Step 1: Planning the project
Say you need a bright yard light way out in your yard or a remote outlet to power a pond pump or electric tools. By far the best way to get electrical power right where you need it is to run underground "line" voltage (120-volt household current). All it takes is a little electrical moxie and a willingness to dig a shallow trench.
In this article, we show the easiest, quickest and cheapest method to safely run electricity outdoors. Our method involves directly burying UF (underground feeder) plastic shielded cable 12 in. below ground level. We'll also show you a no-sweat way to run the line under a walkway and how to hook up the wiring at both ends.
For this project, you should have the fundamental wiring skills it takes to make proper connections and the basic electrical and carpentry tools. While they're not crucial, a trench shovel, a mattock and a sledgehammer (Photos 1 and 2) will make the trench work easier and faster. Allow about a day to complete this project, assuming you collect all the parts in advance.
In our project, we run a line from an existing outdoor outlet on the house to a light and receptacle at the edge of a garden path. It incorporates a combination light switch/outlet (Photo 11). The outlet is always hot, and the switch controls only the light. If you don't have an outdoor box to tap in to, consider installing one on your house.
You can expand the project to include additional outlets, switches and lights. The techniques for running the wire and mounting electrical boxes are the same. However, make sure not to overload the circuit.
While we enclosed the electrical boxes inside a hollow post, you can simply mount a weatherproof box on the side of a 4x4, as long as you protect the cable by running it in conduit from the trench bottom to the box (Figure A).
Underground wiring details
Figure A: House Outlet Wiring
Use an existing outdoor outlet as the power source for the light and outlet.
Step 2: Planning the wire route
Get started by determining where you want the electrical post positioned and then find the nearest existing outlet to supply the power. That outlet must be GFCI protected. We used the closest outlet on the house, but garage outlets are also good candidates. By code, those outlets should be GFCI protected. To make sure the “feeder” outlet you choose is protected, look for the characteristic GFCI buttons, or if it's a standard outlet, check it with a GFCI tester. Standard outlets still may be GFCI protected by being linked to another GFCI outlet elsewhere in the house. If yours isn't protected, simply replace the standard outlet with a new GFCI outlet using the techniques we show in Photo 13 and Figure A. Another option is to cut in, mount and wire a new outside GFCI outlet, feeding it from an outlet mounted on an inside wall in the house. Sometimes that's easier than digging a long trench to a more distant power source.
You also have to make sure the new outlet/light won’t overload the circuit you tie into, and that the box is big enough to handle the additional wire. To determine whether the circuit you want to use can handle the additional electrical demand, first shut off the circuit in the main panel. Then go through the house turning on lights and other electrical items. Add up the wattage of everything that stays off (the items on the circuit). Then add on the wattage of the post light plus the wattage of items continuously powered by the outlet. We recommend a maximum connected load of 1,440 watts for a 15-amp circuit and 1,920 watts for a 20-amp circuit (the amperage is stamped on the breaker or fuse). If the total wattage exceeds these amounts, find a different circuit. If you're not sure, call in a licensed electrician to help with this part.
To figure the minimum box size required by the National Electrical Code, add: 1 for each hot and neutral wire entering the box, 1 for all the ground wires combined, 1 for all the clamps combined and 2 for each device (switch, receptacle or combination device) installed in the box. Multiply this figure by 2 for 14-gauge wire and 2.25 for 12-gauge wire to get the minimum box volume in cubic inches. Plastic boxes have the volume stamped inside.
Once you've selected a power source, choose the easiest path for a 12-in. deep trench for the new cable. Give trees a wide berth to avoid chopping through roots. Approach sidewalks or paths at right angles for easier tunneling and make sure to plan a 3-ft. long straight section of trench on one side of the walkway. That's so you'll have room for driving the conduit beneath it (Photo 2). Mark the route with paint and then call the utility companies to mark any underground lines. (Don't forget the telephone and cable company if those lines are underground.) Last you'll need to get an electrical permit. Your local building inspections department either grants those or will direct you to a state office. The inspector will want to see the trench with the cable in place before you fill it back in, and then again for a final inspection after the project is finished.
Call 811 to have your local utility locate and mark buried lines before you dig.
Step 3: Digging the trench and posthole
The best digging tools to use for the trench itself are a mattock and a trenching shovel (Photo 1). That way you'll be able to dig a fairly narrow trench to avoid moving the mountains of dirt you would with a conventional shovel.
