Is it permissible to connect two-wire ac circuits from ungrounded conductors in branch circuits?

Connecting two-wire AC circuits from ungrounded conductors in branch circuits is permissible under specific conditions, making this the correct choice. According to the National Electrical Code (NEC), two-wire systems can be utilized, but they must adhere to safety standards to mitigate the risk of electrical shock and ensure proper functionality.

Key conditions that typically apply include ensuring that the circuit is properly rated for the intended application and that it carries the necessary protective devices, such as fuses or circuit breakers. Additionally, the circuits must maintain appropriate separation from grounded conductors, and wiring must comply with established ampacity requirements to prevent overheating.

The other options do not accurately reflect the NEC's stance; outright prohibition or limitations in specific building types misrepresent the flexibility allowed when certain safety measures are in place. Thus, option A recognizes the complexities involved while still permitting this electrical configuration under defined circumstances.