Complete Guide to Electrical Cable Sizing
Choosing the right cable size is one of the most important decisions in any electrical installation. Get it wrong and you risk overheating, nuisance tripping, equipment malfunction, energy waste, or fire. This guide explains how engineers size cables and how this calculator applies those rules under IEC, BS 7671 and NEC.
Cable Size for Common Breaker Ratings (Quick Chart)
Smallest single-core cable whose current-carrying capacity covers the breaker rating, reference method C (clipped direct), 30°C ambient, one circuit, no voltage-drop allowance. Use the calculator above for your real conditions.
| Breaker (A) | Copper PVC (mm²) | Copper XLPE (mm²) | Aluminium XLPE (mm²) |
|---|---|---|---|
| 6 A | 1.5 | 1.5 | 16 |
| 10 A | 1.5 | 1.5 | 16 |
| 16 A | 1.5 | 1.5 | 16 |
| 20 A | 2.5 | 1.5 | 16 |
| 25 A | 4 | 2.5 | 16 |
| 32 A | 4 | 4 | 16 |
| 40 A | 6 | 4 | 16 |
| 50 A | 10 | 6 | 16 |
| 63 A | 16 | 10 | 16 |
| 80 A | 25 | 16 | 25 |
| 100 A | 25 | 16 | 25 |
| 125 A | 35 | 25 | 35 |
| 160 A | 70 | 35 | 70 |
| 200 A | 95 | 70 | 70 |
| 250 A | 120 | 70 | 120 |
The Two Tests Every Cable Must Pass
A conductor is only correctly sized when it satisfies both of the following. The larger of the two results is the size you install.
1. Current-carrying capacity (ampacity). The cable must carry the current without exceeding its insulation temperature. The tabulated ampacity is reduced by derating factors, and the result (Iz) must be at least the rating of the protective device (In).
2. Voltage drop. Resistance and reactance cause the voltage to fall along the run. Excessive drop dims lighting, weakens motor torque and trips controls. Limits are typically 3% for final circuits and 5% overall.
Breaker Coordination: Ib ≤ In ≤ Iz
- Ib — design (load) current the circuit must carry.
- In — nominal rating of the fuse or circuit breaker; chosen so In ≥ Ib.
- Iz — the cable's derated current-carrying capacity; must be ≥ In.
If Iz falls below In, the breaker could allow current the cable cannot handle, letting it overheat before the device trips. The calculator selects the breaker first, then the smallest standard cable whose derated ampacity clears it.
Derating (Correction) Factors
Temperature: hotter surroundings reduce how much heat the cable can shed. Factors come from IEC 60364-5-52 Table B.52.14 and NEC Table 310.15(B)(1).
Grouping / bunching: cables packed together in a conduit or tray warm each other. Factors come from IEC Table B.52.17 and NEC Table 310.15(C)(1) (more than three current-carrying conductors).
Installation method and insulation: XLPE (90°C) carries more current than PVC (70°C) for the same size, and clipped-direct runs carry more than cables enclosed in thermal insulation.
Cable Sizing in the UK, US and Australia
- UK (BS 7671): Appendix 4 tables follow the IEC method. Choose IEC / BS 7671 above; the values used are for single-core cables, reference method C.
- US (NEC): choose NEC (AWG). The tool uses Table 310.16, ambient correction, the adjustment for more than three current-carrying conductors, the 240.4(D) small conductor limits (14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A copper), the 125% continuous load rule, and caps 90°C ampacity at the 75°C termination rating.
- Australia and New Zealand (AS/NZS 3008.1.1): ratings assume a 40°C ambient air temperature. Select 40°C in IEC mode for an estimate, and verify against the AS/NZS 3008 tables for your installation method.
NEC Wire Size Chart (Copper, Table 310.16)
Allowable ampacities for not more than three current-carrying copper conductors at 30°C ambient. Small conductor overcurrent limits of 240.4(D) still apply.
