How to Trench and Feed a Yard Post Receptacle
Why this matters
A receptacle on a post at the garden or the fire pit is a simple circuit that goes wrong in the ground, where nobody sees it until a shovel or a frost heave finds it. The callbacks come from cable buried at whatever depth the trench happened to reach, a bare UF riser chewed by a string trimmer, a conduit pulled apart by the post moving, and a GFCI placed at the post itself, where it takes the weather and fails first. This card is the run from the house to the post: picking the wiring method and its depth, where the GFCI goes, how the post is set, and the two readings that prove it.
Before the shovel goes in
Call the one-call center and wait for the marks. Every state has a one-call law requiring notice before excavation, with its own waiting period and its own tolerance zone around each mark inside which you dig by hand. Read your state's rules; the numbers differ. Utilities mark their lines only to the meter. Irrigation, a propane line to a grill, a pet fence wire, low-voltage landscape lighting and an old feed to a shed are private and are not marked, so ask the owner and look for them.
If you strike something. A gas line: stop digging, get everyone away from the trench and upwind, no engines or phones started near it, and call 911 and the gas utility from a distance. An electric line: nobody touches it or the tool that hit it; keep people clear and call the utility or the owner of the line. Do not try to cover or patch either one.
At the panel, the new circuit is installed with the panel de-energized where you are working near other terminals: main off, locked and tagged, and proved dead with live-dead-live on a CAT III or better meter (proved on a known live source before and after, per NFPA 70E Article 120, 120.5 in the 2021 edition and renumbered in 2024; work practices at 29 CFR 1910.333(b)(2)).
Choosing the method and its depth
NEC Table 300.5 (2023 edition, as adopted) sets the minimum cover, measured from the top of the cable or raceway to finished grade, by wiring method and location. Three choices cover almost every yard post. The full table, including under driveways and slabs, is 'Underground Wiring Burial Depth Card'.
| Method | Cover in a lawn | What it depends on |
|---|---|---|
| UF cable direct buried | 24 in | Nothing else |
| PVC conduit | 18 in | Nothing else |
| Any listed method | 12 in | 120 V, 20 A or less, dwelling, GFCI-protected |
Why our default is PVC at 18 in. The 12 in row is legal, and it is the shallowest, but the depth then depends on a breaker. The day a future owner swaps the GFCI breaker for a plain one because it "kept tripping", the cable is too shallow and nobody knows. PVC at 18 in does not depend on anything at the panel, conductors can be pulled out and replaced without digging, and the conduit protects them from a spade. UF at 24 in is fine where the run is long and the soil is easy, but the extra 6 in of trench is most of the work.
If you do run UF, the risers are where it fails. NEC 300.5(D)(1) as adopted requires direct-buried cable emerging from grade to be protected by a raceway or enclosure from the required cover depth (it never has to go deeper than 18 in) up to at least 8 ft above grade or to the enclosure it terminates in. On a post with the box at 18 in, that means a sleeve from the trench floor all the way into the box, with a bushing on the bottom end so the cable is not cut by the sleeve's edge when the soil settles. Schedule 80 PVC is the usual sleeve where a mower or trimmer works around the post. A UF run that comes out of the dirt bare and climbs the post under staples meets none of that and lasts until the first edging season.
Splices underground are permitted only by listed methods for direct burial (300.5(E)), so a UF run that comes up short gets a listed underground splice kit or a junction box above grade, never a wire-nut wrapped in tape and buried.
The section
HOUSE YARD POST
J-box inside rim
| sleeve thru rim joist
[LB] [WP box, WR rcpt,
| PVC riser on wall in-use cover]
| expansion fitting | PVC on post
==|===== finished grade ====================|====
| |
+--- 3/4 PVC, 18 in cover, 55 ft ---------+
sweep at each end, backfill no rock
The steps
1. Size the circuit at the source. The worked job: a 20 A, 120 V circuit, 12 AWG copper (20 A is the 240.4(D) limit for 12 AWG), on a 20 A GFCI breaker. From the panel, 15 ft of 12/2 NM to a junction box on the basement rim joist, where the NM splices to 12 AWG THWN conductors (black, white, green) that run the remaining 55 ft in conduit. Total 70 ft one way. Voltage drop at a 12 A load: 2 x 70 ft x 12 A x 1.98 ohm per 1,000 ft (Chapter 9 Table 8, 12 AWG stranded copper, close enough for the solid NM section) / 1,000 = 3.3 V, which is 2.8 percent of 120 V. That is fine for a receptacle. On a 150 ft run it would be 7.1 V, 5.9 percent, and the answer becomes 10 AWG.
2. Put the GFCI at the source. NEC 210.8(A) as adopted requires GFCI protection for outdoor dwelling receptacles. A GFCI breaker or an indoor GFCI receptacle at the start of the run protects the whole run, the buried part included, and keeps the device out of the weather. A GFCI receptacle at the post satisfies the receptacle rule too, but it is the part most likely to fail from moisture, and it is the one the owner cannot find when it trips in the rain.
3. Go through the wall in a sleeve. From the junction box, a PVC sleeve through the rim joist to an LB on the outside wall, sealed at the wall penetration. Down the wall in PVC, strapped, to a sweep at the bottom of the trench. Put an expansion fitting in the vertical riser: NEC 300.5(J) as adopted requires underground runs subject to settlement or frost to be arranged so the movement cannot damage them, and a slip fitting at the riser is the usual way.
4. Dig to depth and measure the cover, not the trench. For 18 in of cover, the trench floor sits at 18 in plus the conduit's diameter, about 19 to 20 in for 3/4 in. Measure from where finished grade will be, not from the top of a mound or a bed that will be scraped. A trench that is 18 in deep with the conduit lying in the bottom gives about 17 in of cover and fails.
5. Lay the conduit, sweep up at the post, and get it inspected before backfill. Glue every joint, sweep up at both ends rather than using a tight 90, and leave the trench open until the AHJ has seen the depth. Backfill must not contain rock or debris that can damage the raceway (NEC 300.5(F)). A warning ribbon is required by 300.5(D)(3) for service conductors, not for a branch circuit like this; many shops lay one anyway at about half the depth, and on this job one went in at 9 in.
6. Set the post and mount the box to it. A 4 by 4 pressure-treated post set about 30 in deep in tamped gravel (our shop default; set it below your local frost line where heave is a problem). The conduit runs up the post's face, strapped, into the bottom of a weatherproof box screwed to the post; the box is supported by the post, not by the conduit (NEC 314.23 lists the acceptable supports; confirm how your AHJ reads a freestanding post). Mount the box with the receptacle's face about 18 in above grade, high enough to clear mulch and sprinkler spray.
7. Trim it. A weather-resistant and tamper-resistant receptacle (NEC 406.9(B) and 406.12 as adopted; see related: 'Tamper-Resistant and Weather-Resistant Receptacle Rules') under an in-use cover listed as extra duty, which stays weatherproof with a cord plugged in. The receptacle's terminals are torqued to the value marked on it (NEC 110.14(D)).
The finish test, at the post
- Restore and stand clear. Close the panel cover, stand to the side as the GFCI breaker closes, and confirm the breaker holds.
- Polarity and ground. A plug-in tester at the post shows correct wiring. On this job: correct.
- The GFCI trip at the post. Press the tester's GFCI button at the post receptacle. The breaker at the panel must trip. This proves the protection reaches the end of the run, which the breaker's own test button does not show. The tester trips by leaking current to the EGC, so it also proves the green conductor is continuous. Reset at the panel.
- Loaded voltage. Measure the post receptacle with nothing plugged in: 121.4 V. Then plug in a 1500 W space heater (about 12.5 A at 120 V) and measure again: 117.6 V. That is a 121.4 minus 117.6 = 3.8 V drop. The branch run alone should account for about 3.5 V at 12.5 A (2 x 70 ft x 12.5 A x 1.98 / 1,000 = 3.47 V), and the rest is the service and panel. A drop much larger than the calculation, say 8 V or more here, means a loose splice in the junction box or at the LB and gets found before the job is closed.
Write on the panel directory: "Yard post receptacle, GFCