How is a copper cable cross-section/current table read?
Cross-section/current tables give the current a cable can carry continuously without exceeding its permitted temperature. Typical values are around 18 A for 1.5 mm² and 76 A for 16 mm². These values are for an ambient of roughly 30 °C and free-air routing; conduit, bundling, high temperature and insulation class pull the value down, so the table alone is not enough to choose a cross-section.
Read the table as an upper limit, not as a target. The current given is the value the cable can carry continuously without exceeding the temperature its insulation withstands. The footnotes under the table are at least as important as the figures, because they define the routing.
Typical reference points: 1.5 mm² about 18 A, 16 mm² about 76 A. For the cross-sections in between you will find values in different publications that deviate somewhat from one another; this is normal, because every table uses a different routing and insulation assumption.
Factors that pull the value down:
- Ambient temperature: relative to the 30 °C reference, a factor of about 0.87 is applied at 40 °C and about 0.71 at 50 °C.
- Routing in a bundle: loaded strands side by side heat each other, and the single-cable value cannot be reached.
- Routing inside conduit or trunking: heat dissipation becomes harder.
Example: if you are going to use a cross-section shown as 26 A in the table in an engine bay at 50 °C, your practical limit becomes 26 x 0.71 = about 18 A.
Finally, the table says nothing at all about voltage drop. On a long run you must also do the A = 2 x 0.0175 x L x I / ΔU calculation and choose the larger of the two results.
The full topic: Installation, redundancy and field continuity