Wire, that sure is used a lot.!Turns out there are lots of different wire for different uses. During my research for the fuse block kits, I went to a lot of sites trying to get as much info as I could to provide a good, long lasting, and safe kit. Even tho' I have worked in the electrical/electronics field for a long time, I was surprised by how little I knew about the wire we use in our cars to add equipment and modify things. I worked military systems and they use different specs.
Below are a few of the sites I found to have very useful information.
Offroaders for the chart for wire sizes and gage and lots of really good info on relays for extra lights.
http://www.offroaders.com/

I have a few more, but I am having trouble getting them to get here. I'll keep trying,
Tom
EDIT: well, I am having no luck getting these to become links.
You can google for:
about.com auto repair corrosion preventative
automotive wire tables
automotive wire - sponsered by allied wire and cable.
Most informative to me was the codes of different types. Probably the most used wire is primary wire. GPT coded. i think this is what most of us buy at the store. See pics
GXL is a cross linked wire that is better suited for higher temp installations.
There are others you may have a need to check for your project.

Tom, if you want to buy that wire in larger spools, it's often called mtw, or machine tool wire. Same stuff, just has a more industrial name, and usually slightly finer strands. It's about all I use, since I have 500 ft spools laying around.

One of the things I found frustrating is the specs linked to J-1128. The SAE ( society of automobile engineers) specs are listed on many wireing spec sheets, but you have to be a member of SAE to see what the spec says. Does anyone have access that can post a copy or is that illegal to do so by copywrite? Chrysler and Ford have specs also, but same thing, can't read them.
Jeff, MTW wire is rated to 105 degrees C. The GXL is rated to 125 degrees C and is specifically designed for use in auto engine bays. I don't think I would ever use 500 ft. And I would think because of the multiple strands, it is pricey also. GXL 12 ga. 100ft is almost $30.00, 14ga a little less.
Tom