Each surge protective device (SPD) shall be connected to which conductor if installed?

The correct choice is to connect the surge protective device (SPD) to the grounded conductor. This practice is essential because the SPD functions to divert excess voltage—usually from transient overvoltage events such as lightning strikes or power surges—away from sensitive equipment.

Connecting the SPD to the grounded conductor ensures that it is able to effectively shunt surges to the ground, thereby protecting the rest of the electrical system and any connected devices from potential damage. The grounded conductor is intended to provide a low-resistance path to ground, which is critical for the SPD to perform its protective role efficiently.

When evaluating the other options, connecting to an ungrounded conductor would not provide a safe means for the SPD to discharge surge energy appropriately, as there is no defined path to ground. This could leave the electrical system and connected equipment vulnerable. Additionally, connecting the SPD to the neutral or hot conductors would not ensure the same level of protection, as they do not serve the purpose of creating a pathway to ground for the surges. Thus, the best practice for installing SPDs is to connect them to the grounded conductor.