Ok, I am new to the site, but I have looked and looked and not found the answer to my specific questions. Many questions are similar but do not have the same specific circumstances and I have asked many people including electricians and I get different answers from everyone I ask so I am seeking additional advice.
Here is my situation:
I have power coming into one corner of my house. I have been told this is 400 amp service. Coming out of each side of the meter box are Midwest boxes containing 200 amp breakers in each Midwest box. These boxes have space on the other side of the breakers to install additional breakers. From these boxes there is Aluminum cable that runs to the opposite corner of the house in the basement where I have (2) 200 amp panels. I want to tap this setup and supply two 100 amp sub-panels into my workshop (detached) that is 170 feet away from the meter and the two Midwest boxes. I need as much power as I can get into my shop without having to go to the pole to get it. I will be running table saw, heat & air (2 large window type units), planers, welder, dust collection (5hp motor), lights, etc. I intend on balancing my loads off each panel. In other words, my dust collector will be on one panel and wood working equipment on the other. I am concerned about the distance of the run (170 feet) and the voltage loss. I have had people tell me that I must run copper of this size and that size, some say aluminum is fine but needs to be larger to carry the load. I have looked at charts, tables, graphs, etc. and I still don't feel that I have the correct answer. My plan is to insert 100 amp breakers into the Midwest boxes and feed each sub panel in the shop from one 100 amp breaker. I will have two runs of wire 170 feet each. The wire will have to be buried in the ground two feet deep. The two sub-panels will then have a main breaker in each of them at 100 amps.
My specific questions are:
1) If I use Copper wire, what size?
2) If I use Al wire, what size?
3) Depending on which I choose does it have to be in PVC or can it be direct bury?
4) Do I also run a ground wire from the Midwest boxes to the subpanels, or do I ground the panels at the workshop, or both?
5) Given the fact that I am going to have to run large wire how do I make the connection to the 100 amp breaker given that the wire is going to be way to large to slip into the breaker?
6) Do I need a disconnect on the exterior of shop for each run of cable and if so any suggestions on what to use?
Any help would be appreciated.

You can't run two feeders to a remote structure.

225.30 Number of Supplies
Where more than one building or other structure is on the same property and under single management, each additional building or other structure that is served by a branch circuit or feeder on the load side of the service disconnecting means shall be supplied by only one feeder or branch circuit unless permitted in 225.30(A) through (E). For the purpose of this section, a multiwire branch circuit shall be considered a single circuit.

225.30(A) through (E) do not apply here.


I think 100A is PLENTY for what you are doing. You'd be surprised what the actual load is compared to what you think you need.

1) I'd use #2
2) I'd use #1 or 1/0
3) Depends on the cable you use. I would use URD in PVC conduit. Either way you DO need conduit above ground for physical protection.
4) Yes; run two hots, a neutral and a ground.
5) A 100A breaker should accept up to 1/0.
6) You need a disconnect near the point of entry. Needing one outside would be a local code.

Thank you for your answers! Just to clarify about the grounding - The way I understand your answer is that I do not set a ground rod at the shop and use only the ground back to the supply? In which case, my subpanel would have separate ground and neutral (non bonded)? I was thinking that being a detached structure I had to have a ground rod and would also need to have the ground wire going back to the supply side.. .

I never mentioned a ground rod.
You DO Need a ground rod at a detached structure with a feeder. The rest is the same.
The ground rod has NOTHING to do with the grounds and neutral in the panel.