Commercial Electrical 100%

2023 NEC Article 250

Lesson 7: Grounding and Bonding

NEC 250.24, 250.50 to 250.68, 250.104, Tables 250.66 and 250.122 · 14 practice questions

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Grounding and bonding are two different jobs that get mixed up all the time. Grounding ties the electrical system to the earth, which steadies its voltage and gives lightning and surge energy a place to go. Bonding joins the metal parts together, so that when a hot conductor touches a metal enclosure the fault current has an easy path back to the source and the breaker opens fast. Both live in Article 250 of the 2023 NEC. This lesson follows the path from the service to the electrodes and then out through the building. Local adoption governs which edition is enforced (Lesson 1).

Service grounding and bonding for a 400 amp 208Y/120 volt service A 400 amp service disconnect is fed by one 600 kcmil copper conductor per phase. Inside the enclosure the neutral bar is tied to the enclosure by a 1/0 copper main bonding jumper. A grounding electrode conductor leaves the neutral bar and reaches four targets. A jumper of 1/0 copper bonds the exposed structural steel frame, and another 1/0 copper conductor bonds the metal water pipe within 5 feet of where it enters. A ground rod needs no more than 6 AWG copper and a concrete-encased footing electrode of 20 feet of half inch rebar needs no more than 4 AWG copper. An intersystem bonding termination with three or more terminals sits outside the enclosure. Service grounding and bonding, 400 A, 208Y/120 V (2023 NEC Article 250) Service conductors 600 kcmil Cu per phase Neutral: 1/0 Cu minimum Table 250.66 and Table 250.102(C)(1): over 350 through 600 = 1/0 Cu 3 phases and neutral in Service disconnect enclosure Neutral bar Main bonding jumper 1/0 Cu, 250.28(D) Grounding electrode conductor lands here (250.24(A)) Intersystem bonding termination (IBT) Outside the enclosure, 3 or more terminals 250.94(A) 1/0 Cu 1/0 Cu 6 Cu max 4 Cu max Structural steel frame Exposed frame, likely to become energized: bond it Jumper 1/0 Cu (3/0 cap) 250.104(C) Metal water pipe 10 ft or more in earth Clamp within 5 ft of entry Jumper 1/0 Cu (3/0 cap) 250.104(A), 250.68(C)(1) Ground rod 5/8 in. x 8 ft minimum Needs a helper unless 25 ohms GEC 6 Cu is enough 250.66(A), 250.53(A)(2) Footing rebar (Ufer) 20 ft of 1/2 in. rebar in footing, 2 in. of concrete GEC 4 Cu is enough 250.66(B), 250.52(A)(3) All electrodes present are bonded into one system (250.50). Each conductor is sized by what it lands on.
A 400 A, 208Y/120 V service with one 600 kcmil copper conductor per phase. The full 1/0 copper size applies to the main bonding jumper and to the water pipe and steel bonds, while a ground rod or footing electrode alone never needs more than 6 or 4 AWG copper.

One connection at the service

On a grounded system such as 208Y/120 V or 480Y/277 V, the grounding electrode conductor (GEC) lands on the grounded conductor at the service. The connection can be made at any accessible point from the load end of the service conductors to the neutral terminal at the service disconnect (250.24(A)(1)). The neutral is run with the phase conductors to each service disconnect (250.24(D)). Inside that enclosure an unspliced main bonding jumper ties the neutral to the enclosure and to the equipment grounding conductors (250.24(C)).

That jumper is the one intentional neutral-to-ground bond. Past the service disconnect the grounded conductor may not be connected to equipment enclosures or to equipment grounding conductors, and it may not be grounded again, except where Article 250 allows it (250.24(B)). Separately derived systems (250.30) and separate buildings (250.32) have their own rules. A neutral bonded to the case of a downstream panel puts normal neutral current on conduit, cable armor and building steel, so it is one of the most common defects found in commercial rehab.

