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| Application | Typical Cable | Protection | Max Length (VD) | | :--- | :--- | :--- | :--- | | Lighting | 1.0mm² or 1.5mm² T&E | 6A | ~50m | | Sockets (ring) | 2.5mm² T&E | 32A | ~40m | | Sockets (radial) | 4mm² T&E | 20A | ~50m | | Cooker (<15kW) | 6mm² T&E | 40A | ~30m | | Shower (<9.5kW) | 10mm² T&E | 40A | ~25m | | Garage submain | 6mm² SWA | 32A | ~60m |
6A, 10A, 16A, 20A, 32A, 40A, 50A.
The first step is knowing how much current the appliance or circuit will draw. This is the Design Current ( Ibcap I sub b Use the formula The Calculation: Example: A 7kW electric shower would be 2. Choose the Correct Protective Device (In) You must select a circuit breaker or fuse ( Incap I sub n ) that is greater than or equal to your design current ( Ibcap I sub b Rule: how to calculate cable size uk
), the cable must effectively be able to handle 64A to remain cool. 4. Select Cable Size via Current Rating Tables
The cable's tabulated current-carrying capacity ((I_t)) must satisfy: [ I_t \ge \fracI_nC_g \times C_a \times C_f ] | Application | Typical Cable | Protection |
values to ensure the circuit disconnects within the required time (usually 0.4 seconds for socket circuits).
For our 30.43A shower, the next standard breaker is 32A ( 3. Apply Correction Factors (Cf) Choose the Correct Protective Device (In) You must
" value is found in the BS 7671 tables for your specific cable size. If the result exceeds the limits above, you must jump up to the next cable size and re-calculate. 6. Verify Earth Fault Loop Impedance (Zs)
This is a solid, practical breakdown of how to calculate cable size for UK domestic and light commercial installations, based on .