NEC Article 250 — Sections 250.20 through 250.34

PART B. SYSTEM GROUNDING, Slide 159

250.20 Alternating-Current Circuits and Systems
System grounding is the intentional connection of one conductor of an alternating-current system to the earth so as to limit elevated voltage on conductors from high voltage surges imposed by lightning, line surges, or unintentional contact with higher voltage lines and to stabilize the phase-to-ground voltage during normal operation [250.4(A)(1)].

(A) AC Systems of Less Than 50 V. Alternating-current systems that operate at less than 50 V are not required to be grounded unless:
(1) The primary exceeds 150 V to ground
(2) The primary is ungrounded

(B) AC Systems Not Over 600 V. Alternating current systems of the following types must have the neutral (X0) terminal of the power supply bonded to a suitable grounding electrode (earth).
Single-phase, 2- or 3-wire, 120 V or 120/240 V system
3-phase, 4-wire, 208Y/120 V or 480Y/277V wye-connected system
3-phase, 4-wire, 120/240 V delta-connected system (high-leg)

Author’s Comment: Other power supply systems such as a corner-grounded delta-connected system are permitted to be grounded, see 250.26(4).

(D) Separately Derived Systems. Separately derived systems, which are required to be grounded by 250.20(A) or (B), must be grounded in accordance with the requirements of 250.30.

Author’s Comment: A separately derived system is a premises wiring system that derives its power from battery, solar photovoltaic system, a generator, transformer or converter winding, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system. See Article 100 definition for separately derived system.

FPN 1: A generator is not a separately derived system if the neutral is solidly interconnected to the service neutral (transfer switch does not open the neutral). Under this condition, a neutral-to-ground connection must not be made at the generator or at the generator disconnect.

FPN 2: For systems that are not separately derived, see 445.13 for minimum size of conductors that must carry fault current.

Author’s Comment: If the conductors from the generator terminate in a transfer switch that does not switch the grounded (neutral) conductor, then the grounded (neutral) conductor will be required to carry fault current back to the generator. Under this condition, the grounded (neutral) conductor for a generator must be sized no smaller than required for the unbalanced load by 220.22 and no smaller than required by 250.24(B). Figure 445-2 445-13 02.cdr

250.24. Grounding Services
(A) Grounding Neutral. Alternating-current services that are supplied from a grounded electrical system from the utility must have the grounded (neutral) conductor connected to a grounding electrode of the specified in 250.52 in accordance with Part III or Article 250 in accordance with (1) though (5). See 250.24(C).

Author’s Comment: Grounding the grounded (neutral) conductor from the electric utility to the earth helps in limiting the voltage imposed by lightning, line surges, or unintentional contact with higher voltage lines, by shunting the energy to the earth. In addition, this earth connection helps the utility to clear line-to-ground fault. See 250.4(A)(1).

(1) Accessible Location. A grounding electrode conductor shall connect the grounded (neutral) conductor to the grounding electrode at any accessible location, from the load end of the service drop or service-lateral, up to and including the service disconnecting means.

(4) Main Bonding Jumper. The grounding electrode conductor can terminate to the equipment grounding terminal to which the main bonding jumper is connected [250.28].

Author’s Comment: Some inspectors require the grounding electrode conductor to originate at the meter socket enclosure, and other inspectors require this connection in the service disconnect. Some inspectors require the grounding electrode connection to terminate only to the grounded (neutral) terminal and other inspectors only allow the connection to the equipment grounding terminal. However, the Code allows the connection to originate at either the meter socket enclosure or the service disconnect, and the termination can be to the grounded (neutral) terminal or to the equipment grounding terminal.

(5) Neutral-to-Ground Connections. A neutral-to-ground connection shall not be made on the load side of the service disconnecting means except as permitted for separately derived systems [250.30(A)(1)], separate buildings or structures [250.32(B)(2)], or meter enclosures [250.142(B) Ex. 2].

Author’s Comment: If a neutral-to-ground connection is made on the load side of service equipment, objectionable neutral current will flow on the conductive metal parts of the electrical equipment in violation of 250.6(A). Objectionable neutral current can create an electric shock, fire, as well as power quality problems. See 250.142 in this book for more information.

(B) Grounding - Effective Fault Current Path. Where the service supplied alternating current system is grounded at the utility transformer (typical installation), a grounded (neutral) conductor must be run from the electric utility to each service disconnecting means. The grounded (neutral) conductor from the electric utility must be bonded to each disconnecting means enclosure by the installation of a main bonding jumper (screw or strap), which is supplied by the equipment manufacturer [250.28].

Author’s Comment: Bonding of the service disconnecting means enclosure to the utility supplied grounded (neutral) conductor provides the low-impedance path necessary to prevent dangerous touch potential from a ground-fault (line-to-case fault). See 250.4(A)(3).

(1) Minimum Size Grounded (neutral) Conductor. The grounded (neutral) conductor from the electric utility must be sized so that it can safely carry the maximum ground-fault current likely to be imposed on it [110.10] from where a ground-fault may occur in accordance with Table 250.66, based on the total area of the largest ungrounded (hot) conductor. In addition, the grounded (neutral) conductors must have the capacity to carry the maximum unbalanced neutral current in accordance with 220.22.

Question. What is the minimum size service grounded (neutral) conductor, if the service ungrounded (hot) conductors are 500 kcmil and the maximum unbalanced load is 100 A?
(a) 3 AWG (b) 2 AWG (c) 1 AWG (d) 1/0 AWG

Answer. (d) 1/0 AWG
Step 1. Ensure the grounded (neutral) conductor has the capacity to carry fault current.
1/0 AWG, Table 250.66 based on 500 kcmil
Step 2. Ensure the grounded (neutral) conductor has the to carry the maximum unbalanced neutral current of 100 A. 3 AWG has an ampacity of 100 A, Table 310.16
Note: 250.24B1 (routing and sizing) states that the neutral conductor is not allowed to be smaller than the grounding electrode conductor at your service.

(2) Parallel Grounded Conductor. Where service-entrance conductors are installed in parallel, the size of the grounded (neutral) conductor in each raceway shall be based on the total area the largest ungrounded (hot) conductor in the raceway. However, 310.4 requires a minimum of 1/0 AWG when paralleling ungrounded (hot) and grounded (neutral) conductors.

(C) Grounding Electrode Conductor. A grounding electrode conductor shall be used to connect the equipment grounding conductor of the service, the service equipment, and the grounded (neutral) conductor to one of the electrodes as listed in Part III of this Article. The grounding electrode conductor is sized in accordance with 250.66, based on the total area of the largest ungrounded (hot) conductor.

250.28. Main Bonding Jumper
A main bonding jumper is required to bond the service-disconnect enclosure to the grounded (neutral) conductor within the enclosure for each service disconnect.

Author’s Comment: The main bonding jumper is supplied by the equipment manufacturer.

(A) Material. Main bonding jumpers can be constructed of a wire, bus, screw, conductor or other corrosion-resistant material.

(B) Construction. Where a main bonding jumper is a screw only, the screw shall be identified with a green finish that shall be visible with the screw installed.

Part II. Circuit and System Grounding

250.30. Grounding Separately Derived Systems
(A) Grounded Systems. Separately derived systems that operate at over 50 volts [250.20(A) and 250.112(I)] must be grounded to an effective fault current path to ensure that dangerous voltage, which can create a shock and/or fires from ground-faults, will not remain [250.2(A)(3)]. In addition, separately derived systems must be grounded to the earth to stabilizing the voltage during normal operation [250.2(A)(1)].

Author’s Comment: Elevated phase-to-ground voltage of an improperly grounded wye system is beyond the scope of this column, but if you want more information on this subject go to http://www.mikeholt.com/Newsletters/highvolt.htm.

Author’s Comment: A separately derived system is a premises wiring system that derives its power from battery, solar photovoltaic system, a generator, transformer or converter winding, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system. Transformers, except autotransformers, are always considered a separately derived system because the primary supply does not have any direct electrical connection to the secondary. Generator, converter winding, UPS systems, or power from solar photovoltaic system are only considered to be a separately derived system, when the grounded (neutral) conductor in the transfer switch is switched.

(1) Grounding - Effective Fault Current Path. To provide the low impedance path necessary to clear a ground-fault (line-to-ground fault), the metal parts of the separately derived system (equipment grounding conductor) must be bonded to the system grounded conductor (X0 Terminal). The bonding jumper used for this purpose must be sized in accordance with Table 250.66, based on the total area of the largest ungrounded (hot) conductor.

