Grounding

The following are grounding requirements for systems, circuits, and equipment.

Systems to Be Grounded

The following systems that supply premises wiring typically must be grounded:

• The neutral conductor of a three-wire direct current system
• Two-wire direct current systems operating at over 50 volts and up to 300 volts, unless they are rectifier-derived from an alternating current system
• Alternating circuits of less than 50 volts if they are installed as overhead conductors outside of buildings or if they are supplied by transformers and the transformer primary supply system is ungrounded or exceeds 150 volts to ground
• Alternating current systems of 50 volts to 1,000 volts, under the following conditions:
  • If the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts
  • If the system is nominally rated 480Y/277 volt, three-phase, four-wire, in which the neutral is used as a circuit conductor
  • If the system is nominally rated 240/120 volt, three-phase, four-wire, in which the midpoint of one phase is used as a circuit conductor
  • If a service conductor is uninsulated

Certain exceptions exist in which an alternating current system of 50 volts to 1,000 volts does not need to be grounded. These exceptions are explained in detail in CFR 1926.404.

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