According to the NEC, surge protective devices (SPDs) must be?

Surge protective devices (SPDs) must be listed according to the National Electrical Code (NEC). Being listed means that the device has been tested and certified by a recognized testing laboratory to meet specific safety and performance standards. This listing ensures that SPDs have undergone rigorous evaluations to verify their ability to protect electrical systems from voltage spikes, ensuring safety and reliability in various applications.

The requirement for SPDs to be listed is crucial as it helps in maintaining safety standards throughout the electrical installation process. Devices that are not listed may not adequately protect against surges or could potentially introduce hazards into the electrical system. This compliance with recognized standards plays a vital role in the overall efficacy of surge protection strategies.

Other options, while they may describe certain characteristics of SPDs, do not align with the foundational requirements outlined by the NEC. For instance, while some SPDs can be rated for outdoor use or adjustable, these are not universal requirements like listing, which is essential for ensuring that the device is safe and effective for its intended use.