which chart are you referencing? one for building construction? there's a difference between 29A @ 12V compared to 29A @ 120VAC (348watts vs 3480watts)
This is not exactly true. 29A is 29A for the most part. We've ran 26,000 watts on 12 guage wire. Amps matter on wire size, not watts.
Do NOT NOT NOT undersize the wire. You will forget and you will start a fire. If you want to do this, then put a fuse to protect the wire. As long as you fuse it properly, then this would be acceptable.
Watch out for voltage drop too. This will increase your amps. Find a voltage drop calculator. I would guess that anything over 25 feet on 12 volts could start to present problems.
Run parallel runs (multiple wires carrying the current) is a good idea, except that the lugs (the place where you land the wires) are not usually not made for multiple lugging. Look at some device that can combine the wires and make them so only one logical wire is on the lug.