Master Electrician Exam (NEC)Grounding and BondingHard
A master electrician is installing a new 480V, 3-phase, 3-wire delta service for an industrial facility. The service will be supplied by a transformer bank. Which of the following statements regarding the grounding of this service is true?
- AOne of the phase conductors must be intentionally grounded.
- BA corner-grounded delta system is not permitted for services.
- CThe system must be grounded, but the grounding electrode conductor is sized differently.
- DThe system must be ungrounded, with ground-fault detection equipment.
Show answer & explanationAnswer & explanation
Correct answer: A. One of the phase conductors must be intentionally grounded.
For a 3-phase, 3-wire delta system, NEC 250.26(3) requires that if a system is grounded, one phase conductor must be intentionally grounded (a corner-grounded delta). Alternatively, the system can be ungrounded, requiring ground-fault detection equipment per 250.21(B). Option A describes a permitted grounded configuration.
Why the other options are wrong
- B. Corner-grounded delta systems are permitted for services, although less common than wye systems.
- C. The GEC sizing rules are generally similar based on service conductor size, not fundamentally 'different' due to delta, although the connection point will vary.
- D. While ungrounded systems are permitted (250.21(B)), they are not mandatory, and require ground-fault detection, which is not stated as a must for 'grounded' system.
Grounding Delta Systems
3-phase, 3-wire delta systems can be ungrounded, or if grounded, must have one phase conductor intentionally grounded (corner-grounded delta).
- Ungrounded delta systems require ground-fault detectors.
- Corner-grounded delta systems have one phase bonded to ground.
- High-leg delta systems (120/240V, 3-phase, 4-wire) have the midpoint of one transformer phase grounded.
Memory trick: Delta's ground: cornered or ungrounded, never floating.