Surge Protection Calculator

Determine the appropriate surge protection device (SPD) type and rating based on your panel size and equipment value.

Results

Visualization

How It Works

Surge protection devices (SPDs) divert voltage spikes away from your equipment. Surges come from lightning strikes, utility switching, and large motor loads. A whole-house SPD at the main panel is the first line of defense, with point-of-use protectors as a second layer for sensitive electronics.

The Formula

SPD Rating (kA) based on: panel size, lightning density, equipment sensitivity. Expected annual loss = equipment value x probability of surge damage.

Variables

  • kA — Kiloamperes -- the surge current the SPD can safely divert (higher is better)
  • Type 1 — Installed between utility meter and main breaker -- diverts surges before they enter the panel
  • Type 2 — Installed at the main panel (load side) -- most common residential SPD
  • VPR — Voltage Protection Rating -- the let-through voltage during a surge (lower is better)

Example

A 200A panel in a moderate lightning area with $10,000 in electronics: Recommended SPD = 50 kA Type 2. Cost ~$180 installed. Annual surge damage risk ~5% = $500 expected loss. SPD pays for itself in under 1 year.

Tips

  • The 2020 NEC (230.67) requires Type 1 or Type 2 surge protection for all new dwelling unit services.
  • SPDs have a limited lifespan -- they degrade with each surge event. Replace every 5-10 years or after a known lightning strike.
  • Layer your protection: whole-house SPD at the panel PLUS point-of-use surge strips for computers and entertainment systems.
  • Check your SPD has indicator lights showing protection status -- a dead SPD offers zero protection.
  • SPDs do not protect against direct lightning strikes -- they handle the far more common indirect surges and utility transients.