Choosing an electrical cable: ampacity and voltage drop

Determining the cable cross-section looks like a simple task of tables, and precisely for that reason it is often done wrong. The cable must satisfy two independent conditions at the same time, and which of them governs depends on the distance.

The first condition: ampacity

The conductor must carry the current without exceeding the permitted insulation temperature. The values in the tables are for reference conditions; reality rarely matches them. Ambient temperature, the method of installation — in air, in conduit, buried — and especially the grouping of several circuits together significantly reduce the capacity. Ignoring correction factors is the most common and most dangerous mistake, because its consequence is overheating.

The second condition: voltage drop

Over long distances, a cable can carry the current without a problem and still be unsuitable. Voltage drops along the line, and if it drops too far, equipment performs poorly: motors overheat, lighting dims, contactors fail to hold. For this reason, on long lines the cross-section is determined by the voltage drop, not by the ampacity — often two or three steps above what would be enough thermally.

The case of motors

During starting, motors draw several times their rated current. If the line is long, the voltage drop during this period can prevent the motor from developing sufficient torque. At pumping stations this is a common cause of faults that look like an equipment problem when they are in fact a line problem.

How AlbGIS helps

• Electrical Cables — minimum cross-section according to IEC 60364-5-52, with an ampacity check corrected for temperature and grouping and a voltage drop check.
• Overhead Power Lines — for lines from the transformer to the consumer, with ACSR/AAC/ABC conductors and a motor-starting check.
• Photovoltaic Panels — when supply comes from self-generation, sizing starts from the expected output.