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.
Best Answer
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.)
TORQUE ALL LUGS TO SPEC
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!