Your plan is to build a dream shop. Like most of us you have a budget the minimizes extravagance, so you will have to tone it down. What kind of shop, a general trades shop? Some wood working, may even consider some ornamental iron work. What about general repairs to the home, kid’s toys, appliances and tools? Minor mechanical, like oil and lube services, minor tune hopes or a whole classic restoration? Minimum power would dictate a at least a hundred amp up to about a 150-amp service. 100-amp panels can service from a low of 20 to as high as 32 120-volt circuits. Each 240 circuit requires two slots. So, with a hundred-amp service you could run two 240-volt circuits and four 120-volt circuits. This is usually adequate for a small shop. Even with 20 slots you could run separated lines to each wall. You may even want to consider ceiling power drops to avoid the clutter and danger of using temporary extension cords as permanent fixtures.
Which is Better; Aluminum or Copper Wire?
For a shop up to a 100 ft away, voltage drop calculators can be used to determine wire size. Also, you will want to keep your drop under 3 volts. Aluminum is far cheaper than copper but requires larger wire gauge. Given a 100-foot run, 2/0 AWG aluminum wire you will experience a 2.46 voltage drop or a 1.03% drop. Your subpanel will have 237.54 volts available. You would need 1 AWG gauge copper at about $3 per foot for single wire and you will need three wires with a ground. Copper UF or underground feed is neither heavy enough nor economical. You will need to run three aluminum 2/0 AWG with a #1 AWG ground. One pair is your hot lines, one a neutral and the other the ground. You can buy aluminum 3 wire with ground direct burial service entrance cable or you can use individual wires pulled through 2” conduit, preferably schedule 80 plastic. Even with the service cable you will need conduit in the transition from the trench bottom to the service panel, at both ends. Cable needs to be buried in sifted sand or soil. No rocks as these can cut through the sheathing. Also consider running phone and or network wire for internet access from the shop. You will need to install #6 AWG copper (all in-building circuits should be copper) to service a 30-amp 240-volt circuit. All other circuits are 12-gauge AWG solid copper to power 120-volt circuits.
How Do You Light an Outside Shed?
It is suggested that the lighting circuit to be separate from line circuits. Another consideration for the shop is to install double duplex outlets, 4 receptacles per outlet will be appreciated in the shop environment. For lights, 48-inch LED light fixtures will last 20 years without changing a bulb and use 1/3rd the power. The 240-volt circuits can power compressors and/or welders and perhaps even a table saw if wired for 240-volts. You may want to contemplate three 30 amp 240-volt circuits; you will still have more than enough single 120-volt circuits for the typical shop. Better an extra than to come up short.
Electrical Codes of Practice
With a ground wire running to the main panel, the subpanel will be grounded as well back to the main. If grounded through the main the neutral and ground bars are not bonded. With a separate subpanel ground, the ground and neutral bars are bonded. Electrical systems have a lot of variables, NEC (National Electrical Code) is recognized in all 50 states. But each state, county and municipality may have their own additions to the code. Sometimes even the experts disagree to best practices across jurisdictional boundaries. If in doubt it is best to consult an experienced electrician or hire the work out. You can hang the subpanel and even do the interior wiring and dig the trench, but correct installation is crucial for efficient, effective and safe installation.