If I have too much mat or cable, can I cut it?
No, you should never shorten the heating wire. This is because the resistance for your mat or cable is calculated on the length of the heater. Shortening the heater would increase the resistance and thereby the temperature. This may lead to issues down the road.
In extreme situations it is possible to shorten the cable by less than 10% of its length, but it would void Warmup’s warranty. The cable is expected to operate fine at slightly higher temperatures, but it may put undue strain on the end-splice, which may then fail over time.