I have several floor joists in my basement that need to be sistered up but unfortunately the 8 gauge wire that powers the AC compressor runs diagonally through all of them. Disconnecting it from the AC unit or at the panel aren't very good options and would require digging into drywall or siding to free up the cable. My ideal method is to cut the cable, splice in an extra ~10 feet, and reroute it to avoid running though any joists. This would require using two junction boxes in order to add the new section. My question is, what's the preferred method for connecting #8 wires in a j-box? Additionally, would there be any concern in regards to adding 10 more feet of cable or potential loss of capacity with the connections of such large wires? Thank you for any advice.

I have several floor joists in my basement that need to be sistered up but unfortunately the 8 gauge wire that powers the AC compressor runs diagonally through all of them. Disconnecting it from the AC unit or at the panel aren't very good options and would require digging into drywall or siding to free up the cable. 

My ideal method is to cut the cable, splice in an extra ~10 feet, and reroute it to avoid running though any joists. This would require using two junction boxes in order to add the new section.

What's the preferred method for connecting #8 wires in a j-box? Additionally, would there be any concern in regards to adding 10 more feet of cable or potential loss of capacity with the connections of such large wires? Thank you for any advice.