It's been said earlier in this thread but it's worth repeating: When choosing a wire gauge consider not just the amps a device will pull, but also the TOTAL length of the run - i.e., from battery to device to ground.
On a sidecar rig, that could be 15 feet or more.
Note also that the chart DRONE posted [and that was included in my link in first post] is for American Wire Gauge - AWG is a little more robust than automotive SAE gauge wire.
You might also choose to use marine wire - it comes in AWG and the strands are tinned to better resist corrosion; again, a little more robust than plain copper strands.
I'm considerably more conservative than DRONE when it comes to wiring - I'll move to one size larger [or maybe even 2 sizes] than recommended just for my peace of mind . The difference in cost is negligible.
Ask The Google for marine wire - there are LOTS of sources, including Amazon :-)
One of the things I still haven't figured out is what adding a bunch of additional stuff to the bikes OEM wiring does, if anything.
It is better to have ridden and crashed, than to have never ridden at all.