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 exactly how professional engineers size cables, and how this calculator applies those rules using NEC and IEC standards.
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 safely 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
This single inequality underpins safe sizing:
- 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 can't handle, letting it overheat before the device trips. The calculator above selects the breaker first, then the smallest standard cable whose derated ampacity clears it.
Derating (Correction) Factors
Catalogue ampacities assume about 30°C ambient and a single circuit. Real installations are rarely ideal, so the rated value is multiplied by correction factors:
Temperature: hotter surroundings reduce how much heat the cable can shed. Factors come from NEC Table 310.15(B)(1) / IEC 60364-5-52 Table B.52.14.
Grouping / bunching: cables packed together in a conduit or tray warm each other. Reduction factors come from NEC Table 310.15(C)(1) / IEC Table B.52.17.
Installation method & insulation: XLPE (90°C) carries more current than PVC (70°C) for the same size; clipped-direct runs carry more than buried-in-insulation runs.
Voltage Drop Formula Used
This tool uses the AC voltage-drop relationship including power factor and an allowance for conductor reactance, rather than resistance alone:
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. Using power factor and reactance matters on large feeders, where ignoring them can underestimate the drop.
Copper vs Aluminium
Copper conducts better, so for a given current it needs a smaller cross-section and produces less voltage drop — ideal for 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. The calculator includes standard ampacities for both (aluminium from 16 mm² upward).
Standard Cable Sizes & Ampacities
Reference values used by this tool (IEC 60364-5-52, method C, copper at 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 | 151 | 198 | Main feeders |
| 70 | 2/0 | 192 | 253 | Sub-mains |
| 95 | 3/0 | 232 | 306 | Risers |
| 120 | 4/0 | 269 | 354 | Risers / incomers |
| 150 | 300 | 309 | 407 | Main incomers |
| 185 | 350 | 353 | 464 | Main incomers |
| 240 | 500 | 415 | 546 | Transformer / main runs |
These are reference figures for guidance. Actual ampacity depends on the exact 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, 3% drop allowed, PF 0.85:
- Breaker In = 63 A (next standard ≥ 50 A).
- Derating = temp 0.87 × grouping (4 circuits) 0.65 ≈ 0.57.
- Cable for current: needs Iz ≥ 63 A, i.e. tabulated ≥ ~111 A → 35 mm² (125 A × 0.57 ≈ 71 A) clears it.
- Cable for voltage drop over 60 m is then checked; the larger of the two governs.
Enter these values above to see the full result and which criterion governs.
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 ~30–50 m.
- Mixing aluminium conductors with copper-rated lugs.
- Forgetting that voltage drop accumulates across feeder + sub-circuit.