Find the minimum conductor cross-sectional area that keeps voltage drop inside your chosen limit, and get the combined IEC 60364-5-52 derating factor for ambient temperature, circuit grouping and installation method. Resistance is evaluated at your stated operating temperature and the reactive component is included in the sizing.
| Symbol | Meaning | Unit |
|---|---|---|
| Amin | Minimum conductor area for the voltage drop limit | mm² |
| Ib | Design current of the circuit | A |
| It | Tabulated ampacity for the installation method | A |
| Iz | Derated current-carrying capacity | A |
| k | Phase multiplier — √3 three phase, 2 single phase | — |
| ρT | Resistivity at operating temperature | Ω·mm²/m |
| α | Temperature coefficient — 0.00393 Cu, 0.00403 Al | 1/K |
| x′ | Reactance per metre from the formation table | Ω/m |
| ΔUmax | Permitted drop in volts | V |
| kT, kG, kI | Temperature, grouping and installation factors | — |
A 200 A distribution board is fed by a 120 m run of 95 mm² copper, laid in trefoil on open tray at a 40 °C conductor temperature, power factor 0.85, with a 5 % voltage drop allowance.
| Input | Unit | Accepted range | Default |
|---|---|---|---|
| Phase system | — | Single or three phase | Three phase |
| System voltage | V | > 0 | 415 |
| Design current Ib | A | > 0 | 200 |
| Power factor | — | > 0 and ≤ 1 | 0.85 |
| Route length | m (ft) | > 0 | 120 |
| Trial CSA | mm² (kcmil) | > 0 | 95 |
| Permitted drop | % | > 0 | 5 |
| Operating temperature | °C | −30 to 90, not exactly 0 | 40 |
| kT, kG, kI | — | > 0 and ≤ 1 | 0.87, 1.00, 1.00 |
| Ambient temperature | kT — 70 °C PVC | kT — 90 °C XLPE |
|---|---|---|
| 25 °C | 1.06 | 1.04 |
| 30 °C (reference) | 1.00 | 1.00 |
| 35 °C | 0.94 | 0.96 |
| 40 °C | 0.87 | 0.91 |
| 45 °C | 0.79 | 0.87 |
| 50 °C | 0.71 | 0.82 |
| 55 °C | 0.61 | 0.76 |
| Circuits in the group | kG — bunched, touching | kG — single layer on tray |
|---|---|---|
| 1 | 1.00 | 1.00 |
| 2 | 0.80 | 0.88 |
| 3 | 0.70 | 0.82 |
| 4 | 0.65 | 0.79 |
| 6 | 0.57 | 0.76 |
| 9 | 0.50 | 0.73 |
| 12 | 0.45 | 0.72 |
| CSA (mm²) | Approx. kcmil | Cu R at 20 °C (Ω/km) | Al R at 20 °C (Ω/km) |
|---|---|---|---|
| 2.5 | 4.9 | 6.880 | 11.280 |
| 4 | 7.9 | 4.300 | 7.050 |
| 6 | 11.8 | 2.867 | 4.700 |
| 10 | 19.7 | 1.720 | 2.820 |
| 16 | 31.6 | 1.075 | 1.763 |
| 25 | 49.3 | 0.688 | 1.128 |
| 35 | 69.1 | 0.491 | 0.806 |
| 50 | 98.7 | 0.344 | 0.564 |
| 70 | 138.1 | 0.246 | 0.403 |
| 95 | 187.5 | 0.181 | 0.297 |
| 120 | 236.8 | 0.143 | 0.235 |
| 150 | 296.0 | 0.115 | 0.188 |
| 185 | 365.1 | 0.093 | 0.152 |
| 240 | 473.6 | 0.072 | 0.118 |
| 300 | 592.1 | 0.057 | 0.094 |
| 400 | 789.4 | 0.043 | 0.071 |
Cable sizing is a three-part check and the largest answer wins. First, current-carrying capacity: the tabulated ampacity for the installation method, multiplied by the derating factors, must be at least the design current. Second, voltage drop: the conductor must be large enough to keep the drop within the permitted percentage over the route length. Third, short-circuit withstand: the adiabatic check k·S must exceed the fault energy over the protection clearing time. This calculator solves the voltage drop leg and reports the combined derating factor for the ampacity leg.
Derating factors reduce a cable's tabulated current-carrying capacity to account for conditions worse than the reference case. IEC 60364-5-52 gives three principal factors: ambient temperature correction, typically 0.87 at 40 °C for a 90 °C XLPE cable; grouping correction, which falls as more circuits share a route, around 0.80 for four circuits touching; and an installation method or thermal insulation factor. Multiply all three together and apply the product to the table value before comparing against your design current.
No. It sizes the conductor on voltage drop and reports the combined derating factor, but it does not compare your design current against a tabulated ampacity, because that requires the ampacity table for your specific cable construction and installation method. Take the derating product reported here, multiply it by the tabulated ampacity from IEC 60364-5-52 Annex B or your manufacturer catalogue, and confirm the result exceeds the design current. Use whichever is larger — that answer or the minimum CSA reported here.
Aluminium has about 65 % higher resistivity than copper, so for the same voltage drop an aluminium conductor needs roughly 1.6 times the cross-sectional area, usually one or two standard sizes larger. Against that, aluminium is substantially cheaper per metre and about a third of the weight, which matters on long runs and large sizes. Aluminium terminations need bimetallic lugs and correct torque because the oxide layer and higher creep make joints the usual failure point.
Above roughly 150 to 185 mm² the skin and proximity effects begin to raise the effective AC resistance measurably above the DC value, so a calculation based on DC resistivity starts to under-read losses and voltage drop. IEC 60287 defines the ys and yp factors that quantify this. In practice, engineers avoid the problem by running multiple smaller cables in parallel rather than one very large conductor, which also improves handling and termination.
Results are for estimation and preliminary design. Cable selection must satisfy current-carrying capacity, voltage drop and short-circuit withstand together. Verify against project specifications, cable manufacturer data and a qualified engineer before construction.
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