper (250.66(A)).
4. Table 250.102(C)(1) with the same 500 kcmil gives a 1/0 AWG copper main bonding jumper.

Example 2: EGC for a feeder. A feeder is protected by a 150 A breaker. There is no 150 A row in Table 250.122, so use the 200 A row: 6 AWG copper.

Example 3: upsized conductors. A 200 A feeder would normally use 3/0 AWG copper (167,800 circular mils) with a 6 AWG copper EGC (26,240 circular mils). The conductors are increased to 250 kcmil for voltage drop.

39,094 CM is more than 6 AWG, so the EGC goes to 4 AWG copper (41,740 CM).

Common exam mistakes

• Sizing the GEC from the service rating (such as 200 A) instead of the service conductor size.
• Using Table 250.66 for an EGC, or Table 250.122 for a GEC or main bonding jumper.
• Forgetting to add up parallel sets before entering Table 250.66 or Table 250.102(C)(1).
• Rounding down in Table 250.122 when the device rating falls between rows.
• Missing the 6 AWG cap for rods and the 4 AWG cap for concrete-encased electrodes.
• Bonding the neutral to the enclosure in a subpanel.

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