Stumped by the Code? Rules for Connecting a Receptacle Grounding Terminal to an EGC

All questions and answers are based on the 2020 NEC.

Underlined text indicates revisions made in the 2020 Edition of the NEC

Q. What Code rule requires the connection of a receptacle grounding terminal to an equipment grounding conductor?

A. An equipment bonding jumper sized in accordance with Table 250.122 is required to connect the grounding contacts of a receptacle to a metal box that is connected to an equipment grounding conductor, except as permitted in Sec. 250.146(A) through (D). Author’s Comment: The NEC does not restrict the position of the receptacle grounding terminal; it can be up, down, or sideways. Code proposals to specify the mounting position of receptacles have always been rejected.

(A) Surface-Mounted Box. A receptacle having direct metal-to-metal contact between the receptacle strap or yoke and a surface metal box is considered to be connected to the required effective ground-fault current path. To ensure sufficient metal-to-metal contact, at least one of the insulating retaining washers on the yoke screw must be removed.

A receptacle installed on a cover is considered to be connected to the required effective ground-fault current path under both of the following conditions:

(1) The receptacle is attached to the metal cover with at least two fasteners that have a thread locking, or screw or nut locking means.

(2) The cover mounting holes are located on a flat non-raised portion of the cover.

(B) Self-Grounding Receptacles. Receptacle yokes listed as self-grounding establish the equipment bonding between the receptacle yoke and a metal box.

(D) Isolated Ground Receptacles. The grounding terminal of an isolated ground receptacle must be connected to an insulated equipment grounding conductor.
Informational Note: Use of an isolated equipment grounding conductor does not relieve the requirement for connecting the raceway system and outlet box to an equipment grounding conductor.

Author’s Comment: Type AC cable containing an insulated equipment grounding conductor can be used to supply isolated ground receptacles because the metal armor of the cable is listed as an equipment grounding conductor [Sec. 250.118(8)].

Author’s Comment: Interlocked Type MCAP cable with a 10 AWG bare aluminum grounding/bonding conductor can be used to supply isolated ground receptacles because the combination of the metal armor and the 10 AWG bare aluminum conductor is listed as an equipment grounding conductor [Sec. 250.118(10)(b)].

Caution: The metal armor sheath of traditional interlocked Type MC cable containing an insulated equipment grounding conductor is not listed as an equipment grounding conductor. Thus, this wiring method with a single equipment grounding conductor cannot supply an isolated ground receptacle. Type MC cable with two insulated equipment grounding conductors is acceptable because one bonds to the metal box and the other one connects to the isolated ground receptacle.

Author’s Comments:

When should an isolated ground receptacle be installed, and how should the isolated ground system be designed? These questions are design issues and are not answered based on the NEC alone [Sec. 90.1(A)]. In most cases, using isolated ground receptacles is a waste of money. For example, IEEE 1100, Powering and Grounding Electronic Equipment (Emerald Book) states, “The results from the use of the isolated ground method range from no observable effects, the desired effects, or worse noise conditions than when standard equipment bonding configurations are used to serve electronic load equipment [8.5.3.2].”
Few electrical installations truly require an isolated ground system. For those systems that can benefit from one, engineering opinions differ as to what is a proper design. Making matters worse — of those properly designed, few are correctly installed, and even fewer are properly maintained.

These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.