Wires and Cables
by Kurt Küpper*
We have covered parts of this topic before, but so much confusion seems to abound that we thought it appropriate to visit the subject again.
A chain is only as strong as its weakest link, and the often neglected link in an electrical system is the wire or cable connecting the whole lot together.
Firstly, what is the difference between wire and cable? Not much, a single insulated conductor is a wire. Put two or more insulated wires together and they become a cable.
The most efficient wire has a single solid core. However, multi-stranded wire is more easily bendable and more durable as it better resists vibrations, and is thus more commonly used.
Choosing the correct wire for the application is essential for any electrical circuit. It must be capable of delivering the current safely from the power source to the load, with as little loss as possible.
The current has to be forced to travel through the conductor. The potential difference or voltage of the power source drives the current from the positive pole through the circuit to the negative pole of the power source. In doing so it has to overcome the resistance in the conductor to it passing through the conductor.
Electricity flowing through a conductor is much like water flowing through a pipe. The thinner the pipe, the harder it is to force water to flow though it. Water pressure (caused by gravitational head or a pump) forces the water through the pipe, overcoming resistance due to restrictions (narrow pipes, bends etc) and friction on the inside wall of the pipe. The greater the resistance to flow, the greater the drop in water pressure across the system.
Very similarly, electrical voltage drops from the level at the power source due to the resistance in the conductors that it has to flow through. The power that any load can exert is proportional to the current flowing through the circuit (Amperes) and the potential difference (Voltage) at point of the load. It thus stands to reason that the lower the voltage drop between the power source and the load, the more usable power will be available at the load.
For practical purposes, a limit has to be placed on conductor size, for cost, weight and space reasons.
A general rule of thumb that has been commonly adopted is that the voltage drop should not exceed 3%. In a nominal 12V system that means 0.36V. Assuming that one does not let the deep cycle batteries discharge beyond 50%, at which point the battery voltage should be about 12.2V, this means that at worst a usable voltage of more than 11.8V should at all times be available at the load.
The voltage drop across a wire is dependent on the cross sectional area and length of the conductor, the voltage and what material it is made of. For a given material, e.g. copper, this will mean that a formula with a constant factor and three variables can be used to determine minimum cable sizes:
DV = 0.017 x I x L
DV = Voltage drop (Volt).
L = Total length of wire in metres from the power source, to the load and back to the power source, i.e. the complete length of the electrical circuit must be taken into account.
I = Current (Ampere) flowing through the wire
A = Cross sectional area of copper in square millimetres.
0.017 = constant for copper conductors at about 25°C. Varies by about 4% per °C, i.e. add or subtract 4% per degree above or below 25°C respectively.
Using this formula it is very easy to determine what the minimum cross sectional area of the cable to be used should be for a maximum voltage drop of 3%, i.e.:
A = 0.017 x I x L = 0.0472 x I x L for
0.36 12V systems.
Rule of thumb: Half Amps times metres divided by ten for 12Volt and a quarter Amps times metres divided by ten for 24Volt.
Part 2 of this article will appear in July.
*Kurt Küpper is director of Aquavolt Electric Boat Parts. Tel: 02 9417 8455 www.aquavolt.com.au