The answer is conduit. Wires from solar panels need to be inside of metal or plastic pipes. For residential grid-tied systems (what this website primarily deals with), these conductors could be anywhere between 250-550 volts DC.
According to a logical interpretation of NEC 690, if the conduit is indoors, the wires must be inside of EMT, which is metal conduit. If it's outdoors, then it can be in PVC, which is plastic tubing.
The size of the conduit depends on the circumference of the conductors and the number of wires within. Most residential systems that I've installed tend to use 3/4" or 1" conduit, and those have been around 4kw or 5kw. Interior conduit runs typically look much better than ones that run on the outside of the house because they're much more concealed.
Performing This Task
1. Select a spot that will be beneath the system, preferably at an ideal point in the attic to enter. It's okay to have a pull point under an array, like an LB, but you can't put any kind of junction box under there (because of access restrictions).
2. Once you have selected the right area, find the beams. Pilot next to a beam. Check the interior and verify that this is a good spot. Try to make the pilot hole at a point in the middle of a shingle row (not directly above a seam between rows). Once it's verified, get your hole saw out. Pick a hole saw size that is slightly larger than the conduit (for example, if 1" conduit, use 1 1/4" and for 3/4 use 1 1/8" and so on). Get through all of the shingles and the plywood beneath.
3. Now you got a big hole in your roof. That's not such a big deal. Cut the semicircular shape out of the shingles so that the flashing can fit snugly underneath. Get your cat's paw or similar tool out, and loosen up the shingles and remove any nails that will hinder the flashing from fitting all the way beneath the shingles.
4. Get your LB with the threaded rigid conduit attachment. Make sure it's long enough that when it drops in, the pipe will be lower than the beams. For most homes, this usually means getting an 6" piece, a coupling, and then either a 4" or 2" piece of threaded rigid pipe.