9.5 Equipment Grounding Conductors: Types, Sizing and Continuity
Key Takeaways
- NEC 250.118 lists the recognized equipment grounding conductor types, including a copper or aluminum conductor, rigid metal conduit, intermediate metal conduit, electrical metallic tubing, and the armor of Type AC cable.
- Flexible metal conduit qualifies as an equipment grounding conductor under 250.118(5) only with listed fittings, overcurrent protection of 20 amperes or less, and a combined flexible-raceway ground return path not exceeding 6 feet.
- Table 250.122 sizes wire-type equipment grounding conductors from the overcurrent device rating: 14 AWG copper at 15 amperes, 12 AWG at 20 amperes, 10 AWG at 60 amperes, 8 AWG at 100 amperes, and 6 AWG at 200 amperes.
- NEC 250.122(B) requires wire-type equipment grounding conductors to be increased in size proportionally by circular mil area where the ungrounded conductors are increased above the minimum required ampacity.
- NEC 250.148 requires equipment grounding conductors in a box to be spliced or joined so that removal of a receptacle, luminaire, or other device does not interrupt the grounding continuity.
What Counts as an Equipment Grounding Conductor — 250.118
The equipment grounding conductor shall be one or more, or a combination, of the following:
- A copper, aluminum, or copper-clad aluminum conductor, which may be solid or stranded, insulated, covered, or bare, in the form of a wire or busbar of any shape;
- Rigid metal conduit (RMC);
- Intermediate metal conduit (IMC);
- Electrical metallic tubing (EMT);
- Listed flexible metal conduit (FMC) meeting all of the following conditions:
- a. The conduit is terminated in listed fittings;
- b. The circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less;
- c. The combined length of flexible metal conduit, flexible metallic tubing, and liquidtight flexible metal conduit in the same ground return path does not exceed 6 feet (1.8 m);
- d. Where used to connect equipment where flexibility is required after installation, an equipment grounding conductor shall be installed;
- Listed liquidtight flexible metal conduit (LFMC) meeting all of the following:
- a. The conduit is terminated in listed fittings;
- b. For trade sizes 3/8 through 1/2, the circuit conductors are protected by overcurrent devices rated at 20 amperes or less;
- c. For trade sizes 3/4 through 1-1/4, the circuit conductors are protected by overcurrent devices rated not more than 60 amperes and there is no flexible metal conduit, flexible metallic tubing, or liquidtight flexible metal conduit in trade sizes 3/8 through 1/2 in the ground return path;
- d. The combined length in the same ground return path does not exceed 6 feet;
- e. Where flexibility is required after installation, an equipment grounding conductor shall be installed;
- Flexible metallic tubing (FMT) where terminated in listed fittings and meeting the same 20-ampere and 6-foot conditions;
- Armor of Type AC cable as provided in 320.108;
- The copper sheath of mineral-insulated, metal-sheathed (Type MI) cable;
- Type MC cable that provides an effective ground-fault current path in accordance with one or more of the specified constructions;
- Cable trays as permitted in 392.10 and 392.60;
- Cablebus framework as permitted in 370.60(1);
- Other listed electrically continuous metal raceways and listed auxiliary gutters;
- Surface metal raceways listed for grounding.
Note what is not on the list: liquidtight flexible nonmetallic conduit, PVC, ENT, and the earth. Those always require a wire-type equipment grounding conductor.