The neutral-to-case bond can be made at the source of a separately derived system or to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices.

DANGER: Failure to provide a low impedance ground-fault path (no neutral-to-ground bond) for the separately derived system can create a condition where a ground-fault (line-to-case fault) cannot be removed. The result is that all metal parts of the electrical system, as well as the building structure will remain energized with dangerous line voltage if a ground-fault (line-to-case fault) occurs.

CAUTION: The neutral-to-ground connection for a separately derived system cannot be made at more than one location. To do so would create multiple neutral-to-ground connections, which produces multiple neutral current return paths to the grounded (neutral) conductor of power supply, which can create a fire, shock hazard as well as power quality problems from electromagnetic interference. See 250.6 and 250.142(A).

(2) Grounding - To the Earth. A grounding electrode conductor, that connects the separately derived system grounded (neutral) conductor to a suitable grounding electrode [250.30(A)(4)], shall be installed in accordance with 250.30(A)(2)(a) or (b). Grounding electrode conductor taps as permitted by 250.30(A)(2)(b), shall be installed in accordance with the requirements contained in 250.30(A)(3).

Exception No. 2: The size of the bonding jumper for a system that supplies a Class 1 circuit from a transformer rated not more than 1000 volt-amperes, shall not be smaller than the derived phase conductors and shall not be smaller than 14 AWG copper.

(a) Single Separately Derived System. A grounding electrode conductor for a single separately derived system must be sized in accordance with 250.66, based on the total area of the largest ungrounded (hot) conductor. This conductor shall connect the grounded conductor of the derived system to the grounding electrode as specified in 250.30(A)(4). The grounding electrode conductor must terminate at the same point on the separately derived system where the neutral-to-case bonding jumper is installed [250.30(A)(1)].

Exception: A grounding electrode conductor is not required for a system that supplies a Class 1 circuit from a transformer rated not more than 1000 volt-amperes. However, the system grounded conductor must be bonded to the transformer frame or enclosure in accordance with 250.30(A)(1).

(b) Multiple Separately Derived Systems. Where more than one separately derived system is connected to a common grounding electrode conductor as provided in 250.30(A)(3), the common grounding electrode conductor shall be sized in accordance with Table 250.66 based on the total circular mil area of the derived phase conductor from all separately derived systems.

(3) Grounding Electrode Taps. Grounding electrode taps from a separately derived system to a common grounding electrode conductor must connect the grounded conductor of the separately derived system to the common grounding electrode conductor and must be installed in accordance with (a) through (d) below.
(a) Tap Conductor Size. Each tap conductor must be sized in accordance with 250.66 for the derived phase conductors of the separately derived system it serves.
(b) Connections. All connections must be made at an accessible location by an irreversible compression connector listed for the purpose, listed connections to copper busbars not less than 1/4 in. x 2 in., or by the exothermic welding process. Tap conductors must be connected to the common grounding electrode conductor as specified in 250.30(A)(2)(b) in such a manner that the common grounding electrode conductor is not spliced.
(c) Installation. The common grounding electrode conductor and the taps to each separately derived system must comply with 250.64(A), (B), (C) and (E).
(d) Bonding. Exposed structural steel that forms the building frame or the interior metal piping in the area served by the separately derived system must be bonded to the grounding electrode conductor in accordance with 250.104(A)(4).

(4) Grounding Electrode. The grounding electrode conductor must terminate to a grounding electrode that is located as close as practicable, and preferably in the same area of the neutral-to-ground termination of the derived system. The grounding electrode shall be the nearest one of the following:
(1) Effectively grounded metal member of the building structure.
(2) Effectively grounded metal water pipe, within 5 ft from the point of entrance into the building.

Exception: The grounding electrode conductor can terminate any point on the water pipe system for industrial and commercial buildings where, (1) conditions of maintenance and supervision ensure that only qualified persons service the installation, and (2) the entire length of the interior metal water pipe that is being used for the grounding electrode is exposed.

