Fault protection

Adiabatic equation — CPC sizing

Calculates the minimum cross-sectional area of a protective conductor from the prospective earth-fault current and the actual disconnection time of the protective device. Always round up to the next standard conductor size.

Prospective fault current I

Disconnection time t

k value

Results

k applied115

Minimum calculated S4.40mm²

Next standard size6.0mm²

Formula

S = √(I² × t) ÷ k
where I is fault current (A), t is disconnection time (s), k from Tables 54.2–54.6.

Reference

BS 7671:2018+A4:2026 Regulation 543.1.3 and Tables 54.2 to 54.6. Only valid for disconnection times up to 5 s; longer clearance times require the full thermal calculation.

Disclaimer — no liability

These calculators are provided for guidance and educational use only. All results must be verified by a competent person against BS 7671:2018+A4:2026, manufacturer data and the specific site conditions before any design, installation or certification decision is made.

All Cabling Limited accepts no liability whatsoever for any loss, damage, injury, cost or non-compliance arising directly or indirectly from use of, or reliance on, these tools. By using them you accept these terms in full.