Electricity to my shed

Question

I live in the Dallas, TX area and have a 10' x 10' Tuff Shed that is located 3' from my house. I want to run electricity to the shed to power A) internal light (4-tube 4' fluorescent fixture at 120 watts) B) exterior light (LED 14 watt) C) exterior outlet (for casual use for hedge trimmer, weed eater, etc.). So let's call it 2-amps (rounding up a lot) with both lights on and say another 5 amps when I plug something "big" into that outlet. All together a max of about 7-amps.

The plan is to install a 20-amp breaker in the breaker box and run a single 12/2 UF-B cable from the breaker, thru the attic, out the soffit, down the exterior house wall, underground, up the shed wall and finally into the shed. The breaker box is on the other side of the house from the shed, so the length thru the attic will be about 65', plus maybe another 25' going up and down and around.

As the cable comes out of the house soffit (vertical), I will put it inside 3/4" schedule 80 PVC conduit to a 1-gang weatherproof box mounted on the side of the house which will have a 15-amp GFCI outlet rated WR/TR with an in-use cover.

From the external box, the PVC conduit will continue down into the ground 12" with a curving 90 to run the horizontal 3' to the shed, then another 90 curving bend up the exterior side of the shed to another weatherproof 1-gang box. This box will contain an exterior switch (for a cutoff) and the wires will finally run into the shed from the back of the exterior box.

Once inside the shed, I'll continue to use the UF-B 12/2 cable (exposed) to the 2 lights, 2 switches and WR/TR exterior outlet with in-use cover (I don't really need an interior outlet in the shed). I will run all the horizontal wire runs at the top of wall headers (to keep them out of the way) and come down vertical on a stud to the 2 switches, lights and outlet.

My questions about all of this are:

1. Any problem running the 12/2 UF-B cable in schedule 80 PVC 12" below the ground? I think I could run THWN-2 wires in this conduit at 12", but I would rather use the UF-B cable I already have instead of buying THWN-2. Since it's such a short length with only 2 bends and the pipe is 3/4", I don't think I'll have too much trouble pulling the cable wires thru the conduit.
2. Do I really need the cutoff switch on the shed? My understanding is that every remote structure requires its own cutoff switch at the structure. Even though I have a GFCI outlet on the house that is technically a "cutoff switch", it's on the house (3' feet away) and not the shed. And I think I understand the the GFCI is required BEFORE I go underground. Is a simple switch good enough, or should I get like an AC disconnect? And can the switch is on the exterior or should it be inside the shed as the first thing on the circuit? I could also put another GFCI outlet inside the shed as the "cutoff switch", but a switch would be cheaper/easier to install.
3. Do I have to ground the shed with a grounding rod? The shed is sitting on concrete blocks and does not have a foundation, and I only have 1 20-amp circuit. So I don't think I need to do this.
4. Do I have to protect the wiring inside the shed (with for example flexible metal conduit) or can I leave it exposed? I don't plan on covering the walls so the wiring will always be exposed. My "protection plan" is keep the horizontal runs high and the vertical on studs, all stapled, clamped at the boxes, etc.
5. Should the GFCI outlet be 20 amps instead of 15 amps? The breaker in the breaker box will be 20 amps and I'm using 12/2 wire, so I really can do 20 amp outlets. On the other hand, I don't need anywhere near 20 amps for what I intend for this shed, and the wire run from the breaker box into the shed (after all is said and done) will be around maybe 90 feet, so my thought is to never allow a 20-amp device on this circuit. Is this reasonable thinking, or too conservative?
6. Any input/comments to improve anything?

Answer 1

1. UF can be run in conduit, but the size can be tricky. There isn't a chart that says a specific size. NEC Chapter 9, Table 1, Note 9 says:

(9) A multiconductor cable, optical fiber cable, or flexible cord of two or more conductors shall be treated as a single conductor for calculating percentage conduit or tubing fill area. For cables that have elliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter.

And then calculate per the allowed fill of 53% for a single conductor. I think 3/4" SCH40 PVC will work for 12/2 UF, but an inspector might require the math.

Then search NEC Table 300.5 for required cover over conduit in trench.

2. Yes you need a switch, it could be as simple as a snap (light) switch.

3. A single circuit (even a MWBC) doesn't require ground rods.

4. The NEC allows cables to be exposed, cables must be secured to maintain 1.25" clearance from either faces of stud or nail plated. Additionally it requires protection where subject to "Physical Damage". The NEC doesn't define that well, giving local AHJ's broad room to adopt ordinances or arbitrarily decide that in a shed is all susceptible to damage from garden tools.

5. 15A duplex receptacles are rated as 20A devices, you don't need 20A configuration.

6. Other thoughts. I don't put UF in conduit, it's hard to pull and generally a PITA to work with. If you run conduit and THWN wires then for almost no more work you can run a 4 wire MWBC, and double your capacity decreasing the chances of tools tripping lights.

Answer 2

Lots of "big" things take the UL maximum of 1500W for a plug-in appliance, and some things like saws take a lot more. So we're talking 12.5 to 15 amps.

Your distance is well into the zone where we need to actually break out the calculator and check for "voltage drop". That's because voltage drop is twice as concerning when it's a 120V circuit rather than 240V. So I would hitting a voltage drop calculator on the Web, override any silly "3%" defaults and see where your voltage drop lands given a 15A load. If you think that's good enough, then okay.

If you want to bury with 6" of cover, you need to use Rigid metal conduit - however at over $2/foot, this would be prohibitively expen -- wait, did you say 3 feet?

Otherwise you need 18" of cover over top of conduit, or 24" of cover over buried cable.

However, there is an exception that you can direct-bury at 12" (conduit entirely optional) if the circuit is GFCI-protected AT THE SOURCE. So right after you leave the service panel, stop at a receptacle junction box and install a GFCI receptacle there. Then feed the long run off the GFCI's "Load" terminals, which will confer GFCI protection to the circuit. A GFCI outlet at the shed is not good enough for this exception.

If you are only using conduit as a protective sleeve, then size is not a factor. Otherwise the conduit ID must be at least 138% of the widest width of the UF cable - and that is usually pretty punishing with UF, because it is so flat.

A single branch circuit (no subpanel) DOES need a disconnect switch. However it can be a simple "light switch" sort of affair rated for the current. It can be indoors or outdoors, but must be near where the circuit enters the outbuilding.

A single branch circuit to an outbuilding does not trigger the requirement for ground rods. Just the same, it's possible for there to be a voltage gradient between local earth and branch circuit ground, especially if you live near an electric railway or a neighbor is having a problem.

Yes, cabling e.g. in a garage needs physical protection. Face it, it's going to happen anyway, you're gonna nick a wire. Better to physically protect it, then, not worry. Code gives several options for that.

Other thoughts. If your local hardware store doesn't sell THHN by-the-foot, you need a better hardware store. If you want real power and the ability to support 240V, think about that multi-wire branch circuit I mentioned earlier, though that will require 12/3 UF.

Did you know that 10/3 UF (30A) is about the same price as 2-2-2-4 MH feeder (90A)? Interesting science fact. Of course you'd go broke on couplers to get from #2 to #12, so I'm not saying to do that in this case. Food for thought, though.