Sizing — Table 250.122
Equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122, but in no case shall they be required to be larger than the circuit conductors supplying the equipment.
| Rating of automatic overcurrent device | Copper | Aluminum or copper-clad aluminum |
|---|---|---|
| 15 A | 14 AWG | 12 AWG |
| 20 A | 12 AWG | 10 AWG |
| 60 A | 10 AWG | 8 AWG |
| 100 A | 8 AWG | 6 AWG |
| 200 A | 6 AWG | 4 AWG |
| 300 A | 4 AWG | 2 AWG |
| 400 A | 3 AWG | 1 AWG |
| 500 A | 2 AWG | 1/0 AWG |
| 600 A | 1 AWG | 2/0 AWG |
| 800 A | 1/0 AWG | 3/0 AWG |
| 1000 A | 2/0 AWG | 4/0 AWG |
| 1200 A | 3/0 AWG | 250 kcmil |
| 1600 A | 4/0 AWG | 350 kcmil |
| 2000 A | 250 kcmil | 400 kcmil |
The table is indexed on the overcurrent device rating, not on the conductor size. A 200-ampere feeder run with oversized 4/0 conductors for voltage drop still starts at 6 AWG copper from the table — before 250.122(B) is applied.
250.122(A) General. Where conductors are adjusted in size to compensate for voltage drop, equipment grounding conductors, where installed, shall be adjusted proportionately according to the circular mil area.
The Upsizing Rule — 250.122(B)
Where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors.
The purpose is impedance. A long feeder upsized for voltage drop has lower phase-conductor impedance but, if the equipment grounding conductor is not upsized, the fault loop impedance stays high. That slows the overcurrent device and raises touch voltage during a fault.
Worked Example — Upsizing
A 100-ampere feeder would normally use 3 AWG copper (52,620 cmil) with an 8 AWG copper equipment grounding conductor (16,510 cmil). To limit voltage drop, the phase conductors are increased to 1/0 AWG copper (105,600 cmil). What size equipment grounding conductor is required?
Ratio = 105,600 / 52,620 = 2.0068
Required EGC area = 16,510 x 2.0068 = 33,132 cmil
From Chapter 9, Table 8:
6 AWG = 26,240 cmil -> too small
4 AWG = 41,740 cmil -> sufficient
4 AWG copper. Note that you round up to the next available conductor; there is no rounding down.
Parallel Installations — 250.122(F)
250.122(F)(1) Conductors in Parallel in Multiple Raceways or Cables. Where conductors are installed in parallel in multiple raceways or cables, the wire-type equipment grounding conductor, where used, shall be installed in parallel in each raceway or cable. Each parallel equipment grounding conductor shall be sized in compliance with 250.122 based on the overcurrent protective device for the feeder or branch circuit.
The requirement is not divided among the raceways. A 600-ampere feeder in three raceways requires three full-size 1 AWG copper equipment grounding conductors, one per raceway.
250.122(F)(2) Multiconductor Cables. Where multiconductor cables are installed in parallel, the equipment grounding conductors within the cables shall be sized in compliance with 250.122 on the basis of the overcurrent device rating, with a specific allowance permitting sectioned equipment grounding conductors smaller than 1/0 AWG in multiconductor cables under stated conditions.
Other Sizing Rules
250.122(C) Multiple Circuits. Where a single equipment grounding conductor is run with multiple circuits in the same raceway, cable, or cable tray, it shall be sized for the largest overcurrent device protecting conductors in the raceway, cable, or cable tray.
250.122(D) Motor Circuits. The equipment grounding conductor size shall be determined as follows:
- (1) General. Where the overcurrent device consists of an instantaneous trip circuit breaker or a motor short-circuit protector, the equipment grounding conductor shall be sized not smaller than that given by Table 250.122 using the maximum permitted rating of a dual-element time-delay fuse selected for branch-circuit short-circuit and ground-fault protection in accordance with 430.52(C)(1) Exception No. 1.
- (2) Instantaneous-Trip Circuit Breaker. As above.
250.122(E) Flexible Cord and Fixture Wire. The equipment grounding conductor in a flexible cord with the largest circuit conductor 10 AWG or smaller, and the equipment grounding conductor used with fixture wires, shall not be smaller than 18 AWG copper and shall not be smaller than the circuit conductors.
250.122(G) Feeder Taps. Equipment grounding conductors run with feeder taps shall not be smaller than shown in Table 250.122 based on the rating of the overcurrent device ahead of the feeder but shall not be required to be larger than the tap conductors.
