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Grounding versus Bonding



250.20 Systems Required to be Grounded and Bonded

Alternating-current systems (power supplies) must be grounded and bonded as provided in (A), (B), (C), or (D).
Author's Comment: System grounding, the intentional bonding of the electrical supply source to the metal case, provides the low-impedance fault-current path necessary to clear a ground fault. Figure 250.43

(A) Alternating-Current Systems Below 50V. Alternating-current systems operating below 50V aren't required to have the system bonded to the metal case unless: Figure 250.44

(1)The primary is supplied from a 277V or 480V circuit.

(2)The primary is supplied from an ungrounded power supply.

(3)Where installed as overhead conductors outside of buildings.

(B) Alternating-Current Systems Over 50V. Alternating-current systems over 50V that require a grounded neutral conductor must have the grounded neutral terminal [250.26] of the power supply bonded to the system metal parts in accordance with 250.30(A)(1). Such systems include: Figure 250.45

  • 2- or 3-wire, single-phase 120V or 120/240V systems
  • 4-wire, three-phase wye-connected 120/208V or 277/480V systems
  • 4-wire, three-phase delta-connected 120/240V

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