There are several ways to approach this. So...I'll comment on the specific parameters you provided.
a. You have a 200 amp service at your house
b. You want to feed a 100 amp service at your garage
c. Your garage is 275 feet away
As others have stated - voltage drop is a determining factor. However, under your paramaters - you will be running a "feeder circuit" to your garage - not a branch circuit. Your allowable voltage drop is 2% NOT 3% as someone has stated.
The NEC puts it this way: Voltage drop on a branch circuit is 3% with a voltage drop of 5% overall.
A branch circuit is defined as the conductors "from the last overcurrent device to the load." Each circuit "inside" your garage is a branch circuit. The circuit running from your house to the garage is a "feeder circuit."
5% overall voltge drop: 3% for the branch + 2% for the feeder = 5%
If you are running a 240V Feeder to your garage, 2% voltage drop is...4.8V
Here is a simplified equation for figuring voltage dorp: I x R x L
Where: I = Current R = Resistance L = Feet of Wire Note: A circuit has two conductors - if the load is 275 feet away - you will use 550 Feet of Wire
The Details
I would not use conduit. A 275 Foot pull is VERY difficult.
I would use "triplex" direct burrial cable - and I would opt for Aluminum
Your trench must be a min of 2 feet to the top of the cable...so be safe...trench 30 inches deep.
You must protect direct burial cable to a min distance of 18" below the ground at each end where the cable emerges from the trench. No need to protect direct burial cable in-between.
Caculation
Note: The Min Size Alu Conductor For 100 amps is: #1 However, because of voltage drop, your actual wire will be larger.
#1 100 x 0.000829 x 550 = 45.6 Volts Dropped - Can't use #1
2/0 100 x 0.000523 x 550 = 28.8 Volts Dropped - Can't use 2/0
250 kcmil 100 x 0.0002778 x 550 = 15.3 Volts Dropped - Can't use 250 kcmil
500 kcmil 100 x 0.0001391 x 550 = 7.69 Volts Dropped - Can't use 500 kcmil
What's the point?
Under the parameters you stated - You Cannot Size Your Wire Large Enough To Satisify a 2% (4.8 V) Voltage Drop
I would get with your utility company. Explain what you want to do. And let them provide you with options.
Looking from where I sit - a separate service at your garage is the only real option.
A licensed Electrican from your local area may have different/better advice for you.
Respectifully,
GB
THIS ^^^^
You have to meet code minimum. Remember if there ever would be a fire do to wiring issues and you not doing it right can mean a denial of claim by insurance company. May cost more $ upfront but doing it right never hurts. This is no place to look cheap.