Identification — 250.119
Unless required elsewhere in the Code, equipment grounding conductors shall be permitted to be bare, covered, or insulated. Individually covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes, unless it is bare.
250.119(A) Conductors 4 AWG and Larger. Conductors 4 AWG and larger shall be permitted to be permanently identified as equipment grounding conductors at each end and at every point where the conductor is accessible, by one of: stripping the insulation or covering from the entire exposed length; coloring the exposed insulation or covering green; or marking the exposed insulation or covering with green tape or green adhesive labels.
250.119(B) Multiconductor Cable. Conductors in multiconductor cables shall be permitted to be similarly identified where the conditions of maintenance and supervision ensure that only qualified persons service the installation.
Continuity in Boxes — 250.146 and 250.148
250.146 Connecting Receptacle Grounding Terminal to Box. An equipment bonding jumper shall be used to connect the grounding terminal of a grounding-type receptacle to a grounded box unless grounding continuity is provided by one of the exceptions:
- (A) Surface-Mounted Box. Where the box is mounted on the surface, direct metal-to-metal contact between the device yoke and the box, or a contact yoke or device that complies with 250.146(B) and is listed as providing effective grounding, shall be permitted. At least one of the insulating washers shall be removed from receptacles that do not have a contact yoke.
- (B) Contact Devices or Yokes. Contact devices or yokes designed and listed as self-grounding shall be permitted.
- (C) Floor Boxes. Floor boxes designed for and listed as providing satisfactory ground continuity.
- (D) Isolated Receptacles. Where required for the reduction of electrical noise, a receptacle in which the grounding terminal is purposely insulated from the receptacle mounting means shall be permitted, provided the insulated equipment grounding conductor is run with the circuit conductors. This equipment grounding conductor shall be permitted to pass through one or more panelboards without a connection to the panelboard grounding terminal, so as to terminate within the same building at an equipment grounding conductor terminal of the applicable derived system or service. The receptacle shall be identified by an orange triangle located on the face of the receptacle.
250.148 Continuity of Equipment Grounding Conductors and Attachment in Boxes. Where circuit conductors are spliced within a box, or terminated on equipment within or supported by a box, all equipment grounding conductors associated with any of those circuit conductors shall be connected within the box or to the box with devices suitable for the use.
- (A) Connections. Connections and splices shall be made in accordance with 110.14(B) except that insulation shall not be required.
- (B) Grounding Continuity. The arrangement of grounding connections shall be such that the disconnection or the removal of a receptacle, luminaire, or other device fed from the box does not interrupt the grounding continuity.
- (C) Metal Boxes. A connection shall be made between the one or more equipment grounding conductors and a metal box by means of a grounding screw that shall be used for no other purpose, equipment listed for grounding, or a listed grounding device.
- (D) Nonmetallic Boxes. One or more equipment grounding conductors brought into a nonmetallic outlet box shall be arranged such that a connection can be made to any fitting or device in that box requiring grounding.
The key phrase in (B) is "does not interrupt the grounding continuity." Daisy-chaining the equipment grounding conductors through a device's terminals is a violation, because pulling the device breaks the path for everything downstream. Splice the grounds together and pigtail to the device.
What size copper equipment grounding conductor does Table 250.122 require for a feeder protected by a 400-ampere overcurrent device?
A 60-ampere circuit's minimum conductor is 6 AWG copper at 26,240 circular mils with a 10 AWG copper equipment grounding conductor at 10,380 circular mils. The phase conductors are increased to 3 AWG copper at 52,620 circular mils for voltage drop. What equipment grounding conductor is required under NEC 250.122(B)?
Three equipment grounding conductors and a receptacle land in a metal box. Which arrangement satisfies NEC 250.148(B)?
A 600-ampere feeder is installed in three parallel raceways. Table 250.122 calls for a 1 AWG copper equipment grounding conductor at 600 amperes. What must be installed?