Article 285 of the National Electrical Code (NEC) provides the minimum requirements for surge protective devices (SPDs) installed on wiring systems of 1,000V or less. What the NEC does not go into is how to select and tier your SPDs to achieve the desired protection; the reason for this omission is addressed in Sec. 90.1.
The SPD limits circuit voltage by conducting during a surge. When using SPDs, you must connect one to each ungrounded conductor [285.4]. You can connect an SPD betwe