Roger,
What is the possibility that a critter may have chewed the wire? That's pretty uncommon for a siamese to burn through for no reason.
Especially from a 12 VDC power supply. Your wall warts should be 12 VDC with a minimum of 1 Amp for each camera.
The best protection against any overheating would be to use a power supply that has internal fuses or thermal breakers. Most professional power supplies have that. It also makes for a lot cleaner installation to have a single power supply instead of all those wall warts. here is an example of a 16 channel supply = http://www.securitycameraking......-prd1.html