Is this plan for running a circuit in buried conduit correct?

Question

I'm planning on running a single circuit from my sub-panel in the garage to my backyard storage shed. This is an uninsulated, unheated wooden structure (pressure treated lumber, painted T1-11 siding, roof felt/asphalt shingle roof) for storage only - there will be no room for a workbench or tools or potting table or anything else. This is purely an extension of the attic/garage/junk-room storage areas.

I'd like confirmation that my plan is correct, and confirmation on a few details I'm not sure about.

The Load

• 4 - 8 plug in LED strip lights

  • each lamp: 1.1 amp, 60 watts
  • each lamp will be plugged into a NEMA 5-15 or 5-20 outlet at the ceiling (i.e. it will be very inconvenient to plug anything but another light into the other receptacle).
  • All lights controlled by a single switch just inside the door.

• 1 duplex outlet, NEMA 5-15 or 5-20

  • We're running the wiring so may as well put in an "accessible" outlet "just in case".
  • No planned usage beyond possibly running a small heater for short periods in the late-fall/early-spring.
  • If the potential draw over the proposed distances would be too great for any additional loads, I'll nix it.

The Code

• According to the Indiana Association of Building Officials, it looks like we're on NEC 2009 with some state amendments.
  • Welcome to Indiana, we're a decade behind. :/

The Plan

• Either a 15 amp or 20 amp GFCI breaker (yet to be determined)

  • I'm leaning toward 20 amp, so I won't be sorry later. WAF is high on this at the moment.

• 12 or 14 AWG THHN/THWN-2 wire (white, black, green) or 14/2 UF-B (which I already have, but may not have enough of)

  • 14 AWG and a 15 amp breaker or 12 AWG for either a 15 or 20 amp breaker
  • Assume from here that all wire is appropriate gauge for the breaker, I won't be specifying 12/14 at every mention

• Run NM-B from the breaker, through the garage to a junction box inside the building where the exit point will be.

  • Up to the rafters, across the structure and down to the exit point will be 50 feet maximum

• Terminate the NM-B in the JB (with a blank cover plate accessible inside) to feed through the outside wall of the house.

  • Wire nut to the THHN/THWN-2/UF-B in the box with standard "indoor" rated wire nuts

• Exit the building through a short stub of conduit directly into a pull hub on the outside to turn 90° down into the ground

  • The pull hub will be 12-18" above ground level.
  • Seal the hole in the vinyl siding with silicone caulk from the outside.
  • Seal the hole in the OSB sheathing with more silicone from the inside.

• Schedule 40 or 80 conduit vertically down, through a 90° elbow, horizontally about 20-25 feet and 12" or so below the surface, through a 90° elbow, then vertically up through the floor of the shed.

  • Backfill with the dirt that came out of the trench. I'm not worrying about gravel or drainage.

• Once the conduit is through the bottom plate of the wall, it will end and the wiring will run into a standard single-gang box where it will be pig-tailed to NM-B for the remainder of the inside wiring and to the "accessible" outlet using standard "indoor" wire nuts.

• I'll probably use metal "handy-box" type boxes with metal covers since they'll be mounted in the stud bays but exposed (i.e. not behind drywall).

  • I've got some push-in bushings for metal boxes, but will probably need more.

The Conduit Questions

• I'm sure it will be significant overkill for a single circuit, but for the minimal price differential, I'm looking at running 1" conduit. What's the minimum size conduit I can use?

  • This will ease pulling, especially if I've got enough UF on hand to make the run (I understand that pulling cable is much more difficult than pulling wire).
  • Buying and pulling the THHN would be more fun and make me feel professional, but justifying the cost may be a bit tough. (Hmmm... 50' rolls aren't that bad!)
  • This gives the capacity to easily pull another circuit to the shed should the need ever arise.

• Is standard Schedule 40 plumbing pipe acceptable as buried conduit or must I use the stuff in the electrical aisle of my favorite big-box store?

• What kind of fittings are necessary at the garage between the inside junction box and the conduit, and the conduit and pull hub outside?

• With only about 12-18" of conduit between the pull hub and where the conduit will disappear in the dirt, is 1 conduit clamp sufficient?

  • Do I even need a conduit clamp for that short a run?

• I presume, especially for burial, that the conduit should be glued together to keep water and dirt out. Do I use standard schedule 40 type plumbing primer and glue or is there something special for electrical use?

• Once inside the shed, do the wires need to be run in conduit between fixture boxes since they won't be covered by an inside finished wall?

  • i.e. the wiring will all be exposed. It will be neatly wire-stapled to the side of the studs.

• Is there anything glaringly obvious that I've missed?

Answer 1

Your setup seems very well thought-out. You’ve done your homework and gotten 99% of it correct IMO.

