In the near future i want to change my 100amp box to a 200amp box in my basement, i am an industrial electrician but i don't deal with residential that much i want to put the new box next to or with in a couple feet of the old panel my question is if the existing romex is not long enough to reach what is the best (safest) and code compliant way to extend the romex to reach the new panel. any advice would be great or refer to code book so i can look up the section.

jlhmaint, check with your AHJ but mine has let me use a Hoffman juncion box (12"x12")to join in when I had a similar problem. I used the one with pre-punched KO's and worked well.Good luck, us Industrial guy's gotta stick together.

Could you make the original a sub-panel, fed from the new panel?
If so, remember to isolate the neutrals & grounds in the 100 amp panel.

Why not install the new panel in the same location as the old one?
Are your plans to install a new 200 amp service, have it energized and then move the circuits over?
The easiest way would be to install the new panel in the same location and back-feed it from the old panel over the face (you can mount it off to the side temporarily). Then when the service changeover is done, strip out the old service and install the panel cover.

well i would like to use the 100amp in the garage that we plan to build and the old panel is located in my oldest boys bedroom now i would like to move it approx 8 to 10 feet to the left into the storage area of the basement, but if this is going to become more work than its worth i will just put the box in were the old one is and deal with it.

hay spkjpr do you have any good industrial links that i might be interested in checking out.