BS 7671 Adiabatic Equation Calculator

Compute minimum CPC size, maximum disconnection time, or permissible fault current using the BS 7671 adiabatic equation.

Calculation mode

Solve for S Solve for t Solve for I Check installed S

Conductor material & insulation (k)

Use custom k-value

Custom k

CPC size S (mm²)

Fault current I

Disconnection time t (s)

Installed S for check (mm²)

Computed S

—

Computed t—

Computed I—

Rounded recommendation—

Pass/Fail—

How to use

Select the mode (solve for S, t, I, or check). Enter the required inputs along with the conductor k-value. The tool computes the missing value and provides a rounded CPC size when applicable.

Methodology

The adiabatic equation relates fault current, disconnection time, k-value, and conductor area. S is in mm², I is in amperes, t is in seconds, and k is based on conductor material and insulation tables.

Worked example

Compute S from I, t, and k using the adiabatic equation.
For I = 3000 A, t = 0.4 s, k = 115, the calculated S is about 16.5 mm².
Round up to the next standard size; recommendation: 25 mm².

Formulas

\[ I^{2}t = k^{2}S^{2} \] \[ S = \frac{I\sqrt{t}}{k} \] \[ t = \left(\frac{kS}{I}\right)^{2} \] \[ I = \frac{kS}{\sqrt{t}} \]

Changelog

0.1.0-draft — 2026-01-19: Initial draft (review required).

Citations

Add authoritative sources relevant to this calculator (standards bodies, manuals, official docs).

✓ Verified by Ugo Candido

Last Updated: 2026-01-19

Version 0.1.0-draft