What device is used to protect transformers or other devices supplied from power-supply conductors?

The correct choice is an over-current device not rated over 20 Amperes because it aligns with the NFPA 72 guidelines that regulate over-current protection for devices like transformers. An over-current device serves to protect equipment from excessive current, which can lead to overheating and potential failures. Specifically, for smaller transformers and similar devices, the requirement for an over-current device rated not over 20 Amperes ensures adequate protection without risking nuisance tripping or false alarms that can occur with larger ratings.

Using a fuse rated over 15 Amperes or a circuit breaker rated over 20 Amperes could provide protection, but they may not meet the specific criteria or operational characteristics needed for the efficient functioning of smaller devices. Larger ratings might also be unnecessarily restrictive in protecting smaller systems, as they could allow higher currents that don't provide the optimal protection for all instances. The transformer protection relay is typically used for larger applications and is not necessarily designed for the specific protection of smaller devices in this context. Therefore, an over-current device not exceeding 20 Amperes is ideal for the conceptual underpinning of this protective mechanism.