Adiabatic Equation Calculator for CPC Sizing (BS 7671)

Use the adiabatic equation as an alternative to tabulated minimum CSA for protective conductors.

The adiabatic calculation is: s = √(I² · t) / k (s = I · √t / k)

Choosing a preset updates k below. You can still edit the value manually.
Minimum CSA; s:
—
mm²
; Note: If a disconnection time of less than 0.1 s is required, consult the protective device manufacturer’s data for suitability.
View common k values
The k values below match typical BS 7671 tables for adiabatic checks (conductor temp rise assumptions shown).
Material Insulation / Type Size band Initial / Final temp (°C) k

* Bare mineral; insulated (unsheathed): where touchable, use 115 instead of 135.

; Method: BS 7671 adiabatic method. Ensure the chosen k matches the conductor material, insulation system and temperature ratings.

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