How to Wire a Garage From a 100 Amp Breaker in the Main

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Installing a sub-panel in a separate building (garage) is one of the most common additions to an electrical system in residential housing. It provides the ability to install lighting, or even larger equipment such as compressors or welders, in your garage. There are different code requirements depending on the city your addition is in. Cities that have adopted the 2008 electrical codes are slightly different than cities still using the 2006 electrical codes. This article will use the 2008 code rules, as it is correct whichever codes your city uses, it is also more reliable.

Things You'll Need

  • 1 grounding clamp (suitable for direct burial and ½ inch grounding rod)
  • no. 4 bare copper wire approximately 10 feet in length
  • 3 no. 2 copper wires suitable for wet locations usually has a U or W in the name
  • 1 no. 6 copper green suitable for wet locations usually has a U or W in the name
  • 1 ½-inch schedule 40 PVC conduit (for underground between house panel and garage panel)
  • 2 10-foot sticks of 1 ½-inch schedule 80 PVC conduit
  • 2 1 ½-inch PVC couplings
  • 2 1 ½-inch PVC connectors
  • 2 1 ½-inch PVC insulator bushings
  • 2 1 ½-inch schedule 40, 90 degree elbows
  • 2 1 ½-inch 2 hole straps
  • 1 ?-inch 1 hole strap
  • PVC glue
  • Pliers
  • Allen wrench sized to fit the main lugs in the sub-panel
  • Straight screwdriver
  • White Electrical tape
  • Turn off the main breaker in the house panel before working in the panel box. Dig a 20-inch deep trench between the main house panel and the garage sub-panel locations. You can only have four 90 degree bends in a single run between the panels, and there will be two already where your conduit comes up into the panels, so dig your trench with that in mind.

  • Install 1 ½-inch schedule 40 PVC conduit the entire length of the trench and connect them together with PVC glue. PVC conduit is, by National Electrical Code 300.5, required to be 18 inches minimum to the top of the conduit in the trench. 1 ¼-inch conduit is usable in this situation but 1 ½-inch conduit will make the wire pull much easier.

  • Glue the 90 degree elbows to the schedule 40 where the conduit will come out of the ground to both panels and try to line up the conduit with one of the 1 ½-inch knockouts towards the back and bottom of the panels.

  • Using a screw driver and pliers, knock out a 1 ½-inch hole, in line with the conduits, in the bottoms of both panels and as close to the wall as possible. Measure the distance from the 90 degree elbow ends, where they come up below the panels, to the bottom of the electrical panels. Cut your schedule 80 to that length allowing extra length for any couplings (flared ends) built into the conduit.

  • Attach the schedule 80, using PVC glue, to the 90 degree elbow below each panel. Glue the 1 ½-inch PVC connectors (threaded connector with a lock nut) to the end of the schedule 80 where it will connect the conduit to the bottoms of the panel boxes. The only exposed area of conduit, between the dirt and the panels, needs to be schedule 80. Metallic conduit can be used for the exposed area also but schedule 80 meets the code requirement for protection and doesn't require grounding.

  • Insert the schedule 80's threaded connectors into the 1 ½-inch holes you made earlier and thread on the 1 ½-inch lock nut that came with your connectors, from the inside of the electrical panels, to secure the conduit. Install the insulator bushings on the inside of both panels to the threads of the connectors above the lock nut you just installed.

  • Identify one of the No. 2 wires as a neutral by wrapping 2 feet of each end with white electrical tape. Tape all of the wires that will go into the conduit together every couple of feet so that the pull will go easily. Pull your three No. 2 and one No. 6 wires through the conduit leaving at least enough wire in both panels to reach the top inside of the panel boxes. Using the 1 ½-inch 2 hole straps secure the conduit to the wall beneath the electrical panels within three feet of the panel boxes.

  • Disconnect the jumper bond between the ground bus and the neutral bus inside of the sub-panel in the garage to isolate the neutral from the ground. You may need to consult the paperwork that came with the sub-panel to see exactly how to disconnect the bonding jumper for your particular type of panel. Isolating the neutral and ground is always needed when running 4 wire feeds to a sub-panel. One of the buses will be isolated from the metal panel box usually with a plastic stand-off, this is your neutral bus. The other bus will have a green ground screw that needs to be installed to connect the bus to the metal can, this is the grounding bus.

  • Attach the ground wire from your conduit to a ground bus screw inside of the sub-panel. Attach the neutral wire you identified with white tape to a neutral bus screw inside the sub-panel. The neutral has to be marked with white tape inside of both panels for identification purposes as per electrical code. Connect the two remaining No. 2 wires to the two main lugs in the sub-panel using an Allen wrench.

  • Drive a UL listed ground rod (compliant to NEC code 250.70) - 0.500 inch diameter, 10 mil copper rod [Eritech cat. # 611380] into the ground within 3 feet of your sub-panel. It must be driven into the ground so that 8 feet of ground rod is in contact with the soil, the whole ground rod should be buried leaving a shovel full of dirt out for inspection purposes. A UL listing is stamped within 12 inches of the driving end of the rod that should be left visible for the inspector. While driving the rod into the ground if you hit rock bottom you may angle the ground rod by up to 45 degrees. If you still have problems driving the rod into the ground it may be buried in a 30 inch deep trench.

  • Knock out one of the small holes in the bottom of the sub panel to allow the 4 bare copper ground wire access into the panel. By using No. 4 bare instead of No. 6 you can avoid having to put the wire in a conduit for protection, 4 bare can be run without protection per NEC 250.64 (B). Run a 4 bare copper wire from the electrical sub-panels' grounding bus to the ground rod.

  • Connect the 4 bare copper to the ground rod using a ground clamp suitable for direct burial. Pull the other end of the 4 bare copper wire through the small hole you made in the bottom of the panel and connect it to one of the screws on the grounding bus.

  • Secure the bare wire below the sub-panel within 12 inches of the panel box with an ?-inch 1 hole strap or staple.The ground wire should be coming out of the bottom of the sub-panel straight down into the dirt (to keep it out of the way of tripping or mowing). The wire should run about 2 inches under the surface to the grounding rod.

  • Connect the two black No. 2 wires (not identified as neutral) in the main panel to the screw connectors of the 100 amp breaker that will feed the garage using a straight screwdriver (or Allen wrench depending on the brand of breaker). Connect the No. 2 wire you marked with white tape to the neutral bus (where all of the white wires in the main panel are attached) using one of the screws in the bus. Connect the green ground wire to a screw on the grounding bus in the main panel (where all of the bare copper wires are attached).

  • Label the 100 amp breaker in the main panel "Garage sub-panel" with permanent marker. The No 2 wire size was determined using NEC 310.16 for 100 amps.

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