distribution feeder connection T loop supply vs Ring main for site power services

I am looking for some information on T loop configuration for power distribution. The application is an industrial zone which will have multiple power zones. I understand that with T-loop configuration we are limited to 18kVA balanced across two lines and a third line thats empty/whats left of the 18kVA.  I am looking to brush up on my knowledge. do you have any suggestions where I can find reading/listening material on this topic? 

Hi P C-I gave it my best shot at researching this for you, but when I search "T Loop"  and "Ring Main" the only reference I find is your question above.  If you can provide more information I'll try again.  Where did you find these terms? 
-Rick