CPC size calculator (adiabatic equation)

Find the smallest protective conductor that can carry a fault current until the device disconnects.

Fault current (I)

Usually Uo ÷ Zs, e.g. 230 ÷ 0.23 Ω = 1000 A.

Disconnection time (t)

From the protective device's time/current curve at that fault current.

Conductor type (k)

Minimum CPC size

–mm²

Next standard size–

k used–

k = 115 is for a copper CPC inside a 70 °C thermoplastic cable (Table 54.3). k = 143 is for a separate copper CPC with 70 °C thermoplastic insulation (Table 54.2). Check against your copy of BS 7671 or the On-Site Guide before relying on a result.

The formula

S = √(I² × t) ÷ k

S is the minimum cross-sectional area in mm², I the fault current in amps, t the disconnection time in seconds, and k a factor for the conductor material and insulation. It comes from BS 7671 Regulation 543.1.3.

Worked example

A fault current of 1000 A cleared in 0.4 s, with the CPC inside twin and earth (k = 115): √(1000² × 0.4) ÷ 115 = 5.5 mm², so the next standard size up is 6 mm².