Does the National Electrical Code (NEC) require a 480-volt (V), three-phase, 3-wire, delta-connected system to be grounded? No, it is optional. This article examines the NEC’s electrical-system grounding provision. Generally, Code users should understand there are systems that are required to be grounded, systems that are not required to be grounded and systems that must not be grounded. Part II of NEC Article 250 contains electrical-system grounding provisions. Let’s take a closer look at the requirements.
System grounding required
Section 250.20 includes the driving text that indicates electrical-system grounding is required in accordance with sections 250.20(A) and (B), depending on each system’s voltage and phase arrangement. If a system is optional, but the choice is made to ground it, all system-grounding rules in the NEC must be applied.
Section 250.20(A) provides the requirements for grounding systems of less than 50V. The Code requires alternating current (AC) systems of less than 50V to be grounded under any of the following conditions:
- Where supplied by transformers if the supply system exceeds 150V to ground
- Where supplied by transformers if the supply system is ungrounded
- Where installed outside as overhead conductors
Section 250.20(B) addresses the grounding requirements for premises wiring and premises-wiring systems of 50 to 1,000V. Systems in this voltage range have to be grounded under any of the following conditions:
- If the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150V
- If the system is three-phase, 4-wire and is wye-connected and the neutral conductor is used as a circuit conductor
- If the system is three-phase, 4-wire and is delta-connected in which the midpoint of one phase winding is used as a circuit conductor
The preceding requirements apply to many premises-wiring systems installed today. In item one, if the system can be grounded in a way that the phase-to-ground voltage is less than 150V, it must always be grounded. An example of this is a single-phase, 2-wire system with a 120V output (secondary). If one conductor or the other is grounded, the system’s phase-to-ground voltage is 120V.
System grounding optional
Section 250.21(A) provides a list of electrical systems that are permitted, but not required, to be grounded as follows:
- Systems exclusively for industrial electric furnaces for melting, refining, tempering, etc.
- Separately derived systems exclusively for rectifiers supplying only adjustable-speed industrial drives
- Separately derived systems supplied by transformers that have a primary voltage rating of 1,000V or less, if the system is used exclusively for control circuits, if qualified people service the installation, and if continuity of control power is required
- Other systems not required to be grounded in accordance with Section 250.20(B) requirements
Typical systems permitted, but not required, to be grounded include 240V, three-phase, 3-wire and 480V, three-phase, 3-wire, delta-connected systems.
System grounding not permitted
Section 250.22 addresses electrical systems that are not permitted to be grounded. These include circuits for overhead cranes that operate over combustible fibers in Class III hazardous locations. The idea is that a first phase-to-ground fault will not create a shower of sparks or hot particles that could cause a fire due to the accumulations of fibers on the floor below.
This condition is common in textile mills because of the Class III locations associated with those manufacturing processes. Other systems that must not be grounded are isolated power systems used in healthcare facilities. Those requirements are in Section 517.160.
Section 250.22 also prohibits circuits for equipment within an electrolytic cell working zone, as provided in Article 668. Electrolytic cells are commonly used in the aluminum- and chlorine-processing industries.
Secondary circuits of low-voltage lighting system must not be grounded, as indicated in Section 411.5(A). Also, low-voltage lighting systems for underwater pool lighting supplied by isolation transformers must not be grounded. The listed transformers for these systems are of the isolation type with a grounded metal barrier between the primary and the secondary windings. Note that these systems are not permitted to be grounded, but the following are generally required to be grounded: normally noncurrent-carrying metal parts of equipment enclosures and raceways that contain these ungrounded systems conductors and equipment.
Ungrounded systems are also generally required to be equipped with ground-detection systems as indicated in 250.21(B) and such equipment must be marked “Caution: Ungrounded System Operating _____ Volts Between Conductors,” in accordance with 250.21(C).
About The Author
JOHNSTON is NECA’s executive director of codes and standards. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at [email protected]