250.184 Solidly Grounded Neutral Systems
- Service-entrance conductors
- Service laterals or underground service conductors
- Direct-buried portions of feeders
Informational Note: See 225.4 for conductor covering where within 3.0 m (10 ft) of any building or other structure.
Exception: In industrial and commercial premises under engineering supervision, it shall be permissible to size the ampacity of the neutral conductor to not less than 20 percent of the ampacity of the phase conductor.
- A single-point grounded neutral system shall be permitted to be supplied from (a) or (b):
- A grounding electrode shall be provided for the system.
- A grounding electrode conductor shall connect the grounding electrode to the system neutral conductor.
- A bonding jumper shall connect the equipment grounding conductor to the grounding electrode conductor.
- An equipment grounding conductor shall be provided to each building, structure, and equipment enclosure.
- A neutral conductor shall only be required where phase-to-neutral loads are supplied.
- The neutral conductor, where provided, shall be insulated and isolated from earth except at one location.
- An equipment grounding conductor shall be run with the phase conductors and shall comply with (a), (b), and (c):
- The neutral conductor of a solidly grounded neutral system shall be permitted to be grounded at more than one point. Grounding shall be permitted at one or more of the following locations:
- The multigrounded neutral conductor shall be grounded at each transformer and at other additional locations by connection to a grounding electrode.
- At least one grounding electrode shall be installed and connected to the multigrounded neutral conductor every 400 m (1300 ft).
- The maximum distance between any two adjacent electrodes shall not be more than 400 m (1300 ft).
- In a multigrounded shielded cable system, the shielding shall be grounded at each cable joint that is exposed to personnel contact.
Exception: In a multipoint grounded system, a grounding electrode shall not be required to bond the neutral conductor in an uninterrupted conductor exceeding 400 m (1300 ft) if the only purpose for removing the cable jacket is for bonding the neutral conductor to a grounding electrode.