"521.10.1
Non-sheathed cables for fixed wiring shall be enclosed in conduit, ducting or trunking.
Non-sheathed cables are permitted if the cable trunking system provides at least the degree of protection IPXXD or IP4X, and if the cover can only be removed by means of a tool or a deliberate action."
The last bit about deliberate action is a little vague in its intent...