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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"!

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  • Have you considered that you might want a heat pump instead of the space heater + window a/c? Also, is the breaker box on the inside or outside of the house? – ThreePhaseEel Jan 31 at 3:09
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    I have not considered heat pump, but down here in San Antonio, it doesn't get that cold so that is why I am planning on a small space heater as a low cost option. It was 75 deg here today, we barely see below freezing temps. And breaker box is on the outside of the house. – lucky.expert Jan 31 at 3:24
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    Similar question (slightly longer distance, less power need) diy.stackexchange.com/q/122399/18078 Also, given that this IS an office, run at least TWO conduits (networking/communications as well as power.) – Ecnerwal Jan 31 at 4:40
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    Any heat pump is probably going to be an order of magnitude more expensive than a window AC unit and a small space heater. – TylerH Feb 1 at 15:07
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    Considering it's an order of magnitude difference, it's gonna be 20 years at least. :-) especially in a place like Texas where it isn't needed. – TylerH Feb 1 at 18:48
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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.

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  • So you are suggesting running 240v all the way to the office with a subpanel there, correct? And are you also suggesting running 4 separate single wires or a single 8/3? I hadn't considered aluminum so I'll look into that for sure. Half of my wire will be above ground and the other half underground, wonder if I need something direct bury rated the whole way, or maybe it doesn't matter if in conduit? – lucky.expert Jan 31 at 3:45
  • Not a sub-panel. An MWBC - multiwire branch circuit - you do need a "local shutoff" at the shed, but that can be a two-pole 20A switch. It is 240V, but it's split into two 120V circuits that share the neutral. I prefer wires in conduit by a WIDE margin over direct-burial. Cable in conduit is misery - don't do that (use a junction box to change wire types if you run cable until you go underground.) Or just run conduit the whole way. – Ecnerwal Jan 31 at 3:48
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    If you put in a sub-panel, you might need to choose an outdoor one. The required working space in front of a sub-panel is 2.5 feet wide and 3 feet in front, and nothing can be stored in it; it must always be kept clear - which is a significant chunk of a 96 square foot space. – Ecnerwal Jan 31 at 3:56
  • Ohhh I see, so 4 single strand wires from the main panel the entire distance to get duel 120v circuits. I like the simplicity of that a lot. – lucky.expert Jan 31 at 3:56
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    They would need to be wet rated (all outside conduits are defined as wet, and usually live up to the definition) - THWN, XHW, THWN-2 - some rating with a W in it. But not direct burial - the conduit deals with that. At #8, you need one green (or bare), one white or gray, and either two from any of the other colors that are not white, gray, or green (can be the same color or different - red and black are usual, because that's what you'll find in a cable.) And actually your ground wire does not need to be #8 (both hots and the neutral would be.) Most THHN is also marked as THWN. – Ecnerwal Jan 31 at 4:20
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Run 2-2-2-4 MH feeder from the panel to the office.

The reason is the distance and very severe voltage drop on 120V circuits. Normally I'm pretty slack about voltage drop. However 200' at 120V is so bad even I have to stand up and take notice.

And it isn't even a case of 1 wire size bump (to 10/2)... that might work at 100' but at 200', just off the cuff I can tell you that you'll need a second bump to 8/2.

And now we get into a costing problem. 8/2 UF-B is $1.28/foot. Well, heck. 2-2-2-4 MH feeder is $1.27/foot. Why are we putting in 40A/120V cable when we can put in 90A/240V cable for less money? It simply doesn't make any sense!

Of course you can't land #2Al wire on a 120V receptacle (can't land #8 either)... so the cheapest solution for landing #2 on a recep is just running a subpanel.

  • Use a 60A feed breaker (the wire is good for 90A but 60A is the cheapest breaker that'll take #2 wire).
  • The subpanel needs a disconnect switch (being an outbuilding). I would select a 12-space or larger subpanel that is convertible to main-breaker (don't convert it yet).
  • I would simply connect the feeder wires to the subpanel's main lugs and install ONE 20A/240V breaker ($10). That will serve as the main disconnect since it is the only breaker. You can power up to two 120V/20A circuits off of that.
  • Voltage drop will simply be a non-factor for a 120V/20A load.
  • Later on, if you want to grow that into a real subpanel, you simply retrofit the main breaker and install as many breakers as you please.
  • At 200', voltage drop at 240V isn't a big deal. You're allowed to plan to use up to 72A on that circuit, and at 72A you'll have 3.52% voltage drop. That's no big deal.
  • So yeah, if you want to install seven 2000W Cadet heaters to make the building warm up real fast, go for it.
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    Thanks for the idea, it's good to know I could go way big for about the same price as just big enough, I wouldn't have ever guessed that was possible – lucky.expert Feb 1 at 2:03
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    Would it be sensible to use a 240 V heating/cooling unit? – Andrew Morton Feb 1 at 9:46
  • Would it be worth the OP using an RCBO in the house, before the cable heads outside to the outdoor office? – SpacePhoenix Feb 2 at 8:50
  • @SpacePhoenix Always... except the US standard for RCBO/GFCI is 5ma, which is optimized for human-safety protection. And that's awfully sensitive for a whole structure. Would work for 1-2 circuits but if that gets expanded in the future, the nuisance trips may get excessive (which is why Europe uses 30ma). – Harper - Reinstate Monica Feb 2 at 18:12
  • @Harper-ReinstateMonica maybe RCBOs in the "local" panel on the home office. AFAIK it's possible to get RCBOs combined with arc fault detection devices in the same device – SpacePhoenix Feb 2 at 19:44
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I'd run two conduits instead of one

