MC4 cable?

Yes, pigtail MC4 on the panel is often #12. Those extensions included with the solar kit are likely #10, it will be written on it.

The wire should have it written on it what the gauge is. My 230w panel came with MC4 connectors on its two 3 ft terminal wires. They are #12 AWG and are aluminum!!! Unsat. :(

I cut them off short and added onto them fatter copper wire to go the rest of the way to the controller.

How do I determine what the gauge is of the wire I have that came with the 100w panel is?
Thanks for pointing out that "MC4" can come in different gauges.

Yes the wire is a bit stiff for short lengths connected to a circuit board.
Bend the wire to the proper shape before the final controller connection.

Otherwise some #10 from the auto parts store is fine.

I went with the idea of using well defined battery cables with weather protected RING TERMINALS everywhere I could to make connections into my battery bank wiring. I purchased my ANCOR cables in 25-foot lengths along with the proper ring terminal connections and used a 8-ton hand operated hydraulic crimper tool (AMAZON)

I used BLUE SEA components everywhere I could such as SWITCHES, BUS BARS, and battery terminals. All connections are now 4AWG marine HD cables such as ANCOR BRAND and all cables have ring terminals with proper adhesive type heat shrink ends on each end.

Adding solar or any other additional wiring is now a easy function and I can depend on the connections to work, be protected from adverse weather, and be neatly professionally installed everywhere.

In my view the extra professional work pays off in the end.

Roy Ken

Just to clarify om terminology, if I may: MC4 is not a wire gauge, it's a type of connector. In my solar system there are MC4 extensions #10 (mostly), and one piece #8, and the place where I bought them Unlimited Solar there are MC4 extensions as thin as #16 if somebody needs.

Battery wire gauge depends on the maximum current expected and on the length.

I don't like leftovers from those MC4 extensions for a battery wire. This insulation is mechanically strong and good UV protection, but it's not convenient for short length inside wiring, too "springy".

I think you were at 100w and standard #10 wire is fine.
If you have extra from cutting the MC4 cable then by all means use it.

Sure.. depending on the amps. #10 is good for whatever the ampacity chart says it is.