Protective conductor
Adiabatic calculator (CPC size)
Check the minimum protective conductor size with the adiabatic equation, S = √(I²t) / k, from the fault current and the disconnection time.
S = √(I²t) / k from BS 7671 Reg 543.1.3, k from Tables 54.2 to 54.4 (copper). Use the fault current at the conductor and the actual disconnection time.
What the adiabatic check is for
A protective conductor has to survive the fault current that flows through it for as long as the device takes to disconnect, without its insulation cooking. The adiabatic equation gives the smallest conductor that can take that energy. It is the last check in a cable design, after current capacity and volt drop.
The equation
S = √(I²t) / k. I is the fault current in amps, t is the disconnection time in seconds, and k depends on the conductor material and its insulation. A CPC that is a core inside a cable (like the earth in twin and earth) has k 115; a separate copper CPC with 70°C PVC is 143; 90°C thermosetting is 176. Take the fault current at the conductor and the real disconnection time for the device, then round up to a standard size.
When it matters
On most twin-and-earth circuits the built-in CPC already passes, because the tables that set conductor sizes assume it. The check earns its keep on reduced CPCs, SWA where the armour is the CPC, and high fault levels. Get the fault current from the PFC calculator and the time from the max Zs check.
FAQs
What is the adiabatic equation?
S = root(I squared x t) divided by k, where I is the fault current in amps, t the disconnection time in seconds and k a factor for the conductor and its insulation. It gives the minimum protective conductor cross-section.
What k value should I use?
For a CPC that is a core within a cable (such as twin and earth), k is 115 for 70 C thermoplastic. A separate 70 C PVC CPC is 143, and 90 C thermosetting is 176. The values come from BS 7671 Tables 54.2 to 54.4 for copper.
Does twin and earth always pass the adiabatic check?
Usually, because the standard cable tables assume the integral CPC. The check matters most for reduced CPCs, SWA armour used as the CPC, and circuits with a high fault current.