Round-Trip Circuit Length
In a DC circuit the current goes from the source to the load and the same amount returns; therefore the conductor length is taken as double in the voltage drop calculation. For a driver 6 m away from the battery, the length entering the calculation is 12 m. Skipping this factor makes the loss look half of what it is.
This is where the factor of 2 in the formula comes from: ΔU = 2·L·I·ρ/A. The negative conductor shows just as much resistance as the positive one and the two are in series. This fact does not change even if you use a chassis return; only the resistance of the return path then becomes dependent on the quality of the sheet-metal body and the connection points.
- Two-core run: since both cores are copper, the calculation is done directly with 2·L.
- Chassis-return run: because the body has a large cross-section its theoretical resistance is low, but paint, rust and a loose bolt quickly destroy that advantage.
- Measurement tip: measure the drop of the positive line and the negative line separately; this is how you find out which side the problem is on.
On vehicle and work machine installations the battery is usually behind the cab while the remote control or driver is close to the working end. Because the cable route is not a straight line, the real length is often 1.5 times the as-the-crow-flies distance; when calculating the cross-section, actually measure the route with a string or a tape.
The context in which this term is used: Installation, redundancy and field continuity