If I add an extension cable, is keeping the same cross-section enough?
No. Voltage drop is directly proportional to length; when you extend the line, the drop at the same cross-section grows in the same proportion. You have to redo the calculation A = 2 x 0.0175 x L x I / ΔU with the new total length. For example, on a device drawing 20 A at 12 V, a line that needs 9.7 mm² for 5 m requires about 19.4 mm² when extended to 10 m.
An extension is a change that requires the line to be calculated again from scratch. Because L is a direct multiplier in the formula, doubling the distance also doubles the voltage drop.
Numerical example: 12 V, 20 A, target 3%, that is 0.36 V.
- For 5 m one way: A = 2 x 0.0175 x 5 x 20 / 0.36 = 9.7 mm²
- For 10 m one way: A = 2 x 0.0175 x 10 x 20 / 0.36 = 19.4 mm²
So if you add 5 m of the same cross-section to an existing 10 mm² cable and leave it at that, the drop becomes twice the target and the device runs at its limit.
Rules to apply when extending:
- Calculate the cross-section for the new total length; renew the whole line with a larger cross-section if necessary.
- The joint is a new source of resistance. Use a crimp splice sleeve or a suitable connector; do not make twisted-and-taped joints.
- After making the joint, measure the voltage drop across it under load; it should stay below 0.2 V, and above 0.5 V the joint is faulty.
- Choose the fuse according to the smallest cross-section on the line; if the cross-section has changed with the extension, the protection must be reviewed as well.
Full coverage of the topic: Installation, redundancy and field continuity