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Size your circuit protective conductor using BS 7671 Table 54.7 or the adiabatic equation. Covers copper in cable, separate copper, and steel conduit — with instant next-standard-size output.
Table 54.7 applies when CPC and line conductor are the same copper material. Use adiabatic for accurate sizing when fault current is known.
Applies BS 7671 Table 54.7 minimum CPC size rules.
Minimum CPC Size
2.50 mm²
BS 7671 Table 54.7
Next Standard Size
2.5 mm²
use this size or above
Next step
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BS 7671 Regulation 543 provides two methods for sizing CPCs: the simplified table approach and the adiabatic equation for accurate sizing. Both are implemented here.
BS 7671 Table 54.7 gives minimum CPC sizes for installations where the CPC is made of the same material as the line conductor. For phase CSA up to 16mm², the CPC must be the same size. For 16–35mm² phase, the CPC is 16mm² minimum. Above 35mm², the CPC is half the phase conductor CSA. This method is conservative and does not require knowledge of the fault current.
The adiabatic equation is: S (mm²) = √(I² × t) / k. I is the fault current in amps; t is the disconnection time in seconds; k is a material-dependent constant. For copper CPCs within a cable (PVC insulated), k = 115. For separate copper conductors, k = 143. For steel conduit, k = 51. The equation assumes all heat is retained in the conductor — conservative for disconnection times under 5 seconds.
Flat twin-and-earth (T&E) cable typically has a CPC that is one standard size smaller than the line conductors (e.g. 1.5mm² CPC in 2.5mm² T&E). For most domestic final circuits this is acceptable because the fault current is limited by the loop impedance. However, for high-fault-current environments (very low Ze, short circuits), the adiabatic equation should confirm adequacy. If the equation gives a required size larger than the T&E CPC, a separate supplementary earth conductor must be installed.
Both methods give a theoretical minimum. In practice, always round up to the next standard cable size available (1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95mm²). The calculator does this automatically. Never specify a CPC smaller than the calculated minimum, and always verify that the CPC also meets the Zs compliance check (see the Earth Fault Loop Impedance calculator).
WORKED EXAMPLE
An electrician installs a 6mm² radial circuit with a 4mm² T&E CPC (as supplied in 6mm² flat T&E). The board Ze is 0.1Ω, circuit length 10m, fault current approximately 1,800A, disconnection time 0.4s. Is the 4mm² CPC adequate?
Phase conductor
6 mm²
CPC in cable
4 mm²
Fault current I
1,800 A
Disconnection time
0.4 s
Adiabatic min S
9.9 mm²
4mm² T&E CPC
Undersized
Outcome
Run separate 10mm² earth
T&E CPC undersized for this fault current — supplementary copper earth required
Always perform the adiabatic check when fault currents are high relative to the installed CPC size. T&E cable CPC sizes assume typical domestic fault currents. Refer to BS 7671 Regulation 543.1.3.
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Common questions about CPC sizing and BS 7671 Regulation 543.
A circuit protective conductor (CPC), commonly called the earth wire, is the conductor that connects exposed-conductive-parts of equipment to the main earthing terminal. Its purpose is to provide a low-impedance path for fault current so that the protective device (MCB, fuse, or RCD) operates quickly enough to protect against dangerous voltages appearing on metalwork. BS 7671 Regulation 543 covers the selection and erection of protective conductors. In flat twin-and-earth (T&E) cable, the bare or green/yellow core is the CPC.
A CPC (circuit protective conductor) provides a fault current return path for a specific circuit, connecting equipment back to the main earthing terminal. A main protective bonding conductor connects extraneous-conductive-parts (gas pipes, water pipes, structural steel) to the main earthing terminal to equalise potential. Supplementary bonding conductors are used in special locations such as bathrooms. CPCs run with each circuit; bonding conductors are generally run from the consumer unit earthing bar to each service entry point. Their sizing requirements are different — bonding is set in BS 7671 Table 54.8.
In standard flat twin-and-earth cable (to BS 6004), the CPC is one standard size smaller than the line conductor — 1mm² CPC with 1.5mm² line, 1.5mm² CPC with 2.5mm² line, and so on. This is acceptable for most circuits because the CPC does not carry load current in normal operation, only fault current for a fraction of a second. However, BS 7671 requires verification using the adiabatic equation (Regulation 543.1.3) or Table 54.7 that the CPC can carry the fault current without damage. For high-fault-current circuits, the T&E CPC may be undersized and a separate earth conductor required.
Use the adiabatic equation (S = √(I²t) / k) when: the protective device does not operate within the prescribed disconnection time; the circuit has an unusually high fault current (low Zs); you are using a non-standard conductor material or insulation; or the CPC is not the same material as the line conductor. Table 54.7 is a simplified check assuming the protective device operates within the time limits of BS 7671. The adiabatic equation confirms that the thermal energy let through during the actual disconnection time does not damage the CPC insulation.
Yes, steel conduit can be used as a CPC under BS 7671 Regulation 543.2.1(iii), provided its continuity and adequacy for the fault current are verified. The k factor for steel is much lower than copper (51 vs 115–143), meaning a larger cross-sectional area is needed to carry the same fault current safely. Steel conduit impedance must be measured and included in the Zs calculation. In practice, most modern installations use a separate copper CPC within the conduit rather than relying on the conduit itself, due to the difficulty of maintaining continuous low-impedance joints at every coupler.
The adiabatic equation S = √(I²t) / k gives the minimum CPC cross-sectional area in mm². S is the minimum area, I is the fault current in amps, t is the disconnection time in seconds, and k is a material constant that accounts for the thermal capacity and resistivity of the conductor. For copper within a cable (PVC insulation), k = 115. For a separate copper conductor, k = 143. For steel, k = 51. The equation assumes all the heat from the fault is retained in the conductor (adiabatic = no heat loss), which is conservative at the short disconnection times used for final circuits.