The Basics of Isolated Grounding Receptacles

Oct. 1, 2001
An isolated ground receptacle (IGR) can reduce electrical noise, but if installed incorrectly, it can create a dangerous installation. This receptacle differs in construction from its self-grounding counterpart. The grounding terminal for an IGR is insulated from its metal mounting yoke. This means you must connect the grounding terminal directly to an effective fault current path by an insulated

An isolated ground receptacle (IGR) can reduce electrical noise, but if installed incorrectly, it can create a dangerous installation. This receptacle differs in construction from its self-grounding counterpart. The grounding terminal for an IGR is insulated from its metal mounting yoke. This means you must connect the grounding terminal directly to an effective fault current path by an insulated equipment grounding conductor. To ensure the device you're installing is an IGR, look for the Code-required orange triangle located on the face of the receptacle [Sec. 410-56(c)].

The NEC says the insulated equipment grounding conductor for an IGR may originate at the neutral point of the power source, and it may pass through boxes and panelboards without termination, but neither configuration is required [Secs. 250-96(b), 250-146(d), 250-148 Exception, and 384-20 Exception].

The NEC does not require you to run the insulated equipment grounding conductor for an IGR back to the neutral point of the power source. In some cases, running it to this point would be too difficult, impractical, or expensive. Thus, the typical grounding termination point for an IGR in an existing facility is the equipment grounding terminal at the panelboard that supplied the circuit.

Per the Code, the grounding terminal for an IGR could terminate to the metal outlet box that contains it (Figure, point C). The NEC doesn't dictate where you terminate the grounding terminal for an IGR — just that you terminate to an effective fault current path. Nor does the NEC require each IGR to be on its own dedicated branch circuit.

However, the Code does require you to ground the metal enclosure. This is automatic with a regular receptacle; you must provide an additional grounding means when using an IGR (unless you bond the enclosure to the IGR). The Code does not permit you to use interlock, or standard, type MC cable when wiring an IGR because the outer interlock sheath is not recognized as an effective fault current path to ground metal enclosures. However, you can use AC cable with an insulated equipment grounding conductor, because type AC cable armor is listed for use as an equipment grounding conductor.

Some manufacturers require you to “ground” their equipment to an “isolated” ground point in the earth, independent from the building's electrical system. Sec. 250-2(d) prohibits — as application of Ohm's Law — using the earth as the sole equipment grounding conductor or fault current path, because doing so is dangerous. If the metal enclosures were truly isolated, or floated, a line-to-case fault could not clear, and those enclosures would remain energized with dangerous touch voltage.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!

Sponsored Recommendations

Chapter 9 of the NEC — Part 5

Calculating voltage drop with help from Table 8.

How to Calculate Labor Costs

Most important to accurately estimating labor costs is knowing the approximate hours required for project completion. Learn how to calculate electrical labor cost.

8 Types of Electrical Conduit and Their Uses

Electrical conduit is a tube or raceway used to house and protect electrical wires within a building or structure. From data centers to underground subways to ports and bridges...

Champion Strut Catalog

Champion Fiberglass is the most advanced manufacturing facility of fiberglass conduit, fiberglass bridge drain and fiberglass strut systems in the world. Its well-trained and ...