A journeyman is installing a new 480-volt, 3-phase, ungrounded delta system for a motor control center. Which of the following statements regarding the grounding of this system's associated equipment is true?
- AAll non-current-carrying metal parts of the equipment must be bonded to the system frame.
- BThe equipment grounding conductors are not required since the system is ungrounded.
- CThe equipment grounding conductors must be connected to a grounding electrode, but not to the system neutrals.
- DThe equipment grounding conductors must be installed and connected to the equipment grounding conductor terminal bar in the panelboard.
Show answer & explanationAnswer & explanation
Correct answer: D. The equipment grounding conductors must be installed and connected to the equipment grounding conductor terminal bar in the panelboard.
Even for an ungrounded system, equipment grounding conductors (EGCs) are still required to be installed and connected to the equipment grounding conductor terminal bar in the panelboard. While there's no neutral conductor to connect to in an ungrounded system, the EGCs provide a path for ground-fault current to return to the source to operate overcurrent protection, typically through the supply transformer windings.
Why the other options are wrong
- A. While bonding is important, the EGCs must specifically connect to the panelboard's EGC terminal bar for a complete fault path.
- B. EGCs are always required for equipment safety, regardless of whether the system is grounded or ungrounded (NEC 250.4(A)(3) and 250.4(B)(2)).
- C. EGCs are generally connected to a grounding electrode system, but their primary purpose is to provide a fault path to the source, not just to a ground rod. The system has no neutral to connect to.
EGC for Ungrounded Systems
Equipment grounding conductors are required for an ungrounded system to provide a ground-fault current path, even though there is no grounded (neutral) conductor to connect to.
- EGCs are mandatory for equipment in ungrounded systems (NEC 250.4(A)(3)).
- They establish a ground-fault current path to the supply source.
- Connect to the equipment grounding terminal bar in panelboards.
Memory trick: Delta's ground: Equipment still needs a path!