Cable size calculator (twin and earth)
Pick the device rating and how the cable is installed. The smallest twin and earth that works shows as you go.
- V – Voltage
- I – Current
- P – Power
The MCB, RCBO or fuse rating.
Add this to check voltage drop too.
Leave blank to use the device rating.
Minimum cable size
Ratings are for 70 °C thermoplastic flat twin and earth (6242Y) from BS 7671 Table 4D5, single circuit, with correction factors from Tables 4B1, 4C1 and 52.2. Check against your copy of BS 7671 or the On-Site Guide before relying on a result.
How it works
It ≥ In ÷ (Ca × Cg × Ci × Cc)
It is the tabulated current the cable must be able to carry. In is the device rating. The factors reduce what a cable can carry: Ca for ambient temperature, Cg for grouping, Ci for thermal insulation and Cc for a rewireable fuse (0.725). The calculator then picks the smallest twin and earth whose Table 4D5 rating for your installation method is at least It.
The Ci factor only applies when the cable is clipped direct (Method C) but passes through insulation. Methods 100 to 103 already allow for insulation in their ratings.
Worked example
A 32 A radial clipped direct, 30 °C, not grouped: It = 32 ÷ 1 = 32 A, so 4 mm² (37 A) is the smallest that works. Over a 40 m run, 4 mm² drops 14.1 V, which is over the 11.5 V limit, so the calculator steps up to 6 mm² (9.3 V).
The calculator also checks the design current against the device rating, so Ib ≤ In ≤ Iz holds.