Technically it is highly recommended. I am personally guilty of failing to do so on my outdoor enclosure. There is about a foot of cable showing going into the power source. I haven't quite thought of a way to protect it and it be sufficient. The only thing I can think to cover the wires with is some wire loom and duct tape. However, larger gators/crocs would be able to bite through it. Personally, I would just recommend making sure that all cables are out of immediate access. If the cables are RIGHT there in the crocs face, then most likely he is going to bite it and get zapped. However, if the cables are not in the crocs path, then it's nothing that they should be too concerned with. On my indoor enclosure, the wires are covered and protected. Both enclosures have powered items connected to a GFI.
Chris
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Chris Law
U.A.P.P.E.A.L. (Uniting a Proactive Primate and Exotic Animal League)
Herpetoculture Element Representative