Technical Article

NEC 2023 Basics: Equipment Grounding Conductors

August 02, 2023 by Lorenzo Mari

Learn about equipment grounding and types of equipment grounding conductors.

Part VI of NEC’s Article 250 states the rules for equipment grounding and equipment grounding conductors. This part of the NEC lists the equipment grounding conductors’ permitted and not permitted types. The NEC does not allow grounding equipment directly to a grounding electrode. The correct way of grounding equipment is by employing an equipment grounding conductor.

Image used courtesy of Pixabay

Connect the exposed, normally non-current-carrying metal parts of equipment and enclosures likely to become energized to an equipment grounding conductor. Electrical equipment and enclosures must be at earth potential under normal conditions.

When a ground fault hits an ungrounded metal piece, the piece will remain energized, threatening people’s safety. If you are grounded and touch that equipment, a current will flow through your body. The fault current magnitude will probably not be enough to operate the circuit protective device, but it might be sufficient to harm you.

The equipment grounding conductor provides a secure, low-impedance current path to the ground, allowing the flow of a ground-fault current magnitude high enough to blow a fuse or trip a breaker, eliminating the hazard.

Figure 1 shows a single-phase, 3-wire overhead service to a building supplying a single-phase, 2-wire, 120 V load. The load is any power-consuming device connected to the wiring – like a motor, lamp, fridge, or washing machine. The building’s wiring starts at the service equipment.

The conduit connection to the ground shown in Figure 1 is accidental, not by design. This “grounding” occurs when attaching the conduit to a grounded support – technically, this conduit is ungrounded.

The resistance between the accidental ground and the grounding electrode system is high, and the consequence will be a low ground-fault current incapable of activating the overcurrent protective device – the conduit will stay energized at 120 V, posing a hazard to people.

Figure 1. A ground fault in an ungrounded conduit. Image used courtesy of Lorenzo Mari

The same situation arises when substituting the conduit in the figure with any ungrounded metal enclosure.

Figure 2 addresses the consequence of an insulation failure in an ungrounded metal enclosure. If the fuse does not melt, the ground-fault current will circulate through the person’s body, causing a shock and perhaps death.

Figure 2. Shock hazard when employing an ungrounded enclosure. Image used courtesy of Lorenzo Mari

Figure 3 shows an insulation failure with a metal enclosure grounded via a wire-type equipment grounding conductor. This time, there is an effective ground-fault current path, and the current magnitude is high enough to operate the overcurrent protective device – the enclosure, at ground potential, does not pose a shock hazard.

Figure 3. Metal enclosure grounded through an equipment grounding conductor. Image used courtesy of Lorenzo Mari

National Electrical Code Section 250.109 Metal Enclosures

This section permits using the metal enclosures to connect the bonding jumpers, equipment grounding conductors, or both – becoming part of an effective ground-fault current path.

To keep the effective ground-fault current path, metal covers, extension rings, plaster rings, and metal fittings must be attached to the metal enclosures or connected with equipment grounding conductors, bonding jumpers, or both.

Figure 4 shows metal enclosures and raceways employed as the equipment grounding conductor.

Figure 4. An effective ground-fault current path. Image used courtesy of Lorenzo Mari

National Electrical Code Section 250.110 Equipment Fastened in Place or Connected by Permanent Wiring Methods

This section sets forth the situations under which you must connect an equipment grounding conductor to the exposed non–current–carrying metal parts of fixed equipment supplied by or enclosing conductors or components that are likely to become energized.

Connect the equipment grounding conductor to the exposed non-current-carrying metal parts if:

Within 2.5 m vertically or 1.5 m horizontally of ground or grounded metal objects and subject to contact persons.
It is located in a wet location and not isolated.
In electrical contact with metal.
In a hazardous (classified) location.
It is supplied by a wiring method providing an equipment grounding conductor – except as permitted by Exception No. 2 to Section 250.86 for short segments of metal enclosures.
The equipment operates at over 150 V to the ground in any terminal.

Exception 1: Electrically-heated appliances' metal frame is permanently and effectively insulated from the ground.

Exception 2: Distribution apparatus mounted on wooden poles more than 2.5 m above ground or grade level.

Exception 3: Listed equipment protected by a double insulation system or its equivalent.

There is no need to ground exposed, non–current–carrying metal parts of fixed equipment that are not likely to become energized, like metal accessories or screws mounted on nonmetallic enclosures.

National Electrical Code Section 250.118 Types of Equipment Grounding Conductors

Section 250.118(A) Permitted

Generally, the NEC requires only one equipment grounding conductor per circuit, which can be any type specified in this section.

The equipment grounding conductor may run with or enclose the circuit conductors. It must be one or more than one or a combination of the following:

Copper, aluminum, or copper-clad aluminum conductors.
Rigid metal conduit (RMC).

Article 344 covers rigid metal conduit. See Figure 5.

Figure 5. Rigid metal conduit. Image used courtesy of Rexway

Intermediate metal conduit (IMC)

Article 342 covers intermediate metal conduit. See Figure 6.

Figure 6. Intermediate metal conduit. Image used courtesy of SG Industrias Electric

Electrical metallic tubing (EMT)

Article 358 covers electrical metallic tubing. See Figure 7.

Figure 7. Electrical metallic tubing. Image used courtesy of SG Industrias Electric

The NEC does not restrict the size and length of RMC, IMC, and EMT.

Listed flexible metal conduit (FMC), meeting six conditions:

◦ The conduit terminates in listed fittings.

◦ Maximum overcurrent protective device = 20 A.

◦ Maximum conduit size = 1 ¼“.

◦ Maximum combined length = 1.8 m.

An example of “combined length” is a series connection of 0.6 m of FMC plus 0.6 m of LFMC plus 0.6 m of FMT, all three in the same ground return path.

◦ Wire-type equipment grounding conductor or bonding jumper per 250.102(E)(2) required if flex is installed to reduce the vibration transmission from equipment or to provide flexibility for equipment requiring adjustments after installation.

An example is a short run to an electric motor. The FMC will isolate the motor vibrations and allow its periodic belt-tightening.

◦ Wire-type equipment grounding conductor or bonding jumper per 250.102(E)(2) required if flex is constructed of stainless steel.

“Greenfield” is a common term to refer to FMC.

Article 348 covers flexible metal conduit. See Figure 8.

Figure 8. Listed flexible metal conduit. Image used courtesy of Southwire

Listed liquid-tight flexible metal conduit (LFMC), meeting six conditions.

◦ The conduit terminates in listed fittings.

◦ Maximum overcurrent protective device = 20 A for trade sizes 3/8” through ½”. See Figure 9.

Figure 9. Listed LFMC ⅜” through ½”. Image used courtesy of Lorenzo Mari

◦ Maximum overcurrent protective device = 60 A for trade sizes 3/4” through 1 ¼“ and no smaller conduit or tubing in the effective ground-fault current path. See Figure 10.

Figure 10. Listed LFMC ¾” through 1 ¼”. Image used courtesy of Lorenzo Mari

◦ Maximum combined length = 1.8 m.

The NEC recognizes that a listed liquid-tight flexible metal conduit of trade size 1 1/4” and smaller, not exceeding 1.8 m in length, works as a grounding means through its metal core.

◦ Wire-type equipment grounding conductor or bonding jumper per 250.102(E)(2) required if LFMC is installed to reduce the vibration transmission from equipment or to provide flexibility for equipment requiring adjustments after installation.

This limitation is identical to that in FMC for uses requiring flexibility. See Figure 11.

Figure 11. Listed LFMC requiring flexibility. Image used courtesy of Lorenzo Mari

Section 250.102(E)(2) deals with bonding jumpers or conductors and equipment bonding jumpers installed outside a raceway or enclosure. Run these jumpers with the raceway or enclosure.

Hold the bonding jumper installed on the outside close to the conduit to keep a low impedance in the ground-fault current path, fastening it frequently along its way.

◦ Wire-type equipment grounding conductor or bonding jumper per 250.102(E)(2) required if LFMC contains a stainless-steel core.

Article 350 covers liquid-tight flexible metal conduit. See Figure 12.

Figure 12. Listed liquid-tight flexible metal conduit. Image used courtesy of SG Industrias Electric

Flexible metallic tubing (FMT), meeting three conditions.

◦ The tubing terminates in listed fittings.

◦ Maximum overcurrent protective device = 20 A.

◦ Maximum combined length = 1.8 m.

Article 360 covers flexible metallic tubing.

The armor of Type AC cable.

AC cables secure the grounding by relying on a jacket with a wire or a thin strip – their combination makes the equipment grounding conductor.

Article 320 covers armored cables. Figure 13 shows an AC interlocked galvanized aluminum armored cable.

Figure 13. An AC interlocked galvanized aluminum armored cable. Image used courtesy of Nassau National Cable

The copper sheath of mineral-insulated, metal-sheathed cable type MI.

Article 332 covers mineral-insulated, metal-sheathed cables. See Figure 14.

Figure 14. A mineral-insulated, metal-sheathed cable. Image used courtesy of Wikimedia

Type MC cable providing an effective ground-fault current path in one or more of three ways:

◦ Containing an insulated or uninsulated equipment grounding conductor.

◦ Combining the metallic sheath and uninsulated equipment grounding conductor or bonding conductor of interlocked metal tape-type MC cable listed and identified as an equipment grounding conductor.

◦ The metallic sheath or combination of metallic sheath and equipment grounding conductor of smooth or corrugated tube-type MC cable listed and identified as an equipment grounding conductor.

A welded and corrugated metallic sheath or an interlocking tape armor encloses power and control cables. One of the outer interstices in multi-conductors typically includes an insulated or uninsulated equipment grounding conductor.

Article 330 covers metal-clad cables. Figure 15 shows a Type MC metal-clad aluminum interlocked armor cable.

Figure 15. A metal-clad aluminum interlocked armor cable. Image used courtesy of Nassau National Cable

Cable trays.

Article 392 covers cable trays. See Figure 16.

Section 392.60(A) permits using a metal cable tray as an equipment grounding conductor if continuous maintenance and supervision ensure qualified persons service the cable tray system.

Section 392.60(B) permits using a steel or aluminum cable tray if it meets four requirements.

Figure 16. Cable trays. Image used courtesy of Comaple

Cable bus framework.

Article 370 covers cablebus. See Figure 17.

Figure 17. Cable bus. Image used courtesy of LBS

Other listed electrically continuous metal raceways and listed auxiliary gutters.

Examples are metal wireways, covered in Article 376, and strut-type channel raceways, covered in Article 384.

Surface metal raceways listed for grounding.

Article 386 covers surface metal raceways.

The equipment grounding conductor must carry a fault current, which may be very high in magnitude. An effective fault-current path requires good handicraft – loose connections may introduce high resistances that reduce the fault-current level, increasing the time to clear the fault or initiate sparking and arcing.

Installing a wire-type equipment grounding conductor in a metal raceway not isolated to reduce electromagnetic interference on the grounding circuit puts the raceway in parallel with the wire. In such a case, the equipment grounding conductor is the parallel combination of the raceway and wire.

Section 250.118(B) Not permitted

Grounding electrode conductors.

The structural metal frame of a structure or building.

Equipment Grounding Takeaways

An effective ground-fault current path allows a current magnitude high enough to quickly open the circuit overcurrent protective device and remove dangerous voltages in metal enclosures and raceways.
Metal enclosures, bonding jumpers, and equipment grounding conductors must be part of an effective ground-fault current path.
Section 250.118 describes suitable types of conductors, metallic cables, or raceways for equipment grounding conductors.
Listed flexible metal conduit, listed liquid-tight flexible metal conduit, and flexible metallic tubing can be used as equipment grounding conductors if they terminate in listed fittings, the total combined length is not over 1.8 m, and the circuit protection is not above 20 A. The circuit protection in a listed liquid-tight flexible metal conduit may reach 60 A for trade sizes ¾“ through 1 ¼“.
Install the equipment grounding conductor appropriately to avoid high resistance in the ground-fault current path.

To catch up on Lorenzo Mari’s series on National Electrical Code 2023 Basics: Grounding and Bonding, follow these links: