In NEC® Section 250.20(B)(3), what condition must a three-phase, four-wire delta-connected transformer meet?

In NEC® Section 250.20(B)(3), the correct condition for a three-phase, four-wire delta-connected transformer is that it must be grounded. This grounding is essential for safety and system stability, providing a reference point for the electrical system and helping to protect against overvoltages and fault conditions. Grounding the transformer lessens the risk of electric shock to personnel and equipment by facilitating the operation of overcurrent protection devices. Furthermore, grounding helps minimize the potential for floating voltages that can occur with ungrounded systems, providing a more reliable and safer operating environment.

The grounding requirement specifically for delta-connected transformers is especially relevant in three-phase systems where ungrounded configurations may present hazards due to phase-to-ground overvoltages that can occur during faults. As such, the NEC emphasizes the importance of grounding these transformers to maintain safety standards and ensure system protection.