1) Pat.......if unable to visually trace out the circuit path, stay on the exterior and measure again for voltage on both of the 50 amp hot's to ground. Have somebody inside the basement turning the respective double pole breakers to the OFF position in order to determine which breaker is feeding the 2 hot's on the exterior that you wish to disconnect.
2) If the conductors were installed in conduit or Greenfield or Liquidtite, you can also gently tug on the wires from the main panel to determine it's destination or any splice boxes along the path to the exterior while a helper listens for the wires moving about inside any conduit or flexible conduit.
3) You can also determine which conductors originate from which breaker by the size of the hot conductors. A 50 amp requires either 6 AWG Stranded Copper or 4 AWG Stranded aluminum. An 80 amp requires either 3 AWG Stranded Copper or 1 AWG Stranded Aluminum. The 80 amp circuit conductors will be a larger wire size.
The wire gauge will also be labeled on the exterior conductor insulation and you can visually determine which conductors belong to the respective double pole breakers.
Hope this helps………If you have any additional questions, let me know and I’ll be glad to answer them for you.
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The next time you have an electrical question, you can also request for me at:http://www.justanswer.com/home-improvement/expert-your-electrician ……….Thanks…………..Kevin!