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Relationship between cable voltage rating and number of cable cores
Selection conditions for 1kV and below for 1-5 core cables
1. Single-core cables
Longer lines, circuits with higher working currents or when laid underwater, single-core cables can be used in order to avoid or reduce intermediate joints, or when single-core cables have better overall technical economy than multi-core cables; low-voltage DC power supply circuits can use single-core cables when required.
When the neutral point of the power supply is directly earthed at 1kV and below, the number of cable cores in the single-phase circuit should be 2 cores when the protection line and the neutral line are combined with the same conductor; for DC power supply circuits, 2 cores should be used.
When the neutral point of the power supply is directly earthed at 1kV or below, the number of cable cores for single-phase circuits is: when the protective and neutral conductors are independent, a 3-core cable is appropriate.
4 core cables (3+1 cables) of 1kV and below, where the 4th core is used as a protective earth, but also to carry the unbalance current and short circuit current of the power system. Its size is determined by the unbalance current and short-circuit current, but generally shall not be less than 1/2 of the phase line.
1kV and the following three-phase four-wire low-voltage power distribution system, when the protection line and the neutral line together with the same conductor, should be used 4 core cable, and shall not use 3 core cable plus single-core cable combination into a circuit, or directly use the 3 core cable metal sheath or armor layer for the neutral line way. Otherwise, when the 3-phase current is unbalanced, it is equivalent to the operating state of a single-core cable, which can easily cause industrial frequency interference.
5 core cables
1kV and below the three-phase four-wire low-voltage distribution system, some of the higher reliability requirements of electrical installations distribution lines, as well as some of the electrical to ensure that no damage, but also to resist interference grounding communications and automation equipment, it is appropriate to use 5 core line of the TN-C low-voltage distribution system, so that the protection line (PE line) and neutral line (PN line) are independent, cable core cross-sectional area is generally 3 large and 2 small or 4 large and 1 small or 5 large.
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