5.5.2 Hierarchy for Power Supply
Figure 11 shows a typical wiring hierarchy for the equipment power supply cabling in the automotive vehicle
environment.
Notice that the cable ratings are higher than the circuit breakers and fuse ratings. In this example the circuit
breaker rating has been de-rated to 70% of the cable maximum continuous current rating.
Also note that the current rating of the input side of the circuit breaker from the vehicle bus bar. This cable must be
rated to exceed the current rating of the bus bar. In this case 70% de-rating has been applied.
Figure 11: Typical Bus Power Supply Circuit Breaker and Cable Rating
5.5.3 Length Versus Size
For power distribution it is critical that the cable is selected to minimise voltage drop. This selection process shall
consider the peak currents generated during equipment start up. As a guide, it is recommended that the cable
voltage drop uses a peak current of 1A (in a 24V system) and a maximum voltage drop of 1V (including return, i.e.
0.5V each way) when selecting the cable.
All cables have resistance. When comparing cables of the same type and style, then generally, the thicker the
copper the lower the resistance for the same length. If cables are to be used over long lengths, then it may be
necessary to increase the cable size to reduce the voltage drop caused by the resistance of the wire and the load
current.
For the Adept Validator wiring in a 24V system, the following is provided as a guide to maintain an acceptable
voltage drop.
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