Wiring, Harnessing, Connectors and Circuit Protection
Wiring, Harnessing, Connectors and Circuit Protection tests cable selection, voltage drop, fuses, relays, terminal repair and loom routing. You must choose and repair wiring so it carries current safely and survives vibration, heat and abrasion.
Core rules
- Cable must carry continuous current with acceptable voltage drop, typically under 0.5 V for many circuits; the fuse protects the cable, not the device.
- Fuse rating should be above normal load but below cable capacity; continuous loads are often kept to about 80% of fuse rating.
- Relays allow small switches to control large currents; typical pins are 30 battery positive, 87 load output, 85 coil ground and 86 coil positive.
- Crimp terminals with the correct tool, pull-test the joint, and heat-shrink to seal. Avoid soldering where vibration causes fatigue.
- Route looms away from heat, abrasion and moving parts; use grommets, clips and conduit.
Worked example
A 100 W lamp at 12 V draws 8.33 A. A 10 A fuse is loaded at 83%, so it is marginal for continuous use; a 15 A fuse is better if the cable is rated at least 15 A. The cable must also be checked for voltage drop over the run. If the lamp is 6 m from the battery and the chassis is the return, only the positive run may matter for drop; if a separate return wire is used, include it.
Traps
Fitting a larger fuse to stop blowing. Using wire that is too thin for the run. Twisting wires and taping instead of crimping. Leaving a loom rubbing on a sharp edge. Reversing relay pins 85 and 86 on a relay fitted with a coil diode can short the coil.