The Objective: Run a combo Heat/Cool Window unit, 1 switched light and some outlets to a detached storage shed 140 ft from home. Temperature goal is 60F min in winter and 80F max in summer. Located in North Texas. One light and outlets are for rare use. This will 100% be climate controlled storage of personal items to be accessed maybe 2-3 times per month.

What I have: Home built in 2019 with 200a main panel which is 220 ft away from where the subpanel will go in the shed. Distance from main panel to exterior of the home is approx 80ft. Then I have 1 1/2 in. conduit from the house to the shed using sweeping 90 degree fittings buried at least 18 inches. This is 140 ft. Distances are as the wire runs, not the crow flies.

My plan: (Revised based on suggestions) Install XXX amp breaker in main panel. For 240V, run quantity of 4 XX gauge copper feeder wire: two live, one neutral, One ground, a distance of 220 ft to a disconnect at the shed. Disconnect goes to subpanel. Then two branch circuits: One 240v for HVAC. Other for outlets and light circuit (GFCI).

My Primary Question: What size copper wire should I use as the feeder?

I have read differing opinions on this. Some leaning on the very cautious side and some leaning on the "don't overcomplicate it" side.

If I plan for breaking amps (40) at <3% voltage drop, I'll need 2awg? If I plan for normal operating amps (20? guessing) at <3% voltage drop, I'll need 4awg? If I plan for breaking amps (40) at <8% voltage drop, I'll need 6awg? If I plan for normal operating amps (20? guessing) at <8% voltage drop, I'll need 10awg?

Also, I'm fairly sure that shed will need it's own dedicated ground rod. But will it also need a ground feeder back to the main breaker? I have read that is a new requirement but don't understand why it requires both (if correct). If so, what size wire should I use to connect? Is it just naked ground like in romex?

Any other things I need to consider that I haven't will be appreciated. I have tried by best to research existing posts and books but I'm stuck currently on what to do. Thank you.

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    I'll let the experts dive in with all the details. But you should almost certainly run three main wires, not two. (Plus ground unless metal conduit). The reason is that 3 wires lets you run 240V or run twice as much 120V. That is a big deal for HVAC. How much HVAC - and therefore how much power - will depend on the size of the shed and the quality of the insulation. Even if you don't need 240V for for HVAC (but it might help a lot), having 2x power for 1.5x wire (and no extra on conduit, etc.) is a big bonus. And it provides more flexibility in case the shed later becomes a workshop. – manassehkatz-Moving 2 Codidact Jun 5 '20 at 22:38
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    Since the A/C unit is will be your biggest load we need to know about it. 120? 240? nameplate amp draw on the anticipated A/C unit? BTU? There are some here that really do over-kill IMHO on the panel size, but in reality, at the end of the day a larger panel doesn't cost much more. You don't need to feed the panel at it's rated capacity, you can always go lower for the feed, just so your "XX breaker" is of the correct size for the installed wiring. I would suggest a 240 volt feed which requires 2 hots, a neutral and ground (4 wires). A 240 v AC unit would draw half the amperage of... – George Anderson Jun 5 '20 at 23:03
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    dang, ran out of comment space. ....a similar sized 120v unit, meaning you could run smaller wires. The increased cost of a 240 v unit will likely be more than offset by smaller less expensive wire as well as give you additional options down the road in your shed. Anyway, what we need to know now are the spec of the AC unit. EDIT: Yes, you'll need ground rods at the shed and need to isolate (not bond) the neutral from the ground in the sub-panel. – George Anderson Jun 5 '20 at 23:05
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    @ChristopherHaley -- is the existing conduit PVC or metal? – ThreePhaseEel Jun 5 '20 at 23:05
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    Ed Beal will probably weigh in here soon. I don't know a lot about HVAC sizing, but I think you should do some calculations based on building materials and insulation values to get the right sized A/C unit. It might seem logical to go oversize (just to be safe) on the unit but that's not good for A/C. If an A/C unit is over-powered for the space it's cooling it doesn't run long enough to remove moisture from the air. That leaves the space with a "clammy feeling" with high moisture which could lead to mold and mildew. I'm hoping others here with more HVAC exp. will chime in. – George Anderson Jun 6 '20 at 0:30

Given your requirements, you don't need much for HVAC

For a shed the size of yours, just about any HVAC unit you can come up with will do the job. The basic tradeoff, then, is "do you want to do the bit of extra work a subpanel entails for a simple storage shed, especially given that the wiring's in conduit and can be easily replaced in the future, or would you rather save some work and perhaps a bit of money now, at the cost of being limited in what you can use for a HVAC unit?"

