Check a protective conductor (CPC) by the adiabatic equation to BS 7671 Reg 543.1.3.
S = √(I²t) ÷ k. Enter the prospective fault current, disconnection time and installed CPC size.
For design guidance only. Always verify with a qualified engineer.
About this adiabatic cable check calculator
This free adiabatic cable check calculator works out the minimum cross-sectional area a protective conductor (CPC) needs to survive an earth fault, using the adiabatic equation from BS 7671:2018+A2:2022 Regulation 543.1.3. It is built for electricians, designers and inspectors who need a quick conductor-sizing check during design or verification. Enter the prospective earth-fault current, the protective device's disconnection time and the installed CPC size, and the calculator returns the required minimum size S with a pass or fail verdict. All calculation runs in your browser. Nothing is uploaded.
How the adiabatic cable check is calculated
The required minimum conductor size is S = √(I² × t) ÷ k, where I is the prospective earth-fault current in amperes, t is the protective device's disconnection time in seconds, and k is a material and insulation coefficient. The term I²t is the let-through energy the fault deposits in the conductor. The k value is read from BS 7671 Table 54.2 when the CPC is separate or not bunched (initial temperature 30 °C) and from Table 54.3 when it is incorporated in a cable or bunched (initial temperature equal to the conductor operating temperature). For copper this ranges from 115 (70 °C PVC, incorporated) to 176 (90 °C XLPE, separate). The installed CPC passes when its area is at least the calculated S. The method is valid for disconnection times up to 5 seconds; beyond that the equation becomes conservative because heat is lost to the surroundings.
Frequently asked questions
What is the adiabatic equation in BS 7671?
It is the method in Regulation 543.1.3 for sizing a protective conductor against thermal damage during a fault: S = √(I²t) ÷ k. It assumes all the fault energy heats the conductor with none lost to the surroundings, which holds for disconnection times of 5 seconds or less.
Which k value should I use?
Use Table 54.3 when the CPC is incorporated in a cable or bunched with other conductors, as it starts at the higher operating temperature and gives a lower k (the more onerous case). Use Table 54.2 for a separate, non-bunched CPC assumed to start at 30 °C. The calculator picks the correct k once you select the material, insulation class and installation condition.
Does this replace a full cable design?
No. The adiabatic check confirms a protective conductor will withstand the fault thermally, but a complete design must also satisfy current-carrying capacity, voltage drop and earth-fault loop impedance. These k values cover conductors up to 300 mm²; larger sizes use slightly lower figures. Always confirm the result with a qualified engineer.