The main bonding jumper

The main bonding jumper can be a wire, a bus or a screw, made of copper, aluminum, copper-clad aluminum or another corrosion-resistant material (250.28(A)). A screw alone must have a green finish that stays visible after installation (250.28(B)). Its size comes from Table 250.102(C)(1), based on the largest ungrounded service conductor, or the equivalent area where the phases are paralleled (250.28(D)(1)). If the service has more than one enclosure, each jumper is sized from the conductors serving that enclosure (250.28(D)(2)). Above 1,100 kcmil of copper the table note switches to 12.5 percent of the ungrounded conductor area.

The grounding electrode system

Every electrode that exists at the building must be bonded together into one grounding electrode system. If none exist, one or more of the types in 250.52(A)(4) through (A)(8) must be installed (250.50). The ones you will meet are these.

  • Concrete-encased electrode: at least 20 ft of bare or coated rebar, 1/2 in. or larger, in one length or tied together into 20 ft, or 20 ft of bare copper 4 AWG or larger, with at least 2 in. of concrete around it, in a footing or foundation in earth contact (250.52(A)(3)).
  • Metal underground water pipe with 10 ft or more in earth contact (250.52(A)(1)), and metal in-ground support structures such as pilings, also 10 ft in the earth vertically (250.52(A)(2)).
  • Ground ring: 20 ft or more of bare copper, 2 AWG or larger, around the building (250.52(A)(4)).
  • Rod or pipe: at least 8 ft long, rods at least 5/8 in. in diameter unless listed, pipe at least trade size 3/4 (250.52(A)(5)).

Some limits matter. Water pipe must be supplemented by another electrode, and its continuity cannot depend on a water meter or filter (250.53(D)). A single rod, pipe or plate must also be supplemented, unless it tests 25 ohms or less (250.53(A)(2)); multiple rods stay at least 6 ft apart (250.53(A)(3)), and so do rods of different grounding systems (250.53(B)). Rebar may not serve as the conductor between electrodes (250.53(C)). Metal underground gas piping and aluminum are never electrodes (250.52(B)). In an existing building, footing rebar that cannot be reached without breaking concrete need not be added (250.50).

Sizing the grounding electrode conductor

Table 250.66 sizes the GEC from the largest ungrounded service conductor (or the equivalent area of parallel sets). Copper rows are below; aluminum conductors use a larger GEC in each row, so look those up.

Largest ungrounded conductor (copper)Copper GEC
2 AWG or smaller8 AWG
1 AWG or 1/06 AWG
2/0 or 3/04 AWG
Over 3/0 through 350 kcmil2 AWG
Over 350 through 600 kcmil1/0
Over 600 through 1,100 kcmil2/0

Three limits shorten the conductor when it goes to only one kind of electrode. To a rod, pipe or plate that does not continue on to other electrodes, no more than 6 AWG copper is needed (250.66(A)). To a concrete-encased electrode alone, no more than 4 AWG copper (250.66(B)). To a ground ring alone, no larger than the ring conductor itself (250.66(C)). The GEC runs in one unbroken length, with splices only by listed irreversible compression connectors or exothermic welding (250.64(C)), and a steel raceway around it must be bonded to it at both ends (250.64(E)(1)).

Worked example: a 400 A service

A small clinic has a 400 A, 208Y/120 V service with one 600 kcmil copper conductor per phase. The jobsite has footing rebar, a driven rod and metal water pipe.

  1. 600 kcmil falls in the "over 350 through 600" row, so Table 250.66 gives 1/0 copper.
  2. The main bonding jumper uses Table 250.102(C)(1), the same row, so it is 1/0 copper. The neutral is at least 1/0 copper as well, and it must also carry the load.
  3. The conductor to the water pipe and the jumper to exposed structural steel each come from Table 250.102(C)(1), 1/0 copper, and neither is ever required larger than 3/0 copper (250.104(A), 250.104(C)).
  4. A separate conductor to the driven rod alone is 6 AWG copper. One to the footing rebar alone is 4 AWG copper.
  5. The rod and the water pipe are each supplemented by the footing electrode, and everything is bonded together.

Bonding water pipe, gas pipe and building steel

Metal water piping in or attached to the building is bonded to the service enclosure, the grounded conductor at the service, the GEC (if it is large enough) or an electrode used (250.104(A)(1)). The jumper is accessible at its attachment, and it follows Table 250.102(C)(1) with a 3/0 copper ceiling. Interior water pipe can extend the connection to an electrode only within 5 ft of the point of entry, measured along the pipe (250.68(C)(1)). A qualified-persons exception exists for industrial, commercial and institutional buildings, so read the section before you write a correction.

Gas piping and other metal piping likely to become energized are bonded (250.104(B)) with a conductor sized from Table 250.122 using the rating of the circuit that could energize the piping. Exposed structural steel that forms a building frame, is not intentionally grounded, and could become energized is bonded to the service enclosure, the grounded conductor, the GEC or an electrode (250.104(C)).

Bonding the service equipment and the telecom systems

Service raceways, meter fittings and enclosures that hold service conductors are bonded together (250.92(A)). Where a raceway enters through an oversized or concentric knockout or a reducing washer, a bonding jumper goes around the weak connection, and standard locknuts or bushings cannot be the only means of bonding (250.92(B)). Telephone, cable and antenna grounds land on an intersystem bonding termination outside the enclosure at the service or meter, with an accessible set of at least three terminals (250.94(A)).

Equipment grounding conductors

Table 250.122 sizes a wire-type equipment grounding conductor (EGC) from the rating of the overcurrent device ahead of the circuit, not from the size of the wires.

Overcurrent device (A)Copper EGC
1514 AWG
2012 AWG
6010 AWG
1008 AWG
2006 AWG
4003 AWG
8001/0

The EGC never has to be larger than the circuit conductors (250.122(A)). If the ungrounded conductors are made bigger for a reason other than the temperature and adjustment rules of 310.15(B) or (C), such as voltage drop, a wire-type EGC grows in proportion (250.122(B)). One EGC can serve several circuits in the same raceway, sized for the largest device (250.122(C)). Steel conduit and tubing (RMC, IMC and EMT) are recognized equipment grounding conductors (250.118), wire EGCs are needed mainly in nonmetallic raceways such as PVC and where the design calls for a separate conductor.

Key numbers
  • Neutral is bonded to the enclosure only at the service main bonding jumper (250.24(B), (C))
  • Copper GEC and main bonding jumper: 1/0 for 350 to 600 kcmil, 2/0 for over 600 through 1,100 (Table 250.66, Table 250.102(C)(1))
  • GEC to a rod alone: 6 AWG copper at most; to footing rebar alone: 4 AWG copper at most (250.66(A), (B))
  • Concrete-encased electrode: 20 ft of 1/2 in. rebar or 4 AWG bare copper, 2 in. of concrete (250.52(A)(3))
  • Water pipe electrode: 10 ft in earth, supplemented; rod: 8 ft long, 5/8 in. minimum, supplemented unless 25 ohms or less
  • Water pipe and steel bonding jumpers: Table 250.102(C)(1), never required larger than 3/0 copper (250.104)
  • Copper EGC by breaker: 20 A = 12, 60 A = 10, 100 A = 8, 200 A = 6, 400 A = 3 (Table 250.122)

In the field

A GCHS rehab replaces the slab and footings under a small clinic addition. Before the pour, the inspector checks that the footing rebar runs at least 20 ft in one length or is tied into 20 ft, and that a lug or a pigtail stays above the slab for the GEC. After the pour that electrode cannot be added cheaply. At the service, look for the neutral-to-enclosure jumper in the main and none in any downstream panel, a GEC with no splices, water pipe bonded near the entry, and a bonding jumper around any oversized knockout.

Practice questions

1. A Group B clinic has a 400 A, 208Y/120 V service with one 600 kcmil copper conductor per phase. The GEC will run to a metal water pipe electrode and to the building steel. What is the smallest copper GEC under Table 250.66?

  1. 2 AWG copper, because 600 kcmil is close to 350 kcmil
  2. 1/0 AWG copper
  3. 2/0 AWG copper, because the 600 kcmil conductor is at the top of its row
  4. 3/0 AWG copper, the largest size in the table

2. A new Group B office building has a service with 500 kcmil copper phase conductors. One GEC runs only to a driven 5/8 in. by 8 ft rod, and a separate conductor runs to the footing electrode that supplements it. What is the largest copper size the inspector can require for the conductor that lands on the rod only?

  1. 1/0 AWG copper, from the 500 kcmil row of Table 250.66
  2. 4 AWG copper, because rods are treated like concrete-encased electrodes
  3. 8 AWG copper, because rods only need the smallest size in the table
  4. 6 AWG copper

3. During a footing inspection at a new Group B building, the inspector finds two 12 ft lengths of 1/2 in. rebar in the bottom of the footing, joined by steel tie wire, with 3 in. of concrete cover planned. The footing is in direct contact with the earth. Does this qualify as a concrete-encased electrode under 250.52(A)(3)?

  1. Yes, tied together the rebar makes 24 ft, at least 20 ft, and has the required concrete around it
  2. No, the 20 ft of rebar must be one continuous piece, and separate lengths joined with tie wire do not count
  3. No, the rebar must be at least 5/8 in. in diameter to serve as an electrode
  4. No, only bare copper conductor may be used in a footing

4. A small Group M shop has a single 5/8 in. by 8 ft ground rod as its only grounding electrode. It tested at 40 ohms to earth. What does 250.53(A)(2) require?

  1. Nothing more, because one 8 ft rod is a complete electrode
  2. A second rod, but only if placed at least 25 ft from the first
  3. A supplemental electrode of a permitted type, such as a second rod at least 6 ft away
  4. A bond to the metal underground gas piping as the supplement

5. An installer plans to land the GEC on interior copper water piping that runs 14 ft from where the underground pipe enters a small commercial building, to reach the water pipe electrode. Under 250.68(C)(1), where may interior water piping be used to extend the connection to the electrode in an ordinary building?

  1. Anywhere in the building where the pipe is metal and continuous
  2. Within 10 ft of the water meter, on the street side
  3. Within 14 ft of the entry, the same length as the underground run
  4. Within 5 ft of the point of entry, measured along the pipe

6. A Group B building is served by 4/0 AWG copper ungrounded service conductors, one per phase. What is the minimum copper bonding jumper to the metal water piping under 250.104(A) and Table 250.102(C)(1)?

  1. 4 AWG copper, the row for 2/0 and 3/0 conductors
  2. 2 AWG copper
  3. 1/0 AWG copper, the row for over 350 through 600 kcmil
  4. 6 AWG copper, the row for 1 AWG and 1/0

7. A large warehouse in Group S-1 has an exposed steel building frame that is not intentionally grounded. The service uses parallel 750 kcmil copper conductors in each of two raceways for a combined 1,500 kcmil per phase. What is the largest copper jumper the code requires to bond the frame?

  1. 3/0 AWG copper, the ceiling in 250.104(C)
  2. 4/0 AWG copper, 12.5 percent of 1,500 kcmil
  3. 250 kcmil copper, matching the ungrounded conductors
  4. 2/0 AWG copper, the top row of Table 250.66

8. A 400 A feeder breaker in a Group B office supplies a panelboard through copper conductors in PVC conduit. The PVC is not a grounding path, so a wire-type EGC is run. What is the smallest copper EGC in Table 250.122?

  1. 1 AWG copper, the size for a 600 A device
  2. 4 AWG copper, the size for a 300 A device
  3. 3 AWG copper
  4. 6 AWG copper, the size for a 200 A device

9. A 60 A circuit in EMT at a Group B shop uses 4 AWG copper conductors, up from 6 AWG only to satisfy the adjustment factors in 310.15(C)(1). A wire-type EGC is installed. What size EGC is required?

  1. 4 AWG copper, increased in proportion to the larger conductors
  2. 8 AWG copper, one size above the Table 250.122 size
  3. 6 AWG copper, the size of the original conductors
  4. 10 AWG copper, the Table 250.122 size for a 60 A device

10. At a Group B strip center, an inspector finds the neutral bar in a 200 A sub-panel bonded to the panel's steel can by the factory green screw. The panel is fed from the service main through a separate feeder with an EGC. What is the correct action?

  1. Remove the bonding screw and land the EGC on a separate ground bar, since the neutral is bonded only at the service
  2. Leave it, because a bond in every panel is required for fault current
  3. Leave it, but add a second GEC to the sub-panel's enclosure
  4. Replace the screw with a copper bonding strap of 3/0 AWG

11. A service at a Group B strip center has two disconnects in separate enclosures, as 230.71(B) allows. Enclosure A is served by 250 kcmil copper conductors and enclosure B by 3/0 copper conductors. What is the smallest copper main bonding jumper for enclosure B?

  1. 2 AWG copper, sized from the 250 kcmil conductors in enclosure A
  2. 6 AWG copper, the smallest size in the table
  3. 4 AWG copper
  4. 1/0 AWG copper, sized from the combined service

12. An inspector arrives at a new Group B clinic and finds a service raceway entering the meter enclosure through an oversized concentric knockout, held with a standard locknut and bushing on each side. What does 250.92(B) call for?

  1. Nothing more, since the locknut and bushing count as the bond
  2. A bonding jumper around the impaired connection, since locknuts and bushings alone are not enough
  3. A larger locknut and a fiber bushing on each side
  4. A GEC run from the meter enclosure to the water pipe

13. A new Group A church is installing a ring of bare copper around its foundation as the building's grounding electrode. To meet 250.52(A)(4) what is required, and how large need the GEC be if it lands on the ring only?

  1. At least 20 ft of bare 2 AWG or larger copper, and the GEC need be no larger than the ring conductor
  2. At least 10 ft of bare 4 AWG copper, and a GEC of 1/0 copper taken from the full Table 250.66 row
  3. At least 20 ft of bare 6 AWG copper, and a GEC of 4 AWG copper
  4. At least 8 ft of bare 2 AWG copper, and a GEC of 6 AWG copper

14. A new Group B clinic is being wired, and the plans list several possible grounding electrodes. Which one may NOT be used as a grounding electrode under 250.52(B)?

  1. A metal in-ground support structure with 10 ft of vertical earth contact
  2. The metal underground gas piping system
  3. A bare 2 AWG copper ring of 20 ft or more around the building
  4. A metal underground water pipe with 10 ft in earth contact, supplemented by another electrode

Glossary

Grounded conductor
The circuit conductor intentionally tied to earth, usually called the neutral on 208Y/120 V and 480Y/277 V systems.
Grounding electrode conductor (GEC)
The conductor that connects the grounded service conductor and equipment to the grounding electrode system; sized by Table 250.66.
Main bonding jumper
The unspliced connection between the grounded conductor and the service equipment enclosure and equipment grounding conductors.
Equipment grounding conductor (EGC)
The path that carries ground-fault current back to the source; a wire sized from Table 250.122, or a metal raceway such as EMT.
Concrete-encased electrode
At least 20 ft of half-inch rebar or 4 AWG bare copper set in a footing or foundation with 2 in. of concrete, in earth contact; often called a Ufer ground.
Grounding electrode system
All the electrodes at a building (water pipe, footing rebar, rods, ring) bonded together into one system.
Bonding jumper
A conductor that joins metal parts so they stay at the same potential and a fault can open the breaker.
Intersystem bonding termination
An accessible set of terminals outside the service enclosure where telephone, cable and antenna grounds are connected to the system.
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