Cable cross-section — the three requirements

When choosing the cross-section, three independent requirements must be met — the largest of them determines the cross-section.

• 01Current-carrying capacity: the conductor must not get too hot. The permissible current depends on the installation method, grouping and ambient temperature and is looked up in the standard's tables.
• 02Voltage drop: ΔU must be kept below NORM limit (max. 3% lighting / 5% other, table G52.1).
• 03Short circuit: the conductor must be able to withstand the short-circuit current until the fuse trips.

Current-carrying capacity Iz is the current the conductor can carry continuously without exceeding its permitted operating temperature — 70 °C for PVC insulation, 90 °C for PEX/EPR. The basic value Iz0 is looked up in the standard's tables (B.52 series) based on conductor material, insulation, number of loaded conductors and REFERENCE installation method. The basic value is then corrected for the actual conditions on site.

Where and how the cable is laid determines how well the heat dissipates — and thus Iz0. The standard reduces countless real installations to a set of reference methods, each with its own letter (table A.52.3). Your actual installation is mapped to the nearest reference method.

Ambient temperature (Ca)

The tables apply at a reference temperature of 30 °C in air (20 °C in soil). If it is warmer, the conductor cannot carry as much, and the factor Ca (also called k1) is taken from table B.52.14:

Grouping (Cg)

If several loaded circuits are close together, they heat each other, and each one can carry less. The grouping factor Cg (k2) is taken from table B.52.17 based on number of circuits and installation method. For cables together bundled/enclosed (bundled in air/on surface/in duct):

In soil, additional corrections are made for soil temperature, soil thermal resistivity and burial depth according to table B.52.15/16/18 — moist clay conducts heat away better than dry sand. Look up the current factors in the standard. (Source: DS/HD 60364-5-52, table B.52.14–B.52.21.)

Indicative Iz0 extract (Cu, 2 loaded conductors)

Below is a small excerpt of the basic currents Iz0 for COPPER conductors with 2 loaded conductors (1-phase) at reference method B1 (insulated conductors in pipes on wall) and C (cable directly on wall/shelf), for both PVC and PEX/EPR insulation. The values are table values from DS/HD 60364-5-52 table B.52.4 (PVC) and B.52.5 (PEX/EPR). The excerpt is advisory — always use the current version, the correct column (2 or 3 loaded conductors) and the reference method corresponding to your actual installation.

Overcurrent protection must be coordinated with the feeder, so the breaker trips before the feeder overheats. Two inequalities must both hold:

The first inequality ensures that the load does not exceed the fuse, and that the fuse does not exceed the conductor. The second inequality ensures that even the fuse's guaranteed trip current does not heat the conductor.