What Cable Size Do I Need for My Load Current?

Which checks decide the cable size?

Four checks set the conductor size. Two circuits with the same load current can need different conductors, because the limiting check changes with the installation. Consider a short copper feeder on open tray, a long feeder in a hot grouped conduit and a buried aluminium feeder. All three may carry the same load current, but ampacity may govern the first case, voltage drop the second, and installation or fault duty the third.

The table shows the basis for each check only. Take tabulated capacities, grouping factors and limits from the standard edition the project uses, or from the manufacturer's datasheet.

How do correction factors change the continuous current capacity?

Start with the design current and an ampacity method accepted by the project. Cable tables assume defined installation and temperature conditions. When the real conditions differ, correction factors are applied. The factors multiply: corrected capacity = tabulated capacity × ambient factor × grouping factor × other factors. For a background on ampacity and derating, see cable ampacity and derating.

• Reference ambient temperature is 30 °C for cables in air and 20 °C for buried cables (IEC 60364-5-52).
• At 40 °C ambient in air, the correction factor is 0.87 for PVC and 0.91 for XLPE (IEC 60364-5-52, Table B.52.14).
• Grouped loaded cables, thermal insulation, high ambient temperature and poor soil heat dissipation can all significantly reduce capacity.

The insulation sets the maximum conductor temperature. Normal operation is commonly cited at 70 °C for PVC and 90 °C for XLPE (IEC 60502-1 and manufacturer data), so the two materials do not have the same rating in the same installation. Grouping factors depend on the installation method column of the table, so do not copy a grouping factor from a rule of thumb. Take it from the standard or the manufacturer.

How does voltage drop change the answer on long routes?

Long routes can require a larger conductor even when thermal ampacity is adequate. Check the steady-state drop and, for motors, the starting performance. For a three-phase circuit, the usual approximation is ΔU = √3 × I × L × (R cos φ + X sin φ), where I is the load current, L the one-way length, R and X the resistance and reactance per unit length, and cos φ the power factor. Power factor and conductor reactance can matter on larger AC circuits. A worked method is in cable voltage drop calculation.

Resistance sets the drop, so the conductor size matters directly. The maximum DC resistance at 20 °C for plain copper class 1 and 2 conductors is 7.41 Ω/km at 2.5 mm², 1.15 Ω/km at 16 mm² and 0.193 Ω/km at 95 mm² (IEC 60228). Tinned or metal-coated conductors have slightly higher limits. The voltage-drop limit that applies depends on national rules, so confirm it for your project before choosing the size.

What fault and protection checks must the cable pass?

The conductor and the protective conductor must withstand the expected fault energy until the protective device clears the fault. The device must also disconnect within the required time and protect the cable against overload.

A quick adiabatic check uses S = I × √t / k, with S in mm², I in A and t in s. It is valid for faults up to about 5 s. The k values are 115 for copper with PVC insulation, 143 for copper with XLPE, 76 for aluminium with PVC and 94 for aluminium with XLPE (IEC 60364-5-54 and IEC 60364-4-43).

As a worked example, a 10 kA fault cleared in 0.4 s on copper with PVC gives S = 10,000 × √0.4 / 115 ≈ 55 mm². The next standard size, 70 mm², is the minimum that passes this check. For fast fuses and breakers, use the let-through I²t from the manufacturer instead. More on fault duty is in cable short-circuit rating.

What data do you need for a sizing calculation?

• Load current and load type, including any starting current
• Voltage, phase and frequency
• One-way route length
• Installation method and ambient air or soil conditions
• Number of grouped circuits
• Fault level, clearing time and the permitted voltage drop

Calculate the size rather than selecting it from the current alone. Use the same code basis for the tables and the correction factors, and document the limiting check so that a reviewer can follow it. Online rules of thumb are not a substitute for a documented calculation.

Frequently asked questions