I have a 200 amp main panel in my basement. I want to add a 100 amp panel in my detached garage. I have to run the new line 35 feet across my basement, through a block wall then 8 feet underground to get to the garage. 1. What size and type of wiring should I use? 2. Should I run the underground wiring through conduit to the garage? 3. Do I connect in the main to a 100 amp breaker then to the sub 100 amp??

Item 3 would be the correct method to feed the sub panel.

I need to know if you are planning to use copper or aluminium conductors.

If you use rigid metal or intermediate metal conduit, the depth would be 6".

If you use an underground cable, then 24 inches, with a ribbon tape warning at about 12 inches.

You need to protect the cable as it passes through the wall with a sleeve. Conduit would be needed only if you need to protect the cable.

You have to use a four wire cable feed from the main panel to the sub panel. Also, you must keep the Neutral and gound isolated from each other at the sub panel.

You will also need to set up a grounding rod system at the sub-panel.

I want to caution that while rigid conduit can be buried only 6 inches generally speaking, if it crosses under a residential driveway or a parking area, it also must be at 18 inch deep minimum.

While a warning tape is only required by Code over a utility service entrance feed, it is wise to install over any underground wiring, no matter if direct buried cable of conduit.


While technically not allowed by Code, I am sure your local inspector will allow you to use #2 aluminum wire. Worst is he will require a 90 amp breaker instead of a 100 amp.

Since you only need have 8 feet that will be underground, you can use #2 Aluminum 4 wire SER cable from the main panel to the new panel, using 1.5 or 2 " PVC conduit to sleeve the cable where is needs to be underground.

Done right, you will not need any splices or junction boxes between both points.

Again, to use #2 Al and follow the Code to the letter, use a 90 Amp breaker in the Main Panel to protect this feeder.