Buried residential power to a outbuilding or shed.

Same as every project I start, this project was going to be bigger than I originally thought. I started out with the simple idea to bring water out to my garden for the drip system. Previous year gardens I used a standard garden hose that was run across the lawn. I was not a fan of replacing a hose every year due to the sun damage over a full summer. Additionally, I discovered that bacteria will grow in the hose and swim back upstream into the house and contaminate the whole houses fresh water plumbing. After reading of the bacteria issue, I did take precautions and put the timer valve at the hose bibb, which also helped with the longevity of the hose. None the less, it was time to burry a water line and add it to my underground sprinkler system. I decided to have a tee at the end of the run allowing me to have a hose bibb and a 5th sprinkler valve zone for the drip system.

Now I get to the real reason for this post. Because I can’t keep any project simple, I decided to add two low voltage power cables (for landscaping lights and the sprinkler valve) and 120 Volt AC power outlet for my electric raised garden bed rototiller. But why stop there? I already have plans to build a bigger lawn shed, let’s get lights and outlets in that shed. I am a perfectionist, so this project must be done right and to code.

After hours of research reading NEC 2011 code, blogs, and electrician forums, I have uncovered several limitation that anyone attempting to bring residential power to an outbuilding or shed should be aware of.

1. Depth of cover

2. Size of wire & type

3. Number of circuits

4. Conduit sizing and sharing conduit with low and "high" voltage

I will start with depth of cover which seems to be fairly well covered in forums, but there is a ton of miss information spattered around between posts. For this, I will strictly stick to quoting NEC. Many people will attempt to run their power using a direct burial rated cable such as UF cable. There are a couple things I would like to address when it comes to the miss information on burring UF cable. First, it is not called Romex. Romex is strictly prohibited to be buried or installed in conduit by NEC. Second, you can put UF cable into a conduit, but only if you size your conduit per NEC conduit sizing tables in Annex “C”…More on this later.

The depth of cover is measured from the top of the cable or conduit to the final grade. If you plan to get this inspected you will likely have to leave the trench open for the inspector to see. Although, sometimes, If you are not burring a gas line also, you can document the depth with photos and a tap measure for the middle section of your trench (check with your local building code office before burring, they can make you dig it up). The ends of the run must be left open of the inspector to sign off on.

This blog is for residential buried power upto 20 amps, therefore you can get away with less depth of cover for the various types of installations. NEC Table 300.5 covers minimum cover requirements. Note 4 states “Where one of the wiring method types listed in Columns 1–3 is used for one of the circuit types in Columns 4 and 5, the shallowest depth of burial shall be permitted.” Now in my case I have a mix between columns 4 and 5, so the governing depths are based on column 4 “Residential Branch circuits rated 120 Volts or less with GFCI protection and a max current of 20 amps”. If you plan on running more than 20 amps for say a subpanel, you will have to bury per columns 1 thru 3

Table 300.5 Minimum Cover Requirements, 0 to 600 Volts, Nominal, Burial in Inches

Most people don’t want to dig deep trenches. To limit the depth, I had three options.

1. Use a rigid metal conduit (not EMT, EMT is not rated for direct burial) requiring a buried depth of 6 inches.

2. Pour 2 inches of concrete or equivalent over the run, also putting my depth at 6 inches.

3. Use nonmetallic conduit (PVC) which would limit my depth to 12 inches for residential 120v @20 amp max.

I decided, metal conduit would rust out over time and was far more expensive ($15 per 10ft) than the other two options. I also did not want to use concrete. What would I do with the extra soil replaced by the crete? Also, concrete seemed too permanent to me. 12 inches of depth was easy to achieve using a narrow trench shovel. I used 50 ft of PVC conduit so I could pull more wire in the future, besides it was less than $9 at Home Depot and THHN/THWN was cheaper than UF cable and I could use the extra wire to wire my shed (in conduit).

Wire size, number of circuits and conduit sizing are inter twined, but I will try and split them as best I can.

Originally, I wanted two circuits in my shed. One that would branch off the garden outlet for lights and a second dedicated circuit for outlets. I even thought about running 10 AWG wire for a 30 amp subpanel. NEC will not allow you to run two feeders to an outbuilding or shed (NEC 230.2). If you want more than one circuit you will have to put in a subpanel. If you want a subpanel you will have to run ground rods and will require minimum wire size of 8 AWG Copper or 6 AWG aluminum (NEC 230.31(B)). Looks like one branch circuit is all I get.

I sized my wire based on two factors. First, my 20 amp max feeder breaker. Second, the length of my run. NEC table 310.15(B)(16) shows allowable ampacities for insulated conductors rated up to 2000 volts. Please note there are three temperature columns for both copper and aluminum. In general, you will only really be allowed to apply 75°C rated columns. These temperatures are used for two components, wire insulation and lug connection rating. Most insulation is rated for 90°C but most breaker lugs are rated for 75°C heat rise. Hence your limitation of 75°C. Now for 15 and 20 amp residential branch circuit, NEC says you have to use 14 AWG and 12 AWG or greater despite that fact that the table at 75°C allows for 20 and 25 amps, respectively (NEC 240.4(D)). The length of wire is need for the voltage drop. NEC recommends your branch circuit voltage drop plus your feeder to 5%. This allows for about 95 ft of length before I would have to de-rate my amperage from 20 amps to 15 amps, otherwise I would have to increase my wire size from 12 AWG to 10 AWG. Again this is just a NEC recommendation not a safety requirement. Contrary to common belief, the NEC generally does not require you to size conductors to accommodate voltage drop. It merely recommends that you adjust for voltage drop when sizing conductors. The recommendations are in the Fine Print Notes to Sections 210.19(A), 215.2(A)(4), 230.31(C) and 310.15(A)(1). Fine Print Notes are recommendations, not requirements [90.5(C)].

Conduit sizing and sharing conduit with low and "high" voltage is a subject that needs let attention. NEC covers conduit percent of cross section for sizing your conduit in chapter 9 tables. You can look up the wire approximate diameter and area in in^2 from table 5. For my project I used (3) 12 AWG THHN/THWN wires, hot, neutral and ground. The diameter is 0.130" and the area per wire is 0.0133 in^2. Using these numbers you can calculate the percent fill for a give size conduit. Table 1 shows the maximum percent of cross section fill for a conduit. If you have more than 2 conductors in a conduit you are limited to 40% fill. Use the cross section calculation if you are placing multiple gauges of wire in a single conduit. The second method per NEC code to determine the conduit size is per annex C. These tables flat out tell you how many conductors of a single gauge wire you can put into various conduit types. Because I decide to use Schedule 40 PVC, I used table C.10 - Maximum number of conductors or fixture wires in a rigid PVC conduit, Schedule 40. For 12 AWG THHN/THWN NEC limits the number of conductors to 8 for a 1/2" diameter PVC conduit. This was plenty of space for me being that I was limited to one circuit for my shed.

The final determination to make was whether I could use the next size conduit and run my low voltage landscape wiring into the same conduit. The short answer is no. In theory you could put the 12v and the 120v wiring in the same conduit if the insulations are rated for the same voltage. Landscape wire insulation is most commonly supplied with a rating of 150v and romex or THHN/THWN is rated for 600v.