I want to install a subpanel to be used to supply branch circuits to a new kitchen, laundry, garage, and some additional lighting and receptacles. The 200 amp main panel is about 50 ft away but because of the obstructions, we'll need about 85 feet of cable to connect the 100 amp subpanel. Since we'll need to run through the floor joists in the basement for most of the length (50 ft) it would be eaiser to use a smaller copper conductor. My question is, can we use a #4-3 Romex to connect the subpanel? If not, what type and size of copper cable do I need to use? If we have to use aluminum, what is the appropriate size? I've been told SEU 2 ga. By one supply house and SEU 1 ga. By another.

For a residential 100 amp feeder, either #4 copper or # 2 aluminum is allowed, per NEC Table 310.15(B)(6).

Supply house employees should not be consulting anyone about anything that they sell, unless they are licensed and/or trained regarding sizes, ratings, etc.

Be sure this feeder will have four wires, two live and one neutral, plus one bare or green wire for the equipment ground. Keep the equipment ground and neutral separated at the subpanel.

While bhayne's advice would be practical under other circumstances, it seems that he is only guessing for this situation.

I, however, will rely on facts.

If the feeder was to be loaded to it's maximum of 80 amps, a #4 copper would result with a 4.1 volts dropped, well within the recommended 3% maximum, that being 7.2 volts.

#2 Al will give 4.3 volts dropped, again well below the max of 7.2 volts.

Unless you will have motors larger than 2 HP, or a large welder, the #4 or #2 wire will serve you fine.