Cable Size Calculation

About Cable Size Calculation

This calculator determines the recommended cable size for copper or aluminum conductors based on electrical load, either specified directly as current (A) or derived from power (W). It takes into account supply voltage, number of phases, power factor, line length, permissible voltage drop, temperature, and installation conditions. The result helps estimate a suitable conductor size for low-voltage power circuits.

Tips and Tricks

Step 1 - Define the electrical load. The calculation starts either from the specified load current I (A) or from active power P (W). If power is given, the current is calculated using I = P / (U × cosφ) for single-phase systems and I = P / (√3 × U × cosφ) for three-phase systems, where U is voltage (V) and cosφ is the power factor.

Step 2 - Recalculate active power if needed. When current is entered directly, the calculator derives the corresponding active power using the inverse relations P = U × I × cosφ (single-phase) or P = √3 × U × I × cosφ (three-phase). This ensures consistency between current and power results.

Step 3 - Account for installation and grouping factors. Permissible current for a conductor depends on how it is installed. For grouped cables, the calculator applies a reduction factor k to the allowable current. The effective required current becomes Ireq = I / k, which may lead to a larger cross-section.

Step 4 - Select cross-section by current capacity. Using tabulated permissible currents for copper or aluminum conductors, the calculator finds the smallest standard cross-section S (mm²) whose allowable current is not less than Ireq. This approach reflects common practice in IEC 60364-5-52 and related European wiring standards.

Step 5 - Check voltage drop. For each candidate cross-section, the voltage drop is calculated as ΔU = k × I × (R × cosφ + X × sinφ), where R and X are the active resistance and reactance of the line over its full length. The calculator ensures that ΔU does not exceed the permissible percentage of the nominal voltage.

Step 6 - Temperature correction. Conductor resistance increases with temperature. The calculator corrects resistance using a linear coefficient relative to 20 °C, which increases the calculated voltage drop and may influence the final cross-section choice.

Selection logic. The final recommended cross-section is the smallest standard size that satisfies both conditions: permissible current capacity and allowable voltage drop. If either condition is more restrictive, it governs the result.

Typical reference values. For household circuits, cosφ values around 0.95 are common for mixed loads, while purely resistive loads approach 1. Permissible voltage drop is often taken as 3-5% for final circuits. These assumptions align with common European practice under IEC 60364.

FAQs

Why does the calculator sometimes recommend a larger cable than expected?

This usually happens when voltage drop, installation method, grouping, or elevated temperature becomes the limiting factor rather than current capacity alone.

What is the difference between copper and aluminum results?

Aluminum has higher resistivity and lower permissible current for the same cross-section. As a result, aluminum cables generally require a larger cross-section than copper for the same load.

Is the line length entered one-way or round-trip?

The entered length is the one-way distance from the source to the load. The calculator automatically accounts for the full current path when evaluating resistance and voltage drop.

How important is the power factor in cable sizing?

A lower power factor increases current for the same power, which raises both thermal loading and voltage drop. Ignoring cosφ can therefore lead to undersized conductors.

Can this calculator replace a full electrical design?

No. It provides a reliable preliminary estimate. Final cable selection should also consider short-circuit withstand, protective device coordination, national regulations, and manufacturer data.