[tsubaki]

And if you don't believe spare, look and see what size you have going to the engine now.
If you haven't been having any problems with your current wire size in the past you need to consider that you are now doubling that wires distance.
For this additional wire going up at least one wire size might be prudent, going up two sizes may be necessary.

You made no mention of a battery selector switch.
If you are using one this may change things a little especially depending on the location chosen.