My electrical panel is in the basement and all of the wires are running to the panel from above. There isn't any room to run more wires through the top without drilling and hitting the existing wiring. Can I run the wires down the 2x4 next to the panel and enter through the bottom? Is this acceptable? I don't want to have to rip it out later.
You can run cables or conduits into a modern electrical panel anywhere it has a KO of the right size that is accessible and free. You will need to fit a cable clamp (for NM/UF) or cable fitting (for AC/MC) to the KO, though.
It depends. How old is your panel? These days branch circuit wires can enter a panel on any side that is convenient, EXCEPT through the Service section of the panel. The Service section is where you will find the MAIN discconnect (breakers). In the old days the Service disconnect was physically seperate from the panel containing all the branch circuits breakers, set up as two disctinct boxes. Today, the two halves are combined into one panel, but still with a division inside the panel keeping the two halves seperated. The only wires allowed in the Service setion are the three Service wires (usually designated as Black, Red and White), and the Ground wire.