Surge Protection Calculator Formula

Understand the math behind the surge protection calculator. Each variable explained with a worked example.

Use the Surge Protection Calculator

Formulas Used

Clamping Voltage

clamping_voltage = v_clamp

Energy per Surge

energy_joules = surge_energy

System Peak Voltage

peak_system = v_peak

Peak Surge Power

peak_power_kw = v_clamp * surge_current_ka

Variables

VariableDescriptionDefault
system_voltageSystem Voltage (RMS)(V)230
surge_current_kaExpected Surge Current(kA)10
surge_duration_usSurge Duration(µs)20
clamping_ratioClamping Voltage Ratio2.5
v_peakDerived value= system_voltage * sqrt(2)calculated
v_clampDerived value= v_peak * clamping_ratiocalculated
surge_energyDerived value= v_clamp * surge_current_ka * 1000 * surge_duration_us * 1e-6calculated

How It Works

Surge Protection Device Sizing

SPDs must absorb surge energy while limiting voltage to a safe clamping level that protects downstream equipment.

Formulas

V_clamp = V_peak x Clamping Ratio

Energy = V_clamp x I_surge x Duration

The standard test waveform for SPDs is 8/20 microseconds (8 us rise, 20 us to half-value).

SPD Categories (IEC 61643)

  • Type 1: Lightning current (10/350 us), installed at service entrance
  • Type 2: Surge current (8/20 us), at distribution panel
  • Type 3: Combination wave, at point of use

    • Worked Example

    230 V system, 10 kA surge (8/20 us), MOV clamping ratio 2.5x.

    system_voltage = 230surge_current_ka = 10surge_duration_us = 20clamping_ratio = 2.5
    1. 01System peak: 230 x sqrt(2) = 325 V
    2. 02Clamping voltage: 325 x 2.5 = 813 V
    3. 03Energy: 813 x 10,000 x 20e-6 = 162.6 J
    4. 04Peak power: 813 x 10 = 8,130 kW

    Ready to run the numbers?

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