Under the National Electrical Code (NEC), several types of electrical systems are required to be grounded. The most common are AC systems operating under 1,000 volts where the system can be grounded so that the maximum voltage to ground does not exceed 150 volts, as well as DC systems in the 60 to 300 volt range. Beyond these core categories, the NEC also mandates grounding for separately derived systems, communication system entrances, and certain specialty installations like healthcare facilities and solar arrays.
AC Systems That Must Be Grounded
The NEC (Section 250.20) requires grounding for alternating current systems that serve premises wiring when certain voltage conditions are met. The most widely applicable rule covers systems where the grounded conductor can be connected to earth so the maximum voltage to any ungrounded conductor does not exceed 150 volts to ground. In practice, this captures the standard 120/240-volt single-phase systems found in virtually every home and most commercial buildings in the United States.
Three-phase, four-wire wye-connected systems where the neutral is used as a circuit conductor also fall under mandatory grounding. So do three-phase, four-wire delta-connected systems where the midpoint of one phase winding is used as a circuit conductor. These configurations are common in commercial and light industrial settings.
The purpose of system grounding, as stated in NEC 250.4(A)(1), is to limit voltage imposed by lightning, line surges, or accidental contact with higher-voltage lines, and to stabilize voltage to earth during normal operation. For any grounded system, four things are required: electrical system grounding, electrical equipment grounding, electrical equipment bonding, and bonding of electrically conductive materials.
DC Systems That Must Be Grounded
Direct current systems have their own set of rules under NEC Section 250.162. Two-wire DC systems supplying premises wiring must be grounded when they operate at more than 60 volts but not more than 300 volts between conductors. Below 60 volts, grounding is not required. Above 300 volts, the system typically falls into a different set of rules.
For three-wire DC systems supplying premises wiring, the neutral conductor must be grounded regardless of voltage. This ensures a stable reference point and a safe path for fault current.
Separately Derived Systems
A separately derived system is any wiring system that has no direct electrical connection to conductors from another system. The most common example is a transformer: because the primary and secondary windings are electrically isolated, the secondary side is a separately derived system. Generators can also qualify when they supply a transfer switch that opens the grounded neutral conductor.
NEC Section 250.30 requires separately derived systems to be both bonded and grounded. A system bonding jumper must connect the metal enclosure of the transformer or generator to the grounded (neutral) terminal on the secondary side. This bonding jumper must be sized based on the largest ungrounded secondary conductor, using Table 250.66. It can be installed at the source equipment, the first disconnecting means, or anywhere in between, but only at one location.
Each separately derived system also needs a grounding electrode conductor run to a suitable grounding electrode. That conductor must terminate at the same point where the system bonding jumper is installed to prevent stray current from traveling on metal piping, structural steel, or equipment enclosures.
There is one notable exception: separately derived systems rated at 1 kVA or less are not required to be earthed, though they still need a system bonding jumper so that ground faults can be cleared by the overcurrent protection device.
Communication and Low-Voltage Systems
Telephone lines, coaxial cable, fiber optic systems, and network cabling all fall under NEC Article 800 and related articles (770, 805, 810, 820, 830, and 840). These systems must be grounded at the point where they enter a building, using a grounding electrode conductor that connects to the same grounding electrode used by the building’s electrical power system.
This shared grounding electrode keeps communication equipment and power system equipment at the same earth potential. Without it, a lightning strike or power surge on the communication line could create a dangerous voltage difference between your cable modem and your electrical outlets, for example. Section 800.100 specifies minimum conductor sizing and rules for bonding all grounding electrodes together into a single, unified ground.
Portable Generators
Portable generators get a partial exemption under NEC 250.34. A portable generator does not need a grounding electrode (such as a ground rod) if it only supplies equipment mounted directly on the generator or cord-and-plug-connected devices, and the generator’s frame is bonded to the metal parts of that equipment. In this configuration, the frame itself serves as the grounding reference.
Once a portable generator feeds a building’s fixed wiring system, the rules change. The generator then needs to be grounded to an electrode, and the system must meet the same grounding requirements as any other power source supplying premises wiring.
Solar Photovoltaic Systems
NEC Article 690 covers photovoltaic installations. Residential and commercial solar arrays on dwellings require ground-fault protection that can detect faults, interrupt the circuit, and provide a visible indicator of a ground-fault condition. That indicator must be in an accessible location and include a warning label. For AC modules (solar panels with built-in microinverters), a single detection device is permitted to monitor for AC ground faults across the entire array and disable it by removing AC power if a fault is detected.
The equipment grounding requirements for solar systems ensure that all metal frames, racking, and enclosures are bonded together and connected to the building’s grounding system. This protects both the equipment and anyone who might touch the array during a fault.
Healthcare Facilities
Hospitals and other healthcare facilities have stricter grounding requirements under NEC Article 517. In both general care areas and critical care areas, all receptacles must have an insulated copper equipment grounding conductor. A bare ground wire running through metal conduit is not sufficient on its own. Receptacles at patient bed locations must also be equipped with ground-fault circuit interrupter (GFCI) protection and placed for easy access, typically about five feet above the floor.
These enhanced rules exist because patients connected to medical equipment can be vulnerable to extremely small amounts of stray current that would be harmless in other settings. The insulated grounding conductor provides a dedicated, low-resistance path back to the source, reducing the chance of any current flowing through a patient.
Systems Permitted to Be Ungrounded
Not every electrical system requires grounding. The NEC permits ungrounded operation for certain industrial systems, isolated power supplies in healthcare operating rooms, and high-impedance grounded systems used in specific industrial applications. Even ungrounded systems still require equipment grounding, equipment bonding, and bonding of conductive materials. The only thing omitted is the system grounding connection to earth. This means the metal enclosures and raceways are still grounded for safety; the system’s circuit conductors simply have no intentional connection to the earth.