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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)

Fault Current (A)

Disconnection Time (s)

k Preset (material / insulation / size band)

Choosing a preset updates k below. You can still edit the value manually.

Factor k (material/insulation)

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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Scott Little is an expert in electrical services with over 18 years of hands-on experience. Specialising in testing and inspection, he is committed to ensuring the highest standards of safety and efficiency for your electrical systems.His extensive background allows him to diagnose and address a wide range of electrical issues with precision and expertise.

In addition to testing and inspection, his business offers a comprehensive range of services including subcontracting, rewires, smoke alarm installations, consumer unit upgrades and fault finding. Whether it's a routine inspection, a complete rewire, or a critical upgrade, he brings a meticulous eye and a team with dedication to excellence to every job. Your safety and satisfaction are his top priorities, and he takes pride in delivering reliable, high-quality services that you can count on. Let’s work together to keep your electrical systems running smoothly and safely!