BS 7671 design tool

Adiabatic CPC size calculator

Calculate the minimum protective conductor (CPC / earth) size using the adiabatic equation from BS 7671 Regulation 543.1.3. Enter the prospective fault current, disconnection time, and conductor material.

Circuit inputs

Results

Minimum CPC size (S)16.50 mm²

S = (If² × t)^0.5 ÷ k = (3000² × 0.4)^0.5 ÷ 115 = 16.50 mm²
k = 115 for Cu/PVC — BS 7671 Table 54.2

Next standard size ≥ minimum25 mm²

First standard cable size from BS 7671 Appendix 4 that satisfies S ≥ 16.50 mm²

Proposed CPC check — 2.5 mm²Fail — 2.5 mm² is less than the 16.50 mm² minimum

Minimum standard size required: 25 mm²

Assumptions and limitations

What this tool calculates

  • Minimum CPC cross-sectional area using BS 7671 Reg 543.1.3: S = (I²t)^0.5 ÷ k
  • k values from BS 7671 Tables 54.2–54.4 for copper and aluminium conductors

Known limitations

  • The adiabatic equation assumes worst-case energy let-through — for current-limiting devices (Type 2 coordination) the actual energy may be lower
  • Does not verify that disconnection will actually occur within the stated time — use the Zs tool for that check
  • k values for conductors forming part of a cable differ from separate conductors — refer to BS 7671 Table 54.5 for steel conduit/trunking

⚠️ Important notice

This is a preliminary design aid only. Verify all results against the current edition of BS 7671 and manufacturer data before carrying out or certifying any electrical work.

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