Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
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This calculator applies the adiabatic equation from BS 7671 to check that a circuit protective conductor is large enough to survive an earth fault without overheating. It is aimed at UK electricians verifying reduced-CSA CPCs — such as the smaller earth in flat twin and earth — against the actual fault current and disconnection time.
Enter the prospective earth fault current, the disconnection time of the protective device and the k value for the conductor type, and the tool returns the minimum conductor cross-sectional area. It also rounds up to the next standard size from 1mm² to 50mm², since that is what you can actually install.
S = √(I²t) / k
S is the minimum conductor cross-sectional area in mm². I is the fault current in amps that will flow during an earth fault, t is the disconnection time of the protective device in seconds, and k is a factor from the BS 7671 tables (54.2/54.3) that accounts for the conductor material and insulation temperature limits — 115 for a copper CPC incorporated in a PVC cable, 143 for a separate or bare PVC-covered copper conductor, and 100 for a 90°C thermosetting cable. The equation checks that the energy let through during the fault (I²t) does not raise the conductor above its limit temperature.
Type in the earth fault current in amps — typically calculated from the loop impedance (I = 230 / Zs), or use the prospective fault current calculator.
Enter the time in seconds for the device to disconnect at that fault current. 0.4s is the default; for MCBs operating in their instantaneous region the actual time is much shorter, which makes 0.4s a conservative check.
Pick the k factor matching the CPC: 115 for a CPC within a PVC cable (the twin and earth case), 143 for a separate PVC-covered or bare conductor, or 100 for 90°C thermosetting cable.
The tool shows the calculated minimum CSA and the next standard conductor size up. If your installed CPC is at least that size, it passes the adiabatic check.
The adiabatic equation is the BS 7671 method for verifying protective conductor sizing where the CPC is smaller than the line conductor, which is normal for flat twin and earth. The alternative — selecting the CPC from the standard-sizes table — often demands a larger conductor than the calculation proves necessary.
The check assumes all fault energy heats the conductor with no time for heat to dissipate, which is valid for disconnection times up to around 5 seconds and is conservative for the very short trip times of MCBs. Using a realistic I²t (from the device energy let-through data) rather than a worst-case time gives a sharper answer for critical cases.
If the result comes out above 50mm², or your installed CPC fails the check, revisit the design: a lower loop impedance, a different device or a larger CPC all change the outcome. Guidance only — verify against BS 7671 and the On-Site Guide for the actual installation method.
Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
It verifies that a conductor — most often the CPC — can carry the earth fault current for the time the protective device takes to disconnect without exceeding its insulation temperature limit. It is how you justify the reduced-size earth in twin and earth cable rather than defaulting to a full-size protective conductor.
Use k = 115, the value for a copper protective conductor incorporated in a cable with 70°C PVC insulation. A separate PVC-covered or bare copper conductor uses 143, and conductors in 90°C thermosetting cable use 100. The k value comes from Tables 54.2 and 54.3 of BS 7671.
Usually, yes — run the numbers for your circuit. For example, a 1150A fault cleared in 0.4s with k = 115 gives S = √(1150² × 0.4) / 115 ≈ 6.3mm² only if the device really takes 0.4s; an MCB tripping instantaneously clears far faster, so the true I²t is much lower. Always check with the actual fault current and realistic disconnection time for the circuit.
Use the earth fault current at the point being checked, normally calculated as the nominal voltage divided by the earth fault loop impedance Zs. A higher fault current means faster disconnection but more energy per unit time, so use realistic values for both current and time rather than mixing worst cases inconsistently.
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