Find the smallest twin & earth
Picks the smallest cable with Iz ≥ In ÷ (Ca × Cg × Ci) that also meets the voltage drop limit.
Leave a factor at 1 if it does not apply. Methods A and 100–103 already allow for the insulation they describe — use Ci only for cable fully surrounded by insulation (Table 52.2).
| Size | It (A) | Iz (A) | VD (V) | OK? |
|---|
Check against BS 7671 and the manufacturer's data; this is a guide, not a design tool.
Twin & earth current ratings (BS 7671 Table 4D5)
Current-carrying capacity It for flat twin & earth (6242Y and equivalent), single-phase, conductors at 70 °C, ambient 30 °C.
| Size | C | A | 100 | 101 | 102 | 103 | mV |
|---|---|---|---|---|---|---|---|
| 1.0 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 |
- C: clipped direct
- A: in conduit in a thermally insulated wall
- 100: above plasterboard ceiling, insulation up to 100 mm
- 101: above plasterboard ceiling, insulation over 100 mm
- 102: stud wall with insulation, cable touching inner wall
- 103: stud wall with insulation, cable not touching inner wall
How the cable size is chosen
BS 7671 Regulation 433.1.1 requires Ib ≤ In ≤ Iz: the design current must not exceed the device rating, and the device rating must not exceed the cable's current-carrying capacity after correction. Appendix 4 turns that into the check used here:
It ≥ In ÷ (Ca × Cg × Ci)
- It — tabulated current-carrying capacity from Table 4D5 for the installation method.
- Ca — ambient temperature factor (Table 4B1). 1.00 at 30 °C; less in hotter places such as lofts.
- Cg — grouping factor (Table 4C1) when cables run together or are bunched.
- Ci — thermal insulation factor (Table 52.2) when cable is surrounded by insulation and the installation method does not already cover it.
Then voltage drop
The cable must also keep the voltage drop within 3% for lighting or 5% for other uses (public supply). The calculator uses the design current Ib and the Table 4D5 mV/A/m. Example: a 32 A Type B radial clipped direct (method C), 25 m: 4 mm² has It 37 A, so it passes on current; voltage drop is 11 × 32 × 25 ÷ 1000 = 8.8 V, under 11.5 V — 4 mm² works.
What this does not check
It does not check earth fault loop impedance, the cpc size for adiabatic or Zs limits, a BS 3036 fuse (which needs the 0.725 factor), ring final circuits, or derating for several factors at once beyond the simple product. Check the max Zs and the manufacturer's data before you install.
Questions electricians ask
What size cable for a 32 A circuit?
Clipped direct (reference method C), 4 mm² twin and earth has a tabulated rating of 37 A and is often used on a 32 A radial; 2.5 mm² (27 A) is too small for a 32 A device on its own. If the cable passes through insulation or the run is long, a bigger cable may be needed — check with the correction factors and voltage drop.
What is the current rating of 2.5 mm² twin and earth?
From BS 7671 Table 4D5: 27 A clipped direct (method C), 20 A in conduit in an insulated wall (method A), 21 A above a plasterboard ceiling with up to 100 mm of insulation (method 100), and 13.5 A in an insulated stud wall not touching the inner face (method 103).
What are installation methods 100, 101, 102 and 103?
They are the BS 7671 methods for flat twin and earth in thermal insulation: 100 and 101 are above a plasterboard ceiling covered by insulation up to 100 mm or more than 100 mm; 102 and 103 are in a stud wall with insulation, with the cable touching or not touching the inner wall surface.
Do I apply Ci for method 101 or 103?
No. The ratings for methods A and 100 to 103 already allow for the thermal insulation they describe. Ci from Table 52.2 is for cable totally surrounded by insulation in a way the tabulated method does not cover.
More free tools for electricians
Max Zs table — MCB, RCBO and fuse limits + 80% check› Voltage drop calculator — mV/A/m × Ib × L, 3% / 5% limits›Test sheets on the phone
EIC, EICR and Minor Works with live Zs, insulation and RCD checks, printed on the official IET forms.
See Graftum for electriciansCheck against BS 7671 and the manufacturer's data; this is a guide, not a design tool.