I'm having a little trouble figuring out the best way to get power to a backyard office. The office will be 8'x12' and sits about 40' from the house. The problem is the breaker box is on the opposite side of the house. I figure it's about 150' total of wire after going through the attic and up/down the walls etc.
The office will have a computer, lights, window a/c, and a small space heater. I will be burying the wire in pvc conduit from the house to the office. I think 20 amps will be plenty. I am in south Texas so a small space heater running on low will be about all I need.
So what would be the best approach: Do I run 8/3 to a small 240v subpanel on the opposite side of the house (about 100' of wire away) with a 30 amp double pole breaker at the main panel, then just run 120 to the office by putting a 20amp breaker at the subpanel and run 12/2 direct bury to the office?
Or do I run 240 all the way to the office with a subpanel there?
I was originally thinking maybe I would just run a 110v line the whole 150' to the office but the wire size calculators show I would need a #6 wire that far so maybe that's not a good idea.
All I need is 20 amps out in the office but not sure what the best way is to get it there. Any advice is appreciated!
EDIT:
Thanks for all the great detailed responses, I feel like I am taking info from all of them to create my solution so wish I could mark them all as "the answer"!
Best Answer
Even for limited use, you might as well run a MWBC (240V, split) from a two-pole GFCI breaker and get TWO 20A 120V circuits, IMHO. You can split the loads between them. Takes only one more wire.
Might also want to give "small minisplit" a consideration .vs. window AC and resistance heat.
"The wire calculators" are treating 3% as a rule - it's not, since you are not in Canada - and you don't calculate it on breaker trip (20A) you calculate it on your planned load (and your planned load should not exceed 80% of the 20A breaker size, so 16A.) They want to sell more and larger wire. You may not need that. If you split your loads between TWO 20A circuits on a MWBC, you may find that 4 significantly smaller wires (planned for the actual loads, now split between two circuits, so less load on each) will do the job just fine. If it drops 5%, it's not a big deal in Texas - you couldn't plan it that way in Montreal, but you're not in Montreal.
Looking at a 16A load and a 150 foot run, 8 AWG Aluminum (much less expensive than copper, and not scary in this use) would do a fine job for either 120 or 240V service. You'll probably need to go somewhere other than Home Depot/Lowes to buy it (or you can mail-order it) but you should do that anyway (they are not a good deal on most electrical supplies...)
Aluminum for feeders is fine, large gauge aluminum wiring never had the problems that the 10/12 gauge stuff they used for 20/15A circuits did - partly a matter of a particularly bad alloy choice, partly a matter of the connections on receptacles and switches.
That will (probably) connect straight to a 20A breaker; I know mine are rated for it. At the shed end, you can either use a heavy-duty shutoff (gray steel box with a lever) that might take it directly, or you can use a suitable connector (such as a polaris ISR-1/0) to adapt from 8 Gauge aluminum to 12 Gauge copper in a junction box before landing on a 20A two-pole switch (like a normal wall switch, if a tad beefy) as the local shutoff. Then wire the shed circuits with 12 Gauge copper.