You'll want to run two conduits out to this shed instead of just one. Why? At some point, you'll probably want a hardwired data service to your shed, and running a separate conduit for data makes that much easier. As to conduit sizing? A pair of 1.5" PVC conduits provide ample room for most things you'd want to do, including Harper's 2-2-2-4 mobile home feeder. The data conduit can terminate in bell-ends inside the walls of the house and shed, while the power conduit terminates in the shed panel as Harper describes, and transitions to ENT ("smurf tube") going back to the house's panel when it enters the house walls.

GO BIG OR GO HOME

As far as that panel in the shed goes, I echo Harper's suggestion of getting a panel that's convertible from main lug to main breaker. This pretty much automatically puts you in a 12-space or greater panel if you don't want to burn spaces on a backfed main breaker later, but I'd go larger yet as panel spaces are cheap. In fact, a 24-space or 30-space, 125A, convertible main panel would be not at all out of place here, and using a NEMA 3R (outdoor/weatherproof) panel means that you don't have to allocate clear working space for it indoors.

Just remember to get and fit the correct grounding bar for your panel if it doesn't come with one, and also remove the green bonding screw or strap so that neutral and ground are properly separated! You'll also want to install a pair of 8' deep ground rods driven 8' apart and connected to the panel grounding bus with 6AWG bare copper, unless you are putting an Ufer (concrete-encased electrode) in at your shed. That takes care of the grounding electrode system for your shed, which'll be required as soon as you install more than one throw of disconnecting in your subpanel anyway.

As to what goes in those conduits...

While I don't think the voltage drop situation is quite as bad as Harper makes it out to be, you still will want to take it into some account by running 240V to the shed. This means that you'll be using a two-pole breaker in the subpanel as your shutoff, and four stranded THHN wires instead of a UF cable inside the power conduit. However, over this distance, for a 240V run, 12AWG is adequate, with 20A breakers at both the main panel and the shed subpanel; if you want to, you could bump up to 10AWG instead, but you shouldn't need to go further than that.

As far as the data conduit goes, you'll want to pull fiber through it instead of outdoor-rated 4P UTP cables. This has nothing to do with bandwidth, though, and everything to do with lightning. You see, when copper communications cables are run out doors, the NEC requires what's called primary protection at each end of the cable, which is a significant cost and time adder to the project since you can't buy such things at Home Depot. Nonconductive fiber optic cables don't need any of that, though, just a pair of switches with fiber transceivers in them. Of course, if you have adequate WLAN signals at the shed, you don't need to pull the fiber now; in that case, the conduit simply becomes a future provision, which is fine.

And keeping things nice and cozy in there

While your solution of a small space heater (such as an inexpensive 120V baseboard) and a window A/C is not inadequate for a shed of your size, the more efficient solution would be to use a window or through-wall mounted heat pump. This kills two birds with one stone, and provides you with a more efficient heat source than straight electric heat, while still having electric backup heat available to fill in shortfalls in heating.

Notably, while many TTW and window-mount heat pumps run on 230V, the through-the-wall mounted Amana PBH092 runs on 115V and is available for a reasonable price through a variety of online suppliers (Home Depot actually overcharges significantly for this model). You'll need the matching PBWS01A wall sleeve, of course, and that does drive the price up slightly, but it's also not a hard part to find, and lets you upgrade to a higher-capacity unit later when you fit the full subpanel if you wish more grunt from your shed-office's HVAC system.

Finally, instead of throwing good energy dollars at a bad envelope, I'd construct the shed with good insulation and air-sealing now, as it's far cheaper to get it right the first time. A good air/water barrier (fluid applied or fully adhered) over the sheathing, along with good flashing details, is essential to keep hot Texas air, or rain for that matter, from leaking in, and I'd apply 2-4" of rigid insulation board over the top of that to provide a good, continuous thermal barrier as well.

