Ground Elect System NEC

THE GROUNDING ELECTRODE SYSTEM

Prepared by

The Southwestern SectionInternational Association of Electrical Inspectors

1999 Grounding and Bonding CommitteeWilliam B. Brownell, P.E., Chairman

Based on the 1999 National Electrical Code

This document is intended to illustrate typical grounding electrode systems but does not cover all acceptable installations.

The Southwestern Sectionof the

The Purpose of the I.A.E.I.

The objective of the I.A.E.I. shall be:

• To cooperate in the formulation of standards for the safe installation and use of electrical materials, devices, and appliances.

• To promote the uniform understanding and application of the National Electric Code and other electrical codes.

• To promote cooperation between inspectors, the electrical industry, and the public.

• To collect and disseminate information relative to the safe use of electricity.

• To represent the electrical inspectors in all matters which are dealt with nationally and internationally by the electric industry.

• To cooperate with other national and international organizations in furthering the development of the electric industry.

Scott Davis Timothy Owens

Bill Dickey Jim Palma

Doug Effenberger, P.E. Chuck Leisher

Jesus Mosqueda Supot Ying

Iraj Nasrolahi, P.E.

Incorporated1928

Keystone

of the

Electrical

Industry

OF

Prepared by EDBL Publications Southern California Edison, 626.237.06464930 N. Earle Ave., Rosemead, CA 91770ges.fm, 991013.1143

THE PURPOSE OF SERVICE GROUNDING

• The purpose of grounding specific conductors of systems are to:

• Limit voltages due to lightning.

• Limit voltages due to line surges.

• Limit voltages due to unintentional contact with higher voltage lines.

• Stabilize the voltage to ground during normal operation.

• Establish a low impedance path for proper overcurrent device operation.

• Limit voltage to ground on conductive materials enclosing electrical equipment.

250-20: REQUIREMENTS FOR GROUNDING ALTERNATING–CURRENT SYSTEMS OF 50 VOLTS TO 1,000 VOLTS

AC systems supplying premises wiring and premises wiring systems shall be grounded under any of the following con-ditions.

The neutrals must be grounded because the maximum voltage to ground from any ungrounded conductor does not exceed 150 volts.

Where the system is three phase, four wire, wye connected in which the neutral is used as a circuit conductor.

Where the system is three phase, four wire, delta connected in which the midpoint on one phase winding is used as a cir-cuit conductor.

CAUTION: There exists a “high leg” (208V) voltage from the B phase to ground. The B phase is only to be used in 240V 3 phase or 240V single phase applications.

120V120V

120V

208Y/120V, 3Ø 4W

120V

120V120V

120/240V, 1Ø 3W120V, 1Ø 2W

(b) (1)

120V120V

120V 277V

277V277V

208Y/120V, 3Ø 4W 480Y/277V, 3Ø 4W

(b) (2)

208V240V

120V

120V

B

A C

240V

120/240V, 3Ø 4W(b) (3)

IAEI Southwestern Section –1–Southern California Chapter 1999 NECges.fm, 991013.1143

PREMISES WIRING (SYSTEM): That interior and exterior wiring, including power, lighting, control, and signal circuit wir-ing together with all of their associated hardware, fittings, and wiring devices, both permanently and temporarily installed, that extends from the service point of utility conductors or source of power such as a battery, a solar photovoltaic system, or a generator, transformer, or converter windings, to the outlet(s). Such wiring does not include wiring internal to appli-ances, fixtures, motors, controllers, motor control centers, and similar equipment.

SERVICE: The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.

SERVICE POINT: The point of connection between the facilities of the utility and the premises wiring.

SERVICE EQUIPMENT: The necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s) and their accessories, connected to the load end of service conductors to a building or other structure, or an otherwise desig-nated area, and intended to constitute the main control and cutoff of the supply.

Note: Ungrounded conductors are not shown in illustrations.

250–24: GROUNDING SERVICE — SUPPLIED ALTERNATING–CURRENT SYSTEMS

Premises wiring systems that are grounded shall have at each service a grounding electrode conductor connected to a grounding electrode. The grounding electrode conductor shall be connected to the grounded service conductor in one of the following ways.

250-24 (a) 1: The grounding electrode conductor shall be connected to the grounded service conductor at any accessible point from the load end of the service conductors to the busbar for terminating the grounded service conduc-tor.

250-24 (a) 4: If there is a main bonding jumper (250-28) installed between the neutral bus and the ground bus, the grounding electrode conductor is permitted to be connected to the ground bus.

Bus insulated from enclosure

Bus bonded to enclosure

Service Lateral

Supply End

Grounded ServiceConductor (Neutral)

InsulatedNeutral

Bus

Service Equipmentwith Bonding Screw

Bonding Screw(Green) or Jumper

Grounding ElectrodeConductor

MainBondingJumper

N

G

N

G

Service Equipmentwith Main Bonding Jumper

InsulatedNeutral

Bus

EquipmentGroundingBus

250-24 (a) 1 250-24 (a) 4

OR

Service Lateral

–2– IAEI Southwestern Section

1999 NEC Southern California Chapterges.fm, 991013.1143

250-24 (a) 2: Where the transformer supplying the service is located outside the building, at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building.

250-24 (a) 5: A grounding connection shall not be made to any grounded circuit conductor on the load side of the ser-vice disconnecting means.

250-24 (c)

Grounded ServiceConductor (Neutral)

GroundingElectrode

AdditionalGrounding Electrode

N

G

ServiceSub-Panel Insulated Neutral

Bus (not grounded)N

G

N

G

N

Grounding Electrode

ServiceEquipment

Main Disconnect

The grounded conductor (neutral) must beconnected to a grounding electrode

Equipment Grounding Conductor

Grounded Conductor (Neutral)

Grounding electrode conductors: This Conductor shall be used to connect the: • Equipment grounding conductor • Service Equipment enclosure • Grounded conductor (neutral) To the grounding electrode or grounding electrode system.

G

NBondingScrew


250-32 (a): TWO OR MORE BUILDINGS OR STRUCTURES SUPPLIED FROM A COMMON SERVICE

When a common service serves more than one building, the grounded system in each building shall have a grounding elec-trode bonded to each building’s service equipment enclosure.

250-32 (a) Exception: If the second building has only one branch circuit, and the branch circuit includes an equipment grounding conductor for grounding the noncurrent-carrying parts of all equipment, then the second building is not required to have a grounding electrode.

250-32 (b) 1. EQUIPMENT GROUNDING CONDUCTOR

For a grounded system, an equipment grounding conductor per 250-118 shall be run with the supply conductors and con-nected to the grounding electrode and equipment. The grounded conductor (neutral) shall not be connected to the equip-ment grounding conductor. This method must be used when ground fault protection is installed on the common ac service, or when there is a common metallic path between the buildings, such as piping, fire sprinklers, or water lines.

InsulatedNeutral

Bus

Building 1

G

N

G

N

G

N N

G

Building 2

One BranchCircuit Only

GroundingElectrode

Common Service

Building 1

InsulatedNeutral

BUS N

G

N

G

N

G

N

G

Building 2 Building 3



TWO OR MORE BUILDINGS OR STRUCTURES SUPPLIED FROM A COMMON SERVICE

250-32 (b) 2. GROUNDED CONDUCTOR

If no equipment grounding conductor is run with the supply to the building, and there is no metallic path to ground, and no ground fault protection is installed on the supply, then the grounded circuit conductor (neutral) shall be connected to the disconnecting means and the grounding electrode. The neutral shall be used for grounding or bonding.

250-32 (d): Disconnecting Means Located in Separate Building or Structure

For buildings served with a remote disconnecting means, an equipment grounding conductor must be run with the circuit conductors. The bonding of the equipment grounding conductor to the grounding electrode must be made in an enclo-sure located immediately inside or outside the building.