Pile the dirt on a strip of plastic next to the trench. That keeps dirt out of the grass and/or makes cleanup on adjoining patios much easier. Dig trenches that approach walkways a few inches deeper to allow for driving the conduit (Photo 2). After the trench is finished, dig a posthole at the end of the trench about 2 ft. deeper than the trench to receive the light pole post.
Step 4: Running wire under walkway
Get wire under walkways by driving a length of “rigid metal” conduit beneath the surface. (Never use “thin wall” conduit, which corrodes quickly, or plastic conduit, which is too weak.) Then push the cable through the pipe for short runs up to 6 ft. or so, or pull it through with a “fish tape” for longer runs. Use a length of 1/2-in. rigid conduit that's at least a foot or so longer than the walkway's width. Screw couplings onto both ends and then screw plugs into the couplings (Detail, Photo 2). One of the plugs works as a tunneling point and the other protects the pipe from sledgehammer damage. Prop the conduit up on a couple of blocks to keep the end clear of the trench bottom for driving (Photo 2). Make sure the pipe is level and directed toward the middle of the trench on the far side of the walkway so it'll emerge at the right spot.
For longer runs, like under driveways, drive a section up to the end of the trench, then remove the plug and screw on another section into the coupling and start driving again. (The hardware store or home center will cut and thread custom lengths for an additional charge.) Ease of driving depends on soil conditions; it can be a challenge tunneling through even 3 ft. of clay or stony soil, but you can easily drive 20 ft. or even more through sand.
To run the cable, follow Photos 3, 4 and 8 and Figure A. Adding an extension to the box ensures that the extra wires won’t overfill it. If you run directly into an existing exterior box, make sure it has adequate volume.
Step 5: Partially preassemble your light post before wiring it
Our light post is partially preassembled before it's wired. That's so you can stick it in the ground and have full access to the interior for stapling wires and making the hookups. The boxes and the cable are protected by a wooden housing, so standard metal or plastic electrical boxes and cable are all you need once the wire enters the protection of the box. It's a different matter at the house hookup. There you’ll need to use weatherproof boxes and run the cable through conduit that's coupled directly to the box with adapters (Photo 4). If you're simply mounting an outlet on a bare 4x4, you'll have to protect the wire within conduit right up to the box the same way as the house connection and use an exterior-rated box.
There are no height requirements for lights or outlets; build the posts as high or low as you wish. Likewise, pick outlet and light mounting heights to suit you. But make sure that the light fixtures you choose are rated for “wet location.” You’ll find that designation on the fixture box.
Pick up your light fixture before starting the post. You’ll need a fixture with a base no more than 7 in. wide or it will project beyond the post edges. If you have your heart set on one that’s wider, build a wider post just by using wider boards. But you’ll have to rework the post details. Test-fit the fixture on the post before cutting the hole for the round electrical box to make sure it clears all of the trim.
Cross section of post with wiring.
Figure B: Post Outlet/Wiring
This is what the wiring inside the post should look like.
Step 6: Building the post assembly
It only takes a couple of hours to build the post. See Photos 6 and 7 and use the dimensions given in Figure C for cutting and assembling the parts. You can do it all with a jigsaw and a circular saw, but table and miter saws will simplify several cuts.
To have adequate wire volume, we chose a 3 x 2 x 3-1/2 in. metal box. Carefully trace the rectangular electrical box and then cut for a close fit. After you've cut out the opening, dry-fit that board and mark the 4x4 for the top and bottom of the 1/2-in. deep notch that receives the box. Leave that board off and set the post. Then you can do all of the wiring on the exposed inside of the post and assemble the rest of the post later.
Figure C: Post Construction
Refer to Figure C to build the post.
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Step 7: Electrical connection tips
Electrical connection tips
- GFCI outlets have two sets of terminals, “line” and“ load” (Photo 13). Line is for the incoming wires from the power source, and load is for the switches, outlets and fixtures that it supplies. Don't wire it improperly or users won't be protected against shocks caused by damp ground or standing water.
- Cut and dry-fit all of the PVC parts. Before taking them apart for gluing, draw alignment marks on each joint (Photo 4). That way you'll get them back together in the proper orientation.
- Apply “duct seal” putty to the bottom side of the top hub plugs in the extension box at the house (Figure A). That will keep water from entering the box.
- The male adapters and plastic bushings on the ends of pipes are to protect the cable from the sharp pipe edges. They're not only required but also a good idea, so don’t skip that step!