Do NOT use cat5. In 99.95% of all jurisdictions, doing so would be a code violation. As BrianD mentions, you should use type FPL or FPLR (AKA Firewire). It is red, and specifically designed to prevent fire from shorting the wires prior to the alarm sounding. You should be able to get it at your favorite HA/alarm distributor, or at Home Depot/Lowes or an electrical supply house. Also, make sure that the smokes you purchase are compatible with the ELK system. It should be listed in the Elk installation manuel.