| Size | ≈ mm² | 60°C (A) | 75°C (A) | 90°C (A) |
|---|---|---|---|---|
| 14 AWG | 2.08 | 15 | 20 | 25 |
| 12 AWG | 3.31 | 20 | 25 | 30 |
| 10 AWG | 5.26 | 30 | 35 | 40 |
| 8 AWG | 8.37 | 40 | 50 | 55 |
| 6 AWG | 13.3 | 55 | 65 | 75 |
| 4 AWG | 21.2 | 70 | 85 | 95 |
| 3 AWG | 26.7 | 85 | 100 | 115 |
| 2 AWG | 33.6 | 95 | 115 | 130 |
| 1 AWG | 42.4 | 110 | 130 | 145 |
| 1/0 AWG | 53.5 | 125 | 150 | 170 |
| 2/0 AWG | 67.4 | 145 | 175 | 195 |
| 3/0 AWG | 85 | 165 | 200 | 225 |
| 4/0 AWG | 107.2 | 195 | 230 | 260 |
| 250 kcmil | 126.7 | 215 | 255 | 290 |
| 300 kcmil | 152 | 240 | 285 | 320 |
| 350 kcmil | 177.3 | 260 | 310 | 350 |
| 400 kcmil | 202.7 | 280 | 335 | 380 |
| 500 kcmil | 253.4 | 320 | 380 | 430 |
| 600 kcmil | 304 | 350 | 420 | 475 |
Voltage Drop Formula Used
Three phase: VD = √3 × I × L × (R·cosφ + X·sinφ)
R = ρ ÷ A (Ω/m), ρCu ≈ 0.0225, ρAl ≈ 0.036 Ω·mm²/m (at operating temp)
X ≈ 0.00008 Ω/m, A = cross-section (mm²), L = one-way length (m)
VD% = (VD ÷ system voltage) × 100. For a dedicated voltage-drop check, use the Voltage Drop Calculator.
Copper vs Aluminium
Copper conducts better, so for a given current it needs a smaller cross-section and produces less voltage drop, which suits final circuits and tight routes. Aluminium is lighter and cheaper per amp, which makes it popular for large feeders and risers, but it typically needs one to two sizes larger and careful terminations.
IEC / BS 7671 Cable Sizes and Ampacities
Reference values used by this tool (single-core cables, reference method C, copper, 30°C, single circuit):
| Size (mm²) | ≈ AWG / kcmil | PVC (A) | XLPE (A) | Typical use |
|---|---|---|---|---|
| 1.5 | 16 | 17.5 | 23 | Lighting circuits |
| 2.5 | 14 | 24 | 31 | General socket outlets |
| 4 | 12 | 32 | 42 | Heavy appliances |
| 6 | 10 | 41 | 54 | Sub-circuits, AC units |
| 10 | 8 | 57 | 75 | Cookers, large motors |
| 16 | 6 | 76 | 100 | Sub-mains / feeders |
| 25 | 4 | 101 | 133 | Distribution feeders |
| 35 | 2 | 125 | 164 | Feeders |
| 50 | 1/0 | 151 | 198 | Main feeders |
| 70 | 2/0 | 192 | 253 | Sub-mains |
| 95 | 3/0 | 232 | 306 | Risers |
| 120 | 250 kcmil | 269 | 354 | Risers / incomers |
| 150 | 300 kcmil | 309 | 407 | Main incomers |
| 185 | 350 kcmil | 353 | 464 | Main incomers |
| 240 | 500 kcmil | 415 | 546 | Transformer / main runs |
Reference figures for guidance. Actual ampacity depends on the installation method, cable construction and local code — always verify against the manufacturer's data and applicable regulations.
Worked Example
A 3-phase 415 V motor draws 50 A, fed by a 60 m copper PVC cable, 40°C ambient, with three other circuits bunched (4 in total), 3% drop allowed, PF 0.85:
- Breaker In = 50 A (the next standard rating at or above 50 A).
- Derating = temperature 0.87 × grouping (4 circuits) 0.65 = 0.566.
- Ampacity: needs tabulated ≥ 50 ÷ 0.566 ≈ 88 A → 25 mm² (101 A × 0.566 = 57.1 A) clears it.
- Voltage drop over 60 m with 25 mm² is about 4.2 V (1.0%), well within 3%, so ampacity governs: 25 mm².
Enter these values above to see the full result and which criterion governs. To start from kW instead of amps, use the kW to Cable Size Calculator.
Common Mistakes to Avoid
- Sizing to the load current but forgetting the breaker rating (In, not Ib, must be covered by Iz).
- Ignoring derating in hot plant rooms or congested trays.
- Checking ampacity but not voltage drop on long runs — drop usually governs beyond about 30–50 m.
- Using the NEC 90°C column without checking the 75°C termination rating.
- Forgetting that voltage drop accumulates across feeder and sub-circuit.