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  • Thanks for the ideas on the in wall heat pump, I will look into those for sure! I hadn't considered those would use less energy for heating. Also I think I am going to do as you say and go for either 12awg or possibly 10. After running the numbers again through the calculators now that I understand a little more about all this, It does look like those would be sufficient for the load I will be putting on them. I appreciate the detailed answer! – lucky.expert Feb 1 at 1:50
  • @lucky.expert, don't rule out aluminum conductors for this project, even if you're not bumping all the way up to the 2-2-2-4 Harper suggested. 6 AWG Al is still cheaper than 10 AWG copper per foot, and will support 50A instead of 30. And when landing on modern breakers and panel lugs, which are all aluminum rated, it's easy to use. Source for the prices: wireandcableyourway.com/thhn-thwn (copper) and wireandcableyourway.com/xhhw-2 (aluminum) – Nate S. Feb 1 at 18:30
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Subpanel at the Office

You already need, practically speaking, several circuits:

  • Lights and receptacles - which could be one 120V circuit or two. Advantage of 2 is that if you trip the breaker on the receptacles, you still have light.
  • Air Conditioner
  • Heater

Note that you may be able to combine the air conditioning and heating into one heat pump unit. But if you don't, you really do NOT want to put a plug-in space heater on the same circuit as your computer. You may think "I'll only use it on low", but you'll go into the room on a cold morning and want to get started working quickly and flip it to high (1500W) and then turn on the computer and the laser printer and try to print your schedule for the day...and everything goes off because the breaker (correctly) tripped.

Put in a subpanel. The subpanel itself can be pretty much any size, including much "bigger" than the main panel breaker & wire that are feeding it. For example, you might feed with 40A or 60A (enough to power lights, 2 receptacle circuits (so your space heater doesn't share with the computer equipment, air conditioning) but have a big panel (because you then don't have to worry about future upgrades) that has a 100A breaker because it came bundled with it - big orange & blue stores often have bundles that include the panel plus a bunch of "free" breakers.

You will most likely need GFCI protection (in the subpanel or the receptacles) for all receptacles (but not for lights if they are on a separate circuit). You will also need to separate ground from neutral in the subpanel and ground rods + wire at the office (in addition to the main ground wire going back to the main panel).

As noted in some comments, the subpanel needs the same working space as a main panel. The simplest way to do that is to put it on the outside of the office. Typically, conduit comes up from the ground into the bottom of the panel. The building will need a cutoff switch - the "main breaker" on a subpanel satisfies that requirement.

Wire Size/Conduit

The best thing to do about wire size is to put in conduit. By using conduit, you can change the wire later, if needed. You can also put in a second conduit for your network connection - I wouldn't rely on WiFi for that distance. There are a lot of recommendations recently to use fiber instead of copper for remote locations - ask another question if you need more details.

If you think "conduit will really increase the cost", it won't!

  • Cable generally costs a little more than separate wires, and you can't use plain NM (Romex) cable underground.
  • Being able to change/add things later without having to trench again is a big plus.
  • Conduit is cheap! Big orange has 1" PVC schedule 40 for ~ $5 per 10'. So the 40' outside will cost you ~ $20 + fittings. Not much in the grand scheme of things.

If you use rigid metal conduit then you don't need a separate ground wire. If you use PVC then you do. In either case, it doesn't have to be very big. For example, 3/4" conduit can handle 6 AWG x 4 wires. You may even be fine with 8 AWG, and aluminum (generally requires a size larger than copper for the same current capacity) can save a lot of money. The crucial thing is that the breaker in the main panel needs to match the wire size/type. With a large enough conduit, you will have the option to upgrade later, if needed, without needing to dig again.

Voltage Drop

For 150', voltage drop is not normally much of an issue. More importantly, most of the time you won't use anywhere near the full provisioned amount. For example, you won't run the heat and the air conditioning at the same time.

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  • Air conditioner and heater are covered by one circuit, unless you are the sort of person who would run both at once... – Ecnerwal Jan 31 at 3:32
  • @Ecnerwal True with respect to any real load calculation. But they may be connected to physically separate circuits. – manassehkatz-Moving 2 Codidact Jan 31 at 3:34
  • Despite the accepted DIY-SE mantra of "make it huge" I think a simple two-circuit approach of "thermal control" and "lights and computers" is perfectly adequate for a 96 square foot shed. ;^) – Ecnerwal Jan 31 at 3:45
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    I see office as naturally expanding (add a separate server, etc.) as opposed to a simple "shed". – manassehkatz-Moving 2 Codidact Jan 31 at 3:49
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    Do not go direct burial. – manassehkatz-Moving 2 Codidact Jan 31 at 4:19

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