1. Selection Criteria for 1~5 Core Cables for 1kV and Below
Single-Core Cable
Single-core cables are typically chosen when the circuit is long, carries a large working current, or is laid underwater. This helps to reduce or eliminate intermediate joints, and in some cases, single-core cables offer better overall technical and economic performance compared to multi-core options. They can also be used in low-voltage DC power supply circuits where necessary.
Two-Core Cable
In 1kV and below power systems where the neutral point is directly grounded, a two-core cable is recommended for single-phase circuits when the protective conductor and neutral conductor are combined into one. For DC power supply circuits, a two-core cable is generally required.
Three-Core Cable
When the power system's neutral point is directly grounded, and the protective conductor and neutral line are separate, a three-core cable should be used for single-phase circuits.
Four-Core Cable
A four-core cable (3+1) is commonly used for 1kV and below systems, where the fourth core serves as a protective ground. It can carry unbalanced and short-circuit currents, with its size determined by these factors. Typically, it should not be smaller than half the cross-sectional area of the phase conductors.
In three-phase four-wire low-voltage distribution systems, if the protective and neutral conductors are combined, a four-core cable must be used. Combining a three-core cable with a single-core one or using the metal sheath of a three-core cable as a neutral line is not advisable. Otherwise, an unbalanced three-phase current may cause interference, similar to a single-core cable operation.
Five-Core Cable
For three-phase four-wire low-voltage systems with high safety requirements, such as in communication centers or automation equipment, a five-core TN-C system is often used. This separates the protection (PE) and neutral (PN) lines. The cross-sections are usually 3 large and 2 small, or 4 large and 1 small, or all five large.
Second, Selection of Core Numbers for Medium and High Voltage Cables
For 3~35kV three-phase power supply circuits, single-core cables are preferred when the current is high or the cable is laid underwater. Otherwise, a three-core cable is used, either in a common bundle or as a stranded construction. For 110kV three-phase circuits, unless installed in lakes or seawater, or when the cross-section is small, three-core cables are acceptable, but single-core cables can also be used per circuit.
For three-phase power supply circuits above 110kV, single-core cables are mandatory. In high-voltage AC systems like electrified railways, two-core cables or two single-core cables per return path are recommended.
For high-voltage direct current transmission systems, single-core cables are standard. However, in underwater installations such as lakes or seas, coaxial two-core cables can also be considered.
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