You don't say where you are.
In the US the wires in MC cable would normally be black white and green.
White would be neutral if wired properly.
Black and Red are usually hot lines and would be used in a 220v system
By connecting a 14/2 wire to the system you would lose the ground. That is not recommended. If you are sure the wire is 8 g then I wouldn't suggest running a smaller size wire.