6-AWG is a little fat for even the most demanding applications, but when listed as "bulk welding cable" it is only $1 a foot:
http://endless-sphere.com/forums/viewto ... 11&t=59899
For temperature sensors, small 30ga wire should be adequate, but teflon insulation is recommended due to the heat involved. Here is on eplace to get it that has been used by several ES members. edit: I have gone to using thicker 24-ga wire for halls and temp sensors, because...anything smaller is very hard to manipulate with my fat fingers when crimping or soldering.
http://www.powerwerx.com/wire-cable/ptf ... -wire.html
Heat shrink insulation
For any wire that is exposed to the weather, use 3:1 heat shrink, marine grade
(for boats), because it has heat-activated glue that truly seals up the wire joint. Anything indoors (or inside the battery bag) I use the dirt-cheap Hobby King heat-shrink (typically 2:1 shrink ratio, if it starts out at 4mm diameter, it ends up at 2mm diameter). I even use Hobby King heat-shrink to change the color of the last inch of a wire (by the connector).
But...what size to order for a specific job?
On the Hobby King web-catalog, scroll down the left-side column and go to "Hardware & Accessories", then "Shrink Tube"
Here are their "relative" sizes, metric to inches.