Run length

Have you considered a wireless connection, like the below from SVS? They seem to be getting pretty common.  I have a similar situation, and have been wondering about it.


https://www.svsound.com/blogs/subwoofer-setup-and-tuning/how-to-set-up-a-wireless-subwoofer

Not too long if your source and end point are balanced.

Mr ibm, isn't your subwoofer self powered? Does it have a balanced input? Passive subs are much rarer but, if it is drop the wire to 12 or 10 gauge twisted pair or Kimber Kable 12 TC if you don't mind spending a lot of money. If the subs are active you can run a balanced line 300 feet if you want. Subwoofer drivers usually demand a lot of current so linear resistance matters. Lower gauges have lower series resistance. You want to keep induction as low as possible in a long run like that which is why you should use a twisted pair format vs Zip chord. 

Sub is a JBL B380 non-powered. Using a wireless setup would mean climbing over furniture to turn on the local amp and wireless receiver.

I need to run speaker and CAT x in an outside wall. I know what that means and I still have some paint used on the wall. I plan on using an electrician to do the work.

An electrician will probably not want to run bare wire like that. Use whatever he recommends. Wire going to main speakers handling midrange and up is incredibly important. This is not that. Use whatever.