Published: 2026-08-23
Cable cross-section selection mainly involves calculating the current carrying capacity. There is a quick mnemonic for on-site estimation: **Ten below five, hundred above two, 25/35 three-five four-three levels, 70/90 five double and a half** — for cables under 10 mm², estimate 5A per mm²; for cables over 100 mm², estimate 2A per mm²; for 25/35 mm², use 4A/3A; for 70/95 mm², use 2.5A.
But the mnemonic is just a reference; in actual use, three adjustments must be made:
First is temperature correction: when the ambient temperature exceeds 40°C, the current-carrying capacity should be reduced by 10-20%;
Secondly, correction of laying methods: conduit, cable tray, and underground installation have worse heat dissipation than open laying, so the current-carrying capacity needs to be reduced;
Third, multi-core derating: multi-core cables dissipate heat less effectively than single-core, and the more cores there are, the greater the derating.
For example: a 30kW motor at 380V, the current is about 60A. According to the rule of thumb, choose 16 square millimeters (actually, using the 5A range, calculating as 4A is more reliable). But if it is installed in a conduit and the ambient temperature is above 40°C, you need to increase to 25 square millimeters for safety.
When selecting the cross-section of special cables, you also need to additionally consider: voltage drop for long-distance power supply (voltage drop = current × line length × resistivity, if it exceeds 5%, the cross-section should be increased) and cumulative temperature rise from frequent start-stop operations. Choosing a smaller cross-section saves little money, but a failure can stop a large production line.
Shanghai Suoqi supports calculating cross-section and voltage drop based on load power, line length, and laying method, and can provide selection calculation tables for special operating conditions.
About Shanghai Suoqi
Shanghai Suoqi Electric Wire & Cable Co., Ltd. is a specialized cable enterprise focusing on the research and development, design, production, and sales of special flexible cables. The company's products cover a series including reel cables, flat cables, drag chain cables, encoder cables, polyurethane cables, seawater-resistant cables, seawater-resistant network cables, and high-altitude vehicle cables, which are widely used in ports machinery, mining, industrial automation, marine engineering, and other fields. For more product information or inquiries, please call 13311756197 (same number on WeChat).