Cross-section of incoming cable to the distribution box

The cross-sectional area of the incoming cable to a distribution box depends on the total load current, installation method, and applicable derating factors, with a typical 60A single-phase load requiring a 16 mm² copper cable.

Determining Cable Size

The main factors for selecting the cross-sectional area of a cable include: Load Current (IB): The total current expected when all sub-circuits are in use. For example, a 60A single-phase distribution board requires a cable that can safely carry 60A continuously.

Current-

Carrying Capacity (IZ): The maximum current a cable can carry without exceeding its temperature rating. This depends on conductor material (copper or aluminum), insulation type (PVC, XLPE), and installation method (buried, in conduit, or on a cable tray). Voltage Drop: The cable must maintain voltage within acceptable limits over its length.

Longer runs may require a larger cross-section to reduce voltage drop.

Short-

Circuit Rating: The cable must withstand short-circuit currents for the duration of the protective device operation. Derating Factors: Adjustments for ambient temperature, grouping of cables, or installation conditions. For example, multiple cables in a bundle or high ambient temperatures reduce the effective current-carrying capacity, requiring a larger cross-section.

Practical Example

For a single-phase 230V distribution board rated at 60A, supplying lighting and socket circuits: Phase and Neutral cables: 16 mm² copper is typically used. Earth conductor: Usually sized according to protective device rating, often 16 mm² for a 60A board. Sub-circuits: Individual circuits (e.g., 10A, 16A, 20A) are connected with appropriately sized cables (e.g., 1.

5 mm² for lighting, 2. 5 mm² for sockets). Standards and ToolsIEC 60364-5-52 and BS7671 provide tables and formulas for calculating minimum cable sizes based on current, voltage drop, and installation conditions.

Online cable sizing calculators can simplify the process by considering load, installation method, ambient temperature, and derating factors.

Summary

To select the correct cross-sectional area for the incoming cable: Calculate the total load current of the distribution board. Determine the installation method and ambient conditions. Apply derating factors as needed.

Choose a cable whose IZ ≥ IB and meets voltage drop and short-circuit requirements. For a typical 60A single-phase board, a 16 mm² copper cable is commonly used, but larger loads or special installation conditions may require a larger cross-section. Always verify with standards and, if necessary, consult a qualified electrician.