Tim, I am not aware of any national electric code rules thatwould not allow 8/3 to be joined in an outside junction box...BUT hopefully you were gonna run the wire thru pvc or metal conduit into a weather tight exterior junction box. If you run the wiring thru conduit into and out of a weather tight junction box... this should be safe and per what the code requires and allows. I will check my National Electric Code manuals later and see if I have overlooked something. Anyone else more familiar witht he NEC can comment too please. Good luck to you , if I find something in the code book that says its a NO-NO I will pass it on.