Leon,
Here’s a link to a site which has a voltage drop calculator for DC electrical circuits
https://www.voltagedropcalculator.co...bmit=Submit%21
You will need to know -
Voltage in volts
Cable or conductor size in mm2
Cable length
Current in amps
Note: you may have to check in the notes whether cable length is related to only the positive leg or both positive and negative legs , if so double the cable length for a twin + - conductor cable.
Max voltage drop is generally 5% of nominal voltage , so in a 12volt system you can drop 0.6 volts between supply point (battery or SM power supply) and the load or device. Increasing conductor size 1 or 2 gauges usually resolves your Vdrop issues if you dealing with lengths up to 5m or so in a 12v dc system however it’s all dependent on current draw.
Hope this helps
Martin