Building 1

N

N N N

N

Building 2 Building 3

`

RemoteDisconnect

N

G

Building

EnclosureImmediately

Inside or OutsideBuilding

RemoteDisconnect

G

N

G

N

Building

EnclosureImmediately

Inside or OutsideBuilding

N

G

InsulatedNeutral

BUS

InsulatedNeutral

BUS

G

N

N

BondingScrew

BondingScrew


250-32 (e): Where livestock are housed, underground equipment grounding conductors shall be insulated. Dairy ani-mals are sensitive to as little as one volt and will cease milk production if exposed.

250-50 GROUNDING ELECTRODE/GROUNDING ELECTRODE SYSTEM

If available on the premises, each grounding electrode listed below and any made electrodes in accordance with 250-52 shall be bonded together to form the grounding electrode system.

1. A metal underground water pipe in contact with the earth for no less than ten feet.2. The metal frame of a building where effectively grounded.3. A no. 4 (1/2”) bare or galvanized rebar or a No. 4 AWG (minimum) bare copper conductor encased in no less

than two inches of concrete. The electrode must be located near the bottom of the footing. This is also called the Ufer system.

4. A No. 2 AWG (minimum) bare copper ground ring encircling the entire building. It must never be less than 20 feet in length.

5. Any made electrodes.

Insulated or Covered CopperEquipment Grounding Conductor

Enclosure

G

Grounding Electrode

Rebar orNo. 4 AWG bare copper

1. 2. 3. 4.



If available, all the electrodes specified in 250-50 and any made electrodes per 250-52 (c) & (d) shall be bonded together to form the grounding electrode system. Interior metal water piping located more than 5 feet from the point of entrance to the building shall not be used as part of the grounding electrode system or as a conductor to interconnect electrodes that are part of the grounding electrode system.

250-50: All the grounding electrode conductors shown above are sole connections to the service equipment, therefore the exceptions to sizing apply. Minimum sizes for copper grounding electrode conductors are:

No. 8 through 3/0. Use Table 250-66 for sizing.

No. 8 through 3/0. Use Table 250-66 for sizing.

No. 8 through No. 4 Use Table 250-66 for sizing. It is never required to be larger than No. 4 regardless of the size of the service conductors, 250-66 (b).

The sole connections never have to be larger than the ground rings, 250-66 (c).

At least No.2 bare copper conductor. It must encircle the building and never be less than 20 feet. 250-50 (d)

At least 20 feet of No.4 rebar (½") or No.4 (minimum) AWG bare copper conductor. Rebar or conductor must be encased with no less than 2" of concrete. Epoxy coated rebar is not acceptable as electrode. 250-50( c)

Metal frame must be effectively grounded. 250-50 (b)

Connection must be made within 5 feet of point of entrance of piping.250-50, -(a)

Metal UndergroundWater Pipe

Ground Ring

Rebar

1

2 3

4

10 feetor more

1

2

3

4


250-50: In the following illustration, only (2) is the grounding electrode conductor sized per Table 250-66. Both (1) and (3) are bonding conductors, per 250-50, sized per Table 250-66. (4) is a sole connection to the ground ring, sized per 250-66 (c).

250-50 (a) 2:

ServiceEquipment

MetalWater Pipe

Metal Frame

Rebar

Ground Ring

2

31

4

Service Equipment


If ten feet of metal underground water pipe is available, it must be used as a grounding electrode; however, it must always be supplemented by an additional electrode.

Supplemental Electrode


Service Equipment

Supplemental Electrodes

Note: Water pipe grounding electrode connection must be within 5 feet of point of entrance of water to building, 250-50 (a).

or



250-50 (c)

250-50:

If both are available, the steel frame of the building and a concrete encased rebar must both be used to form the ground-ing electrode system.

The metal frame of the building must be effectively grounded, 250-50 (b).

The connection of the grounding electrode conductor to the steel frame may be by exothermic welding, 250-70.

The grounding electrode conductor between the service equipment and any convenient part of the grounding electrode system (to the first electrode) must be installed in one continuous length without a splice unless spliced by means of listed irreversible compression connectors or exothermic welding, 250-64 (c). It must always be suitably protected against corro-sion, 250-62.

