Wiring a Subpanel
When I was a builder, my partner was a licensed electrician. We did a lot of the work ourselves on the houses we built, and I got a pretty good apprenticeship in house wiring. Consequently, I’ve wired a number of panels and service connections on new houses, but until building my own garage recently, I had never done a subpanel. Like any electrical work, this is something I recommend you tackle only if you have a thorough understanding of the process, and that you pull the proper permits.
Subpanels have both similarities and differences with main panels. Both require properly sized entrance cable, from the main disconnect in the case of main panels, and from the main panel in the case of a subpanel. The conductor size depends on the amperage the subpanel’s main breaker can handle and the distance from the main panel. There are any number of online references to find that information. I had to buy about 60 ft. of cable, and considered both copper and aluminum conductors. Because copper is a better conductor than aluminum, a smaller diameter wire can be used, which makes it a lot easier to pull through conduit. However, even though I had to buy a larger gauge of aluminum conductor, it was substantially cheaper than the copper. One critical thing when using aluminum wires is to coat their ends thoroughly with Noalox to keep the aluminum from corroding where it ties to the panel. Corroded aluminum doesn’t conduct electricity well, and the extra resistance creates heat, which has caused fires.
Because the subpanel is fed from a new 100 amp breaker in the main panel, no other disconnect was required in my case, so tying in was a simple matter of clamping the wire ends to the lugs. The biggest difference between a subpanel and a main panel is that the ground and neutral buses on a subpanel have to be separated. Most panels come with a bar joining the two, which is easily removed. Code requires subpanels to have a ground connection that’s independent of the main panel’s. In my town, two 8 ft. ground rods separated by at least 6 ft. are required. With our rocky soil, they are a challenge to drive. One tip that’s served me well is to chuck the rods into a hammer drill and spin them into the ground. The combination of rotary motion and impacts seems to move the rod around most obstructions, and will even set a rod in frozen ground. And since the rods are 8 ft. long, I drive them at a 45 degree angle to avoid having to stand on a ladder initially.
Because the ground and neutral bars are separate, all the grounding conductors have to go the grounding bus and all the neutral conductors to the neutral bus. Beyond that, there’s no difference between wiring from a subpanel and a main panel. One thing I find very helpful in either case is to mark the incoming cables with the name of the circuit they serve. That helps not only in labeling the panel, but in balancing the load between the two hot busses.