Wiring a 200 4 Wire Breaker Box

• 1). Check the rating of your panel box. You need a 125-amp main panel to install an extra 40-amp, double pole breaker off your main. You'll have to upgrade the main panel in an old house with a 100-amp main or less before installing an extra 40-amp, double pole breaker. The main box will state the rating on the inside of the panel cover.
  
  
• 2). Dig a trench between your house and garage. Check with local utilities to make sure it is safe to dig without disturbing other underground utility lines. Your trench should be 18 to 24 inches deep, depending on code and frost lines, and about 18 inches wide. Turn off your main panel's power and double check with a tester to make sure all power below the entry point is off.
  
  
• 3). Drill a hole through your garage wall, about 18 inches below the bottom of your main panel. Do this with your drill and 1-inch hole-saw bit. Mount a large junction box over this hole. Line up your hole with one of the punch-out holes in the junction box. Use a hole punch to remove the punch-out hole. Mount the junction box to the wall, using drywall screws. Run a section of 1-inch conduit from the bottom of the panel to the top of the junction box. Punch out your holes and connect each end with a 1-inch conduit connector.
  
  
• 4). Attach a 1-inch conduit elbow to the back of the junction box, run a section of 1-inch conduit from the elbow down into the trench and connect another 1-inch conduit elbow. Run a length of 1-inch, watertight conduit through the entire length of the trench. Once you are at the garage, you will repeat the process of installing a junction box and running sections of conduit from the back of the junction box down into the trench. The garage junction box should accepts breakers, while the junction box below the panel is just a wire-pulling box. Install both ends of your rigid conduit with the watertight conduit, using a combo-fitting that is designed to connect those two types of conduit.
  
  
• 5). Install a 40-amp, double pole breaker in both your main panel box and in your garage junction box. Use an electrical fish-tape, which is a wire pulling steel tape, to pull your #8 gauge wire through all your conduits. Then pull a #6 ground wire through all the conduit runs, as well. Codes will vary in regard to wire size, box size and so on. Please consult with an inspector or contractor.
  
  
• 6). Cut back the wire sheath in all the wire runs that you have pulled into the garage breaker (junction) box and the main panel. Install one hot lead to each pole of your 40-amp, double pole breaker. Connect the ground wire to the ground bar and the neutral lead to the neutral bar. Place the covers over both the garage junction box and the main panel. Turn on the main power and make sure all power connections and breakers work properly. Fill in your trench.