If you want the cheapest way possible to do this, I would use a 20A, 2-pole breaker in the panel, feeding 4 12AWG copper THHNs (black, black or red, white, green or bare) in the conduit, and then use a reducer to interface the existing conduit to a non-fused air conditioner disconnect at the shed to provide the required disconnecting means. This means you'll need a 115V HVAC unit, which are rather thin on the ground, though: the Amana PBH092G12CC is what I'd recommend as it's a full-fledged heat-pump in a through-the-wall package (instead of being a simple "air conditioner with a toaster element attached" arrangement). This is plugged into a single 5-20R that goes to one side of the MWBC, while the other side feeds the light switch/fixture and a GFCI convenience receptacle. Finally, at the disconnect, all the grounds go into the bonded bar provided, while the neutrals simply are connected together with a wirenut as they need to float.

The alternative gives you a 230V HVAC unit, but requires you to install a subpanel, complete with main breaker to serve as a disconnect, (our advice: go big or go home on subpanel spaces. A 24-space, 100A or 125A subpanel would not be at all out of place here.) and a grounding electrode system at the shed in addition to the equipment grounding wire in the conduit. Fortunately, two 8' ground rods driven 6-8' apart and connected to the subpanel with 6AWG bare copper suffices in most places, with a 15A or 20A 2-pole breaker in the subpanel for the single NEMA 6-20R for the HVAC unit and a pair of 20A, 1-pole breakers, one for the light circuit and the other for the GFCI. All of this would go with 4 10AWG (black, black or red, white, green or bare) wires in the conduit and a 30A, 2-pole breaker for the feeder. (The size of the subpanel's main breaker doesn't matter since it's just being used as a disconnect switch in this application.)


There's one other thing you'll want to do here, and that's use an inch-pound torque wrench or torque screwdriver to tighten all the electrical connection setscrews to their manufacturer specified torques. This is required by 2017 NEC 110.14(D), and also is a good way to make sure your electrical system won't give you the loose lugnut!


Base your ampacity Canadian style: on real load or 80%

Never base voltage drop calcs on breaker trip. That's just a dumb idea (but the wire salesmen sure like when you do).

When punching numbers into the voltage drop calculator, always use your actual, practical load if you know it, and feel free to make assumptions like 12.5A for common large-draw 120V plug-in appliances like heater-fans, saws, dust collectors etc.

(don't assume 12.5A for a laptop computer, for Pete's sake; look at its nameplate.)

If it is totally impracticable to project real load, then go with 80% of projected (ideal) breaker trip if you had a nanny breaker to protect you from voltage drop. The reason is you're required to provision 125% of any planned load you might have. This is just the inverse of that.

Disregard 3% but don't exceed 8%.

3% is a wire salesman's gimmick. It has two nuggets of truth: first, NEC makes a vague suggestion of that. Second, Canada requires 3% but it requires it on the above practical ampacity, not breaker trip.

8%, on the other hand, is generally where people get alarmed: "Are you sure you want to do that?" Beyond 8% your local inspector may yellow-flag or even red-flag you (depending on what you are doing).

For instance let's say I have a string of post lights going 2000' down a right-of-way. I'm using LED post lights that are deliriously happy on anything from 100V to 277V. (Japan socket voltage vs US commercial lighting voltage). I run it with #14 wire and 240V. My farthest light has 47% voltage drop, getting a mere 127V measured. We happy? Yes. Code doesn't care and the lights don't care. Now if you put a socket on the other end, different story.

Remember this about voltage drop: Your actual voltage drop is proportional to actual current draw at the moment. If you're drawing less current practically, your voltage drop will be less.

No nanny breakers in the USA

Suppose you aim for 40A service load on a feeder (50A breaker nominally). The remote load is an RV subpanel with its own 50A/30A/20A breaker for various RVs and a 120V socket. You decide to ante up for 3% drop at 40A, but due to distance that makes the wiring #1 Al (nobody really uses copper for long feeders). Well #1 Al can be breakered at 100A. What happens?

In Canada, you must breaker for 50A as planned. I call that a "nanny breaker", the breaker is "protecting you from voltage drop", or to be more precise, the State protecting you from yourself.

In the USA, yeah, you can breaker that at 100A. You'll get beat up by voltage drop but only if you actually pull enough current to matter. Let's say you fit two RV stands off that feeder, and you have two 50A RVs. Both of you are pulling 10A for various loads and HVAC, so voltage drop is only 1.5% at the time. One of you starts a dryer load of 22A. Now voltage drop is 3.2%. You stick your head outside and search for Canadian mounties, none in sight. So you're OK.

So next time you see a soldier, salute: They are the ones holding the line against Canadian mounties rampaging through this country tearing out RV stands!

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