Cable size calculator: mm² for any circuit
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Circuit | Electric shower |
| Supply | Single phase |
| Load | 9.5 kW |
| Power factor | 1 |
| Protective device | Type B MCB or RCBO |
| Device rating | 45 A |
| Cable type | Flat twin and earth (6242Y) |
| Installation | C: clipped direct to a surface |
| Cable length | 15 m |
| Thermal insulation | Not in insulation |
| Ambient temperature factor | 1 × |
| Grouping factor | 1 × |
| Thermal insulation factor | 1 × |
| Device type factor | 1 × |
| Ratings from your cable maker's data | Size 1: size 6 mm², current rating for your method 47 A, mv/a/m 7.3 mV/A/m; Size 2: size 10 mm², current rating for your method 64 A |
| Voltage drop limit | 5 % |
| Supply voltage | 230 V |
Smallest size that passes: 6 mm² (indicative)
- Within the guidance Design current within the device rating (Ib ≤ In): 41.304 A UK-wide: BS 7671:2018+A4:2026 Reg 433.1.1. Check with the source
- Within the guidance Device rating within the cable rating (In ≤ Iz): 45 A UK-wide: BS 7671:2018+A4:2026 Reg 433.1.1. Check with the source
- Within the guidance Voltage drop: 1.966 % UK-wide: BS 7671:2018+A4:2026 Appendix 12. Check with the source
| Result | Value |
|---|---|
| Smallest size that passes (indicative) | 6 mm² |
| Design current (Ib) | 41.3 A |
| Current rating needed (It = In ÷ factors) | 45 A |
| Tabulated rating of that size | 47 A |
| Rating after correction factors (Iz) | 47 A |
| Voltage drop | 4.52 V |
| As a share of the supply | 1.97% |
| Size (mm²) | Tabulated rating (A) | Covers It? |
|---|---|---|
| 6 | 47 | Yes |
| 10 | 64 | Yes |
Show the working
- Design current Ib: 9,500 W ÷ (230 V × 1) = 41.3 A
- Rating needed It: 45 A ÷ (1 × 1 × 1 × 1) = 45 A
- Sizes checked (your maker's figures): 6 mm² 47 A, 10 mm² 64 A
- Smallest size with a tabulated rating of at least 45 A: 6 mm² (47 A)
- Iz after the factors: 47 A × 1 = 47 A
- Voltage drop: 7.3 mV/A/m × 41.3 A × 15 m ÷ 1000 = 4.52 V (1.97% of 230 V)
How it works
- Design current Ib = P ÷ (V × pf), or P ÷ (√3 × V × pf) for three phase
- Rating needed It = In ÷ (Ca × Cg × Ci × Cc): ambient, grouping, thermal insulation and device-type factors
- Choose the smallest size whose tabulated rating is at least It, then check Ib ≤ In ≤ Iz (Iz = tabulated rating × the factors)
- Voltage drop = mV/A/m × Ib × one-way length ÷ 1000, compared with the Appendix 12 limit
- Smallest protective conductor S = √(I² × t) ÷ k (adiabatic equation)
The 0.5 factor is the rule for a cable surrounded by thermal insulation for more than 0.5 m; shorter runs have their own factors in Appendix 4.
Indicative only: an electrician checks the design against BS 7671 and the cable maker's data, including fault protection and disconnection times.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Voltage drop limit: Custom circuit | 5 % | Regulation or HSE figure BS 7671:2018+A4:2026 Appendix 12 (other uses) Not yet checked against the source | Source to be added by the Researcher Brief v2 E2. Researcher to confirm against A4:2026. |
| Voltage drop limit: Lighting | 3 % | Regulation or HSE figure BS 7671:2018+A4:2026 Appendix 12 (lighting) Not yet checked against the source | Source to be added by the Researcher Brief v2 E2. Researcher to confirm against A4:2026. |
| Supply voltage: Single phase | 230 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal single-phase supply voltage (brief v2 E2). Researcher to cite the statutory source. |
| Supply voltage: Three phase | 400 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal three-phase line voltage (brief v2 E2). Researcher to cite the statutory source. |
| Device type factor: BS 3036 rewireable fuse | 0.725 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 E1: Cc for a BS 3036 semi-enclosed (rewireable) fuse. Researcher to cite BS 7671:2018+A4:2026 Appendix 4. |
| Thermal insulation factor: Surrounded for more than 0.5 m | 0.5 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 E1: the 0.5 rule for cable surrounded by thermal insulation for more than 0.5 m (Reg 523.9). Researcher to confirm against A4:2026. |
Regulations and standards
- BS 7671:2018+A4:2026 Reg 433.1.1 (Ib ≤ In ≤ Iz)
- BS 7671:2018+A4:2026 Appendix 4 (current ratings, correction factors, mV/A/m)
- BS 7671:2018+A4:2026 Reg 523.9 (cables in thermal insulation)
- BS 7671:2018+A4:2026 Reg 543.1.3 (adiabatic equation for protective conductors)
- BS 7671:2018+A4:2026 Appendix 12 (voltage drop limits)
Explained: BS 7671 (the IET Wiring Regulations), Approved Document P (electrical safety), The IET On-Site Guide.
BS 7671 is used across the UK. Who may do the work differs: Part P applies in England and Wales; Scotland and Northern Ireland have their own rules.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Tabulated current-carrying capacity (It) by cable, reference method and size | BS 7671:2018+A4:2026 Appendix 4 (Table 4D5 for flat twin and earth; Researcher to confirm the tables for SWA and flex) | UK-wide | 0/75 values entered Not yet checked against the source |
| Voltage drop (mV/A/m) for flat twin and earth cable | BS 7671:2018+A4:2026 Appendix 4, Table 4D5 | UK-wide | 0/7 values entered Not yet checked against the source |