Where effectively grounded metal member of the building structure or effectively grounded metal water pipe is not available, then one of the following electrodes must be used:
The metal frame of the building or structure, where effectively grounded, see 250.52(A)(2).
An electrode encased by at least 2 in. of concrete, located within and near the bottom of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 20 ft of one or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than in. in diameter, or consisting of at least 20 ft of bare copper conductor not smaller than 4 AWG, see 250.52(A)(3).
A ground ring encircling the building or structure, in direct contact with the earth, consisting of at least 20 ft of bare copper conductor not smaller than 2 AWG, see 250.52(A)(4).
Rod and pipe electrodes not less than 8 ft in length, see 250.52(A)(5).
Plate electrode expose not less than 2 ft2 of surface to exterior soil, see 250.52(A)(6)
Other local metal underground systems or structures such as piping systems and underground tanks, see 250.52(A)(7).

FPN: Interior metal water piping in the area served by a separately derived system must be bonded to the grounded (neutral) conductor at the separately derived system in accordance with the requirements of 250.104(A)(4).

(5) Equipment Bonding Jumper Size. Where an equipment bonding jumper is run with the derived phase conductors from the source of a separately derived system to the first disconnecting means, it shall be sized in accordance with Table 250.66, based on the total area of the largest derived ungrounded (hot) conductors.

(6) Grounded (neutral) Conductor. Where a grounded (neutral) conductor is installed and the neutral-to-case bond is not located at the source of the separately derived system, the following must apply:
(a) Routing and Sizing. The grounded (neutral) conductor must be routed with the secondary conductors, and it cannot be smaller than the required grounding electrode conductor as specified in Table 250.66 based on the largest ungrounded supply conductor, but it is not required to be larger than the ungrounded derived phase conductor. For secondary phase conductors larger than 1100 kcmil copper or 1750 kcmil aluminum, the grounded (neutral) conductor is not permitted to be smaller than 121/2 percent of the area of the largest derived phase conductor.
(b) Parallel Conductors. If the secondary conductors are in parallel, the grounded (neutral) secondary conductor must be sized based on the total circular mil area of all of the parallel conductors per phase. Where installed in two or more raceways, the size of the grounded (neutral) conductor in each raceway must be,
based on the total area of the largest derived ungrounded (hot) conductors, but in no case smaller than 1/0 AWG are required by 310.4.

Author’s Comment: When the neutral-to-case bond is at the separately derived system, the NEC does not provide guidance on how to size the equipment grounding conductor between the separately derived system and the first system disconnecting means or overcurrent device. I suggest you size the equipment grounding conductor in accordance with Table 250.66.

250.32. Grounding Separate Buildings and Structures
(A) Grounding - To the Earth. Metal parts of the electrical system in separate buildings or structures supplied by a feeder, building or structure must be grounded to the earth to prevent the destruction of electrical components from superimposed voltage from line surges, unintentional contact with higher voltage lines, and voltage transients and to help prevent the build-up of static charges on equipment and material [250.4(A)(2).

Exception. A grounding electrode at a separate building or structure is not required where only one branch circuit (with an equipment bonding conductor) supplies the building or structure. Slide 209

DANGER - Failure to ground the metal parts of the electrical system to the earth can result in electric shock, fires and the destruction of expensive electronic equipment from lightning or high voltage line surges.

(B) Grounding - Effective Fault Current Path. To protect against electric shock from a ground-fault (line-to-case fault), dangerous voltage on metal parts of the electrical system must be removed in less than 1 second by opening the circuit overcurrent protection device. To accomplish this, the impedance of the fault current path must allow the ground-fault (line-to-case fault) current to raise to a value of at least 5 times and preferably 10 times the rating of the overcurrent protection device.

In order for the circuit overcurrent device to open and prevent dangerous voltage from remaining on metal parts of electrical equipment, noncurrent-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment must be bonded together and to the grounded (neutral) terminal at the electrical supply source in a manner that establishes an effective ground-fault current path.

(1) Equipment Grounding Conductor. An equipment grounding conductor [250.118], sized in accordance with 250.122, shall be installed with the feeder conductors to the separate building or structure to bonding equipment, structures, or frames .

Author’s Comment: The feeder equipment grounding conductor provided the low impedance path for fault current to ultimately return to the electrical supply source as required by 250.2(A)(3).

