Parasitic Resistance
This is the sum of the small resistances carried by each of the elements outside the cable — connectors, fuse holders, terminals, switches and lugs. Individually they are in the milliohm range, but if a line has fifteen connections their total can exceed the resistance of the cable.
The cross-section calculation only takes the conductor into account; in a real circuit the current also passes through many contact points. A clean and tight connector pin typically shows a few milliohms, while an oxidised or loosened one goes many times above that.
- Total effect: on a line carrying 20 A, a total parasitic resistance of 25 milliohms means a 0.5 V drop; in a 12 V system that alone is a 4% loss.
- Where it accumulates: fuse holder clips, quick-fit connectors, switches wired in series, intermediate joint points.
- Reducing it: lowering the number of unnecessary intermediate connections, using quality terminals, running a single-piece cable instead of a joint.
- Measuring: rather than measuring every connection separately with an ohmmeter, measuring the voltage drop section by section under load gives a more accurate result in practice.
Running a thick cable over a long distance and then splitting the line with three intermediate connectors gives back what you gained from the cross-section calculation. Limiting the number of connections at the design stage is the cheapest improvement.
Context in which this term is used: Installation, redundancy and field continuity