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 For 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.
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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.
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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
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