what size wire for 100 amp subpanel?

[Rob Russell]

The above is exactly why I suggest contacting your inspection authority before following advice from those you will not be siging off on your installation.

In my neck of the woods a 36" depth is a minimum. "Appropriate" could mean one of many different things. IMHO, short of ridgid Aluminum conduit, metal should not be considered for burial. Although if you live in a very arid climate, you may get away with something that would normally rot.

Remember that "your neck of the woods" is north of the border and the Canadian Electrical Code is what applies. The National Electrical Code is the basis for what most jurisdictions have adopted in the USA. My comments are based on specific sections of the NEC and I'm perfectly willing to provide section cites if need be.

I do agree that checking with the AHJ is prudent.

[Josiah Bartlett]

Aluminum?!

I thought that stuff was banned...

Aluminum is used for panel feeds where the connectors are designed to be torqued with a wrench and used with anti-corrosion compounds. It is not for use in branch circuit wiring, especially with press fit connections.

It is used because it is far cheaper than copper in large gauges.