Back in my utility days we used the following for max loading
1/4 guy wire 2835 lbs
5/16 5400 lbs
3/8 9700 lbs
If you are going to just slack span these should do the trick. If y ou are going to pull the cable up tight there are some additional caculations required.
[This message has been edited by Bob (edited 07-01-2005).]