UN TPI for large holes

[sharafe]

Hello,

I'm trying to find out:

1. When to use UN and when to use UNC in a drawing.

2. How to determine the UN threads per inch designation for holes greater than 6 inches as in the Machinery's Handbook 27th Ed. Specifically, I have a hole that is 7.5" and want to know what TPI would make it UN/UNC/UNF etc.

Thanks.
Sharafe