12 or 14 AWG THHN/THWN-2 wire (white, black, green) or 14/2 UF-B (which I already have, but may not have enough of)

Cost difference in UF 12 vs 14 is enough to care about. Cost difference in THHN 12 vs 14 is not significant.

UF in conduit is usually a bad idea. It requires much larger conduit because of its fine aspect ratio (thin but wide), and it’s stiff as all getout compared to stranded THWN. This would be such a cakewalk in THHN.

The only gotcha with THHN is the conduit must be continuous with junction boxes at the start and end of the THHN.

Minimum conduit size will be 1/2” for THHN, and 3/4” for UF. (Did I mention UF makes you use exceedingly large conduit)?

No planned usage beyond possibly running a small heater for short periods in the late-fall/early-spring.

Well any portable heater will be 1500W because that’s the UL limit. But if you say possibly that knocks it down to 900W, and if you say “for short periods” that knocks it down to 400W — just kidding, provisioning does not work that way.

I would say you should expect and plan for one 1500W load. That with the lights makes a strong case for a 20A circuit vs 15.

If the potential draw over the proposed distances would be too great for any additional loads, I'll nix it.

Your length sounds like about 80’. If you are running 80% of circuit capacity, you’ll be somewhat shy of 3% voltage drop. If you ran at breaker trip, you’ll be around 3%, maybe just a tick over. You won’t have to worry about voltage drop; the breaker will not let you have a problem there.

This gives the capacity to easily pull another circuit to the shed should the need ever arise...

Negative, Ghost Rider. You can only have one circuit to an outbuilding. There are a few exceptions:

• The circuit is a different voltage (say you have a 120/240V split-phase MWBC that serves two 120V legs and also serves 240V receps; that IS NOT a 120V circuit so you can have one of those also. So there, you have three 120V circuits out there.

• The circuit is controlled differently, e.g. one circuit is on a light switch at the house (keep leaving the shed lights on, silly me!)

Is standard Schedule 40 pluming pipe acceptable as buried conduit or must I use the stuff in the electrical aisle of my favorite big-box store?

No, it’s gotta be gray electrical conduit.

What kind of fittings are necessary at the garage between the inside junction box and the conduit, and the conduit and pull hub outside?

I use threaded fittings (that go from conduit to a thread), then I thread it into the knockout.

I presume, especially for burial, that the conduit should be glued together to keep water and dirt out.

Yes, it should be, but you don’t have to do such a bang-up job of it. Electrical conduit is going to fill with water. It happens everytime and that’s why you use wire insulated for water exposure. In fact, go ahead and fill the conduit with water once the wires are in; it’ll remove all doubt about this question.

As such you might as well do a semi-shoddy job of gluing so water has a chance to leave.

Answer 2

1" conduit is fine, but it needs to be conduit

Your plan to use 1" PVC conduit is alright, but it needs to be conduit, not plumbing pipe, with an expansion joint at both the ground-end and at the shed-end to control for ground motion. You'll want to use prefabricated sweeps for the underground bends, with a LB for the aboveground direction change where the run goes down the house from where it exits. You'll also need a couple of male adapter fittings and matching locknuts to attach the conduit to boxes at each end. You'll need very little for support, though, given the short distances involved (the support requirement for PVC conduit of this size is "every 3'")

For the wire itself...

Running NM to the indoor box and transitioning to conduit there is a good plan; inside the conduit, though, black-white-green THHN of the appropriate gauge is the way to go. The wiring inside the shed can also be NM, given that it's not likely to be subject to excessive physical damage if routed in a sane way (i.e. not low down on walls or strung across stud bays).

Positioning wiring devices smartly helps with this physical damage problem, too. I would use a 4" square or two-gang metal box where the wiring enters the building, and put both the convenience receptacle, which needs to be a GFCI, and the light switch, which should not be on the GFCI-protected circuit, there. The two-gang approach also gives you flexibility if you want to use an occupancy sensor for the lights in the shed.

From there, the shed wiring simply runs up the wall and into the shed's rafters to the boxes for the fixtures, which can be 4" octagons. Of course, you'll need to use the proper strain-relief fittings (the plastic buttons work just fine for this); also, I would be OK with the grey fiberglass or thick plastic style of nonmetallic boxes for this application, which'd save you the need for grounding pigtails.

Making the transition into the shed

The key part you've overlooked is that you will need a disconnect at the shed; the simplest means to provide that is a $10 non-fused AC disconnect box. In this, the hot from the house routes into the line-end of the disconnect, with its matching load-end connected to the shed side of the circuit. Both grounds go to the ground bar; however, the neutrals do not, and are simply wirenutted together instead. The conduit can simply come up into the bottom of the disconnect, by the way, with a short length of conduit coming out of the back of the disconnect into the switchbox inside; this does mean that the disconnect will need to be near the door into the shed.