Why is cable length counted as a round trip?
Current leaves the source, reaches the load and must return to the source, so the circuit contains twice as much copper as the cable run. That is why the voltage drop formula is ΔU = 2 x L x I x ρ / A, where L is the one-way distance. Forgetting the factor 2 halves the calculated drop, and the selected cross-section ends up roughly half of what it should be.
This is the most common calculation mistake. If the device is 6 m away from the battery, the copper the current sees is not 6 m but 12 m: 6 m of positive conductor and 6 m of return path.
That is why you can write the formula in two different ways, and both are the same:
- With the one-way distance: ΔU = 2 x L x I x ρ / A
- With the total circuit length: ΔU = Ltotal x I x ρ / A
To avoid mixing them up, pick one method and stick to it. The common one is the first, meaning L is always the one-way distance between the device and the source.
The cost of the mistake is concrete: at 12 V with 20 A, a one-way run of 5 m and a target of 0.36 V, the correct cross-section is A = 2 x 0.0175 x 5 x 20 / 0.36 = 9.7 mm². If you forget the factor 2 you get 4.9 mm², you pull 6 mm², and the real drop comes out roughly twice the target.
Special case: the factor 2 still applies when you use the vehicle chassis as the return path. The chassis is not an ideal conductor; paint, corrosion and the path through spot welds add extra resistance. On a good chassis path, the drop measured under load is expected to stay around 0.05 V.
Full coverage of the topic: Installation, redundancy and field continuity