CAUTION: To prevent dangerous neutral current from flowing on the metal parts of the electrical system, in violation of 250.6(A), the grounded (neutral) conductor at the separate building or structure must not be bonded to either the equipment grounding conductor or to the grounding electrode system.

(2) Grounded (neutral) Conductor. Where (1) an equipment grounding conductor is not run with the supply to the building or structure, and (2) there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved, and (3) ground-fault protection of equipment has not been installed on the common ac service, the grounded (neutral) conductor run with the supply to the building or structure shall be connected to the building or structure disconnecting means and to the grounding electrode(s). The size of the grounded conductor shall not be smaller than the larger of:
(1) That required by 220.22 (maximum unbalanced neutral load), or
(2) That required by 250.122 (equipment grounding conductor size).

Author’s Comment: When an equipment grounding conductor is not run to a separate building or structure, the grounded (neutral) conductor must be used to provide the effective fault current path required to clear any ground-faults (line-to-case faults) in addition to carrying any unbalanced neutral current [250.4(A)(3)].

DANGER - The use of the grounded (neutral) conductor for equipment bonding is permitted by the NEC, but it is a dangerous practice and should not be done. For more information, see the dangers of an open neutral for service equipment in Section 250.24.

(E) Grounding Electrode Conductor. The grounding electrode conductor for a building or structure supplied by a feeder shall be sized in accordance with 250.66, based on the largest ungrounded (hot) feeder conductor. The grounding electrode conductor must be installed in accordance with the requirements contained in Part III of Article 250, specifically 250.62 through 250.70.

Author’s Comment: Where the grounding electrode conductor is connected to rod, pipe, or plate electrodes, that portion of the conductor that is the sole connection to the grounding electrode, it is not required to be larger than 6 AWG copper [250.66(A)]. Where the grounding electrode conductor is connected to a concrete-encased electrode, that portion of the conductor that is the sole connection to the grounding electrode is not required to be larger than 4 AWG copper [250.66(B)].

250.34 Generators-Portable and Vehicle-Mounted
(A) Portable Generators. The frame of a portable generator is not be required to be grounded to the earth if:
(1) The generator only supplies equipment or cord-and-plug-connected equipment through receptacles mounted on the generator, or both, and
(2) The metal parts of generator and the grounding terminals of the receptacles are bonded to the generator frame.

(B) Vehicle-Mounted Generators. The frame of a portable generator is not required to be grounded to the earth if:
(1) The generator frame is bonded to the vehicle frame.
(2) The generator only supplies equipment or cord-and-plug-connected equipment through receptacles mounted on the generator, or both, and
(3) The metal parts of generator and the grounding terminals of the receptacles are bonded to the generator frame.

FPN: Portable generators that supply fixed wiring systems must be grounded in accordance with 250.30 for separately derived systems if they supply a transfer switch that switches the neutral.

250.36. High-Impedance Grounded Neutral Systems
High-impedance 3-phase, 480 V grounded neutral systems insert a resistor or inductor in-line with the neutral-to-case bonding jumper to limit ground-fault current to a very low value, typically 7 A. These systems are only permitted where all of the following conditions are met:
(1) Conditions of maintenance and supervision ensure that only qualified persons will service the installation.
(2) Continuity of power is required.
(3) Ground detectors are installed on the system.
(4) Line-to-neutral loads are not served.

(A) Grounding Impedance Location. The grounding impedance shall be installed between the grounding electrode conductor and the system neutral derived from a grounding transformer.

(B) Bonding Conductor. The conductor from the neutral point of the transformer or generator to the grounding impedance shall be fully insulated and have an ampacity of not less than the maximum current rating of the grounding impedance. In no case shall the neutral conductor be smaller than No. 8 copper.

(C) System Neutral Connection. The system neutral shall not be connected to ground except through the grounding impedance.

FPN: The impedance is normally selected to limit the ground-fault current to a value slightly greater than or equal to the capacitive charging current of the system. This value of impedance will also limit transient overvoltage to safe values. For guidance, refer to criteria for limiting transient overvoltage in Recommended Practice for Grounding of Industrial and Commercial Power Systems, ANSI/IEEE 142-1991.

If you have any comments or suggestions on how I can improve this, please let me know. Mike@MikeHolt.com.

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