You can use copper between buildings but you will have to worry about lightening strikes. If it leaves from one building to another it needs to be a PE type cable. This needs to be terminated within 50' of the entrance on protectors. Because of the high smoke of this cable, it can be extended past that point if it is in a rigid conduit. This also needs to be grounded and bonded.
Telephone cables (Cat 3 or Cat nothing) are done this way all the time. Trouble is, this is expensive by the time you add the cost of Cat ?? rated protectors. This is why most people use fiber for their data needs between buildings. Fiber is glass and not a conductor of electricity.
Hope this helps.
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