Let me show you or anybody else that's interested, how to calculate wire size. I just took the ABYC marine electrical re-certification test on Friday. I had to calculate a few of these questions on the test.

Attached is a file that contains the specs on the Raymarine ACU's and the ABYC table for calculating a 3% voltage drop which I recommend for your autopilot. To calculate wire size from the table you need two numbers. One is wire length and the other is current draw. Looking at the ACU 100 spec, it says maximum current is 7 amps. You said it's about 9ft from the distribution panel to the ACU 100. Wire length is the length to the device and back. So you double the 9ft and you now know the wire length is 18ft. The table starts at 10ft and goes up 5ft intervals each time until it reaches 30 and then goes up by tens. What you do if the length falls in between a set of numbers is round up. So we round up 18ft to 20ft. Same with the current. If the current falls between two numbers, you round up. So in your case we round up the current to 10amps. If you look at the intersection of 20ft and 10amps in the table, you see you would need a 10ga wire. Don't forget to fuse the wire. 

David