The three checks every cable has to pass
BS 7671 (the IET Wiring Regulations) sizes a cable with three checks, in this order:
- Design current (Ib). What the circuit actually draws. A 9.5 kW shower at 230 V draws 9,500 ÷ 230 = 41.3 A.
- Protective device (In). The breaker (MCB or RCBO) must be at least Ib. Common ratings are 6, 10, 16, 20, 25, 32, 40, 45, 50 and 63 A, so the shower needs a 45 A device.
- Cable rating (Iz). The cable's current-carrying capacity, for the way it's installed and after any correction factors, must be at least In.
That's the rule written as Ib ≤ In ≤ Iz. On top of that the voltage drop at Ib must stay within the limit for the circuit, and the earth fault loop impedance must be low enough for the breaker to disconnect in time (your electrician checks that with the actual Zs).
Installation method changes everything
The same cable carries much less current when it can't lose heat. BS 7671 Table 4D5 gives flat twin and earth ratings for each reference method. These are the figures the Cable Sizing Calculator uses:
| Cable | C clipped / in plaster | A conduit in insulated wall | 100 above ceiling ≤100 mm insulation | 101 above ceiling >100 mm | 102 stud wall, touching face | 103 stud wall, not touching | Volt drop mV/A/m |
|---|---|---|---|---|---|---|---|
| 1 mm² | 16 | 11.5 | 13 | 10.5 | 13 | 8 | 44 |
| 1.5 mm² | 20 | 14.5 | 16 | 13 | 16 | 10 | 29 |
| 2.5 mm² | 27 | 20 | 21 | 17 | 21 | 13.5 | 18 |
| 4 mm² | 37 | 26 | 27 | 22 | 27 | 17.5 | 11 |
| 6 mm² | 47 | 32 | 34 | 27 | 35 | 23.5 | 7.3 |
| 10 mm² | 64 | 44 | 45 | 36 | 47 | 32 | 4.4 |
| 16 mm² | 85 | 57 | 57 | 46 | 63 | 42.5 | 2.8 |
Look at 2.5 mm²: 27 A clipped to a joist, but only 13.5 A when it's buried in an insulated stud wall away from the plasterboard. Get the method wrong and the cable can be undersized by half.
Correction factors: Ca, Cg and Ci
The table ratings assume 30 °C and a cable on its own. Where that isn't true, divide the breaker rating by the factors to find the tabulated rating you need: It ≥ In ÷ (Ca × Cg × Ci).
- Ca, ambient temperature (Table 4B1): 0.94 at 35 °C, 0.87 at 40 °C, 0.79 at 45 °C. Think loft spaces in summer or near a boiler.
- Cg, grouping (Table 4C1): cables bunched together heat each other. Two circuits 0.80, three 0.70, four 0.65, six 0.57.
- Ci, thermal insulation: a cable totally surrounded by insulation for more than 0.5 m can drop to half its clipped rating (Table 52.2). Methods 100 to 103 already allow for insulation, so don't apply Ci on top of them.
Example: a 20 A radial clipped direct but bunched with two others (Cg 0.70). 2.5 mm² is 27 × 0.70 = 18.9 A, less than 20 A, so it fails. 4 mm² is 37 × 0.70 = 25.9 A and passes.
Voltage drop: 3% for lighting, 5% for everything else
BS 7671 Appendix 4 suggests a maximum volt drop, on a public supply, of 3% for lighting (6.9 V at 230 V) and 5% for other uses (11.5 V). Work it out from the table's mV/A/m figure:
Volt drop (V) = mV/A/m × Ib × length (m) ÷ 1000
On long runs this, not the current rating, decides the size. A 20 A radial to a garage 35 m away on 2.5 mm²: 18 × 20 × 35 ÷ 1000 = 12.6 V, which is 5.5%, over the 5% limit. 4 mm² gives 11 × 20 × 35 ÷ 1000 = 7.7 V (3.3%), so the garage needs 4 mm² even though 2.5 mm² could carry the current.
Worked examples
| Circuit | Ib | Breaker (In) | Method, length | Cable | Rating Iz | Volt drop |
|---|---|---|---|---|---|---|
| 9.5 kW electric shower | 41.3 A | 45 A | C (clipped / in plaster), 12 m | 6 mm² | 47 A | 3.62 V (1.6%) |
| Same shower, stud wall touching the face | 41.3 A | 45 A | 102, 12 m | 10 mm² | 47 A | 2.18 V (0.9%) |
| 11 kW cooker with socket (after diversity) | 26.3 A | 32 A | C, 15 m | 4 mm² | 37 A | 4.35 V (1.9%) |
| Same cooker, above a ceiling under 270 mm insulation | 26.3 A | 32 A | 101, 15 m | 10 mm² | 36 A | 1.74 V (0.8%) |
| Garage radial | 20 A | 20 A | C, 35 m | 4 mm² (volt drop) | 37 A | 7.70 V (3.3%) |
| Lighting circuit | 4 A | 6 A | 101, 30 m | 1.0 mm² | 10.5 A | 5.28 V (2.3%, limit 3%) |
Two lessons stand out. The shower in a stud wall where the cable doesn't touch the plasterboard (method 103) has no answer at all up to 16 mm²: the cable has to be rerouted. And the cooker run that goes over an insulated ceiling needs 10 mm² where 4 mm² would do clipped to a joist. Routing the cable to avoid insulation is often cheaper than upsizing it.
The cooker figure uses the usual diversity for a household cooker: the first 10 A, plus 30% of the rest, plus 5 A if the cooker control unit has a socket. 11,000 W ÷ 230 V = 47.8 A, so Ib = 10 + 0.3 × 37.8 + 5 = 26.3 A.
What about ring final circuits?
A 32 A ring in 2.5 mm² twin and earth is allowed by Regulation 433.1.204 as long as the cable's rating for its installation method is at least 20 A. That's fine clipped direct (27 A) or above a ceiling with up to 100 mm of insulation (21 A), but not above a ceiling under thicker insulation (method 101, 17 A) or in a stud wall away from the face (method 103, 13.5 A). For volt drop on a ring with the load roughly in the middle, take a quarter of the figure you'd get running the whole ring length as a radial.
The Cable Sizing Calculator works out radials, so for a ring check those two points by hand, and size any spurs as radials.
Get it checked
This guide and the calculator are design aids, not a substitute for a qualified electrician. New circuits, consumer unit changes and work in bathrooms are notifiable under Part P of the Building Regulations in England and Wales, and must be designed, installed, tested and certified by a competent person to the edition of BS 7671 in force (currently BS 7671:2018+A3:2024). Earth fault loop impedance, RCD protection, the type of breaker and the cable route all affect the final choice.