Common ratings given for wires:
Rating | Meaning | Relevance |
|---|---|---|
AWG, mm2 | The cross sectional area of the wire. Small AWG values have a large cross sectional area. | A larger cross sectional area will have less resistance according to the equation R = pL/A |
Solid/Stranded | The internal make-up of the wire | Solid wires have a single conductor, basically a copper rod. |
Voltage | Maximum voltage that the insulation can withstand. | Do not use a higher voltage than specified, and include a good safety factor (1.5 or 2). |
Temperature | Maximum temperature of the insulation material. | Don’t exceed it either through the current power dissipation or the ambient temperature. |
Insulation Material | Commonly Silicone or PVC. Silicone is much more flexible, but is more expensive. | |
Minimum Bend Radius | If the wire is bent sharper than this, the internal conductor will be damaged, and thus the wire will have higher resistance, higher power dissipation, higher temperature, and higher chance of failure. |
Considerations
Wire Use Case
Signal and power wires have very different consideration.
For signal wires, it is important to minimize noise pickup and signal distortion through twisted pair and shielding to preserve the integrity of the signal. Signal wires typically have low current, thus the wire size is not very important. For differential signal, including CAN, the characteristic impedance should be matched with the termination resistors (120ohms for CAN)
Power wires the noise is less important, but the resistance is a much higher priority as more current flows through the wires which causes power loss, and we want a high efficiency system.
Max Current
The maximum current a wire can allow. This is usually given in table for a certain wire gauge. These ratings are based on the maximum temperature, and the temperature rise based on the power dissipation in the wire. A good rule of thumb for our systems is the current rating for Chassis wiring in the powerstream chart in the resources section, divided by 2. So for 16AWG, a current of 11A would be great.
To find the operational current in a wire, the load at the end of the wire must be fully understood, and the wire sized appropriately for the maximum load (current draw) that will be applied.
Voltage Drop
The voltage drop in a wire is proportional to the current through the wire, and can be calculated using Ohms’s Law. An acceptable level of voltage drop is typically defined based on the minimum input voltage of the components on the other end of the wire, and the minimum input voltage to the system.
Power Dissipation
The power lost in a wire is P = I2R. This power is lost as heat in the wire as the current flows through it. For a high efficiency vehicle, it is important to minimize the power loss.
Shielding
A shield around a wire is used to avoid electromagnetic noise (generated from motors, switching devices, etc.) coupling on to the wires. This is very important for signal wires such as CAN, but less important for power wires as small fluctuations in power can be filtered out with capacitors.
Twisted Pair
Twisted pair wires are another way to eliminate noise coupling in to the wires. The idea behind it is that the EMI will and crosstalk will essentially cancel out over the length of the wires.
Resources
Wire Size Tables: https://www.powerstream.com/Wire_Size.htm
Resistivity Tutorial: https://www.electronics-tutorials.ws/resistor/resistivity.html
Twisted Pair: https://www.flukenetworks.com/blog/cabling-chronicles/physics-twisted-pair-cabling