Good night to you too!Okay, let's break down the SPD requirements for your specific scenario. This situation involves some interpretation of the NEC (National Electrical Code), and consulting your local AHJ (Authority Having Jurisdiction) is always recommended for a definitive answer. However, here's how I interpret it and what the NEC says

The Scenario:
320A Continuous Meter

This is your service entrance.

Double Lugs

This meter socket is designed to feed two separate services.

Two Separate Mains

You have two separate main disconnects coming directly from the meter.

NEC Guidance



NEC 230.67(A) Surge Protection

For services of dwelling units, surge protection must be provided.

NEC 230.91 Surge Protection

Surge protection must be provided for service equipment rated 1,000 V or less.

My Interpretation and Recommendations

Considering that you have a single service meter but that meter has double lugs, and that those lugs feed two separate mains, here is my approach:1. SPD at the Meter (Recommended)

In this case, I believe that because the meter has double lugs and is therefore rated to feed two separate services, installing an SPD at the meter enclosure is the most NEC-compliant approach. This single SPD would protect BOTH main panels. If you take this approach, the SPD must be listed for that use.2.

SPD for Each Main Panel

An alternative would be to install an SPD at each of the two main panels. Each SPD would be sized for the panel it protects. If you take this approach, ensure that the SPD installed protects all conductors from the panelboard to ground.

Important Considerations



Local AHJ

This is the most important point. Local jurisdictions can have amendments to the NEC, or interpretations that affect the rules.

ALWAYS consult with your local electrical inspector or AHJ. Tell them your scenario and ask how they want it handled. Their word is final. SPD Listing

Make sure any SPD you use is UL-listed or otherwise approved for its intended application.

Manufacturer's Instructions

Follow the SPD manufacturer's instructions for installation. These instructions are crucial for proper performance and safety.*

Distance from the Panel

Keep the SPD as close as practically possible to the main panel. Longer conductor lengths can reduce the SPD's effectiveness.

In summary

My recommendation is to install an SPD that protects all conductors from the panelboard to ground within each main panel. If you intend to install a single SPD at the meter location, this solution is highly dependent on local codes and the approval of the AHJ.

Always check with your local electrical inspector! Flag for review