Section 90.2(A) of the National Electrical Code is clear that the purpose of the code is the practical safeguarding of people and property from hazards arising from electricity use. This clause goes further to indicate the NEC is not a design specification or instruction manual for untrained people. There are recognized training programs that provide appropriate instruction in the classroom and on-the-job training that serve this purpose. Those programs include specific training on applying codes and standards to electrical installations and systems. 

With that in mind, this article takes a closer look at systems that are restricted from grounding (either as a rule or as an allowance) and how equivalent and effective safety is achieved and backed up by the words in the code. 

First, this column relates to systems less than 1,000V AC and 1,500V DC, meaning the requirements in Article 250 have general application for wiring and protection of these systems. For systems more than 1,000V AC or 1,500V DC, the grounding requirements are provided in Article 270—and, yes, this article is based on the 2026 NEC.

From the earliest years of electricity use, there have been discussions and heated debates about the benefits of operating grounded systems compared to ungrounded systems. The NEC rules today often make those determinations. Some electrical systems must be grounded, some are permitted to be grounded, and some are not permitted to be grounded. The NEC provisions are broken down for each system application. Part II of Article 250 provides the requirements for electrical system grounding. The specific rules for grounding electrical systems are found in 250.20.

Before moving on to the requirements, it is important to review the purpose of grounding an electrical system. Section 250.4 describes the purpose of system grounding, equipment grounding and what is intended to be accomplished by grounding a system, grounding equipment and bonding equipment. A thorough understanding of these performance requirements is essential in understanding the prescriptive grounding and bonding requirements that follow in the NEC.

When grounding is not permitted

Systems not permitted to be grounded are addressed in specific NEC rules, such as those isolated power systems installed to supply operating rooms in healthcare facilities. These requirements and prohibitions appear in Section 517.160, which modifies the general rules in chapters 1–4, in this case to be more restrictive and prohibit grounding the supply system. The reasoning, found in 90.2(A), is for protecting people from hazards arising from electricity use. 

When the current exceeds a set determined value (5 milliamperes), the system continues to provide power, but there is annunciation (audible and visual through a required line isolation monitor) so qualified personnel can respond to the event and clear it. The power for the operating room remains in service to allow for safe cessation of procedures, which also protects the patient. 

The NEC recognizes another ungrounded system: industrial furnaces. In this case, using the allowance provided in Section 250.21(A)(1), (B) and (C), the system operates ungrounded and is monitored for leakage current. 

This example is for industrial applications where a first phase-to-ground fault could cause interruption of power to a large industrial furnace, which would undoubtedly cause significant damage to equipment, property and possibly personnel. In this case, the system is designed and operated ungrounded and ground fault detection monitors are actively monitoring the system for faults and will annunciate for qualified personnel to respond. This allows for orderly shutdown that generally results in protection of the equipment (property in this case) and as such, meets the provisions of 90.2(A). 

Required or permitted grounding

Whether the NEC requires system grounding or permits system grounding by choice, all requirements must be applied whenever systems are grounded; they are not optional. 

Methods of grounding can vary and include solid grounding, impedance or resistance grounding, grounding through surge arresters and grounding through an inductor. The method is not always a matter of choice or a design consideration, but, in many cases, is required by the NEC. 

Ungrounded systems provide unique protection for people and property and must generally be provided with ground detection systems. The sensing equipment for ground detection systems must be installed as close as practical to where the system receives its supply. See Section 250.21(B) and (C) for the actual requirements related to ground detectors and equipment markings.

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