250-50:

• The steel frame of a building may be used as a sole electrode where effectively grounded, 250-50 (b).

• The concrete encased electrode (Ufer) may be used as the sole electrode, 250-50 (c).

• The ground ring may be used as the sole electrode, 250-50 (d).

• A metal underground water pipe shall not be used as a sole electrode, 250-50 (a) 2.

Service Equipment

If the rebar is available, it must be used as an electrode.

The connection to the rebar does not have to be accessible if encased 250-68 (a) Ex.

ServiceEquipment

Rebar No. 4or larger(½" diameter)


250-52: MADE AND OTHER ELECTRODES

Where none of the electrodes specified in section 250-50 is available, one or more of the electrodes specified in (b) through (d) shall be used. The most commonly used electrode is the ground rod.

Flush withgrade

Eight footground rod

Where the grounding electrode conductor is con-nected to made electrodes permitted in section 250-52(c) or (d), the size shall not be required to be larger than No. 6 copper wire. 250-66 (a) It must be a sole connection under this condition.

The connection device must be listed for direct soil burial (250-70). It does not have to be accessible (250-68 (a) Exception).

An eight foot ground rod must be flush or below grade because the NEC requires at least eight feet to be in contact with the soil (250-52 (c) (3)).

A standard ten foot ground rod can be used, with no less than eight feet in contact with the soil. Any rod projecting above the surface must be protected against physical damage (250-52 (c) 3, 250-10).

A single electrode consisting of a rod, pipe, or plate which does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified in section 250-50 or 250-52. Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall be not less than six feet (1.83m) apart.

Resistance tests are not required on the supplemental electrode.



250-52 (c) 1, 2, and 3: ROD AND PIPE ELECTRODES

Where rock bottom is encountered, the electrode shall be driven at an oblique angle not to exceed 45 degrees from the vertical.

250-52 (d) PLATE ELECTRODES

The NEC does permit made plate electrodes providing they expose no less than two square feet of surface to exterior soil. The wording of “exterior” requires that they be buried outside of the building not under the building.

Grounding ElectrodeConductor

Rod

45°

Rod

be buried in a trenchnot less than two feetsix inches deep

2'-6"-or-

Corrosionresistantconnection

Exothermic Weld Connection

Plate electrode shall be installednot less than 2½ ft. (762mm) belowthe surface of the earth. No less than 12" x 12". This

dimension will provide onesquare foot from each of thetwo surfaces.

Minimum thickness: Iron or steel¼", nonferrous metal 0.06 inch.

Aluminum electrodes shall not bepermitted, 250-52 (e).


250-64 (d):

250-64 INSTALLATION AND PROTECTION REQUIREMENTS FOR THE GROUNDING ELECTRODE CONDUCTORS

The Grounding Electrode Conductormay be tapped for multiple serviceequipment enclosures.

MAINMAIN

300 kcmilPhaseConductors

Meter sockets are not shown

3/0 No. 3

B C D

A

1,000 kcmilService Conductors

SteelFrame

MAIN

Size is 2/0 The grounding electrode conductor is based on Table 250-66 for 1,000 kcmil ser-vice-entrance conductors.

Size is No. 2 The grounding electrode conductor tap is based on Table 250-66 for 300 kcmil service conductors.

Size is No. 4 The grounding electrode conductor tap is based on Table 250-66 for 3/0 service con-ductors.

Size is No. 8 The grounding electrode conductor tap is based on Table 250-66 for No. 3 service

A

B

C

D

Grounding Electrode Conductorsized No. 4 or larger shall beprotected if exposed to severephysical damage.

A No.6 grounding electrodeconductor that is free from physicaldamage does not require racewayprotection. It must be securelyfastened to the building.

A No.8 conductor shall be protectedby a raceway or cable armor. Any metallic raceway or cable armor mustbe bonded at both ends.

250-64 (b)

250-64 (b)



Ground Elect System NEC

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