90 ft? Are you saying you will have two 45ft runs for both speakers? I would suggest looking at the wire table for the below link. The table shows you recommended gauge to use for certain ohm speakers and length runs. You may have to read the section on over 50ft runs, which is not as recommended. I would also not hesitate to just go to your local home improvement store and buy right off the spool per foot, you save money and not pay higher unneeded costs. I would not recommend your friends XP flat cable for this application.
http://www.roger-russell.com/wire/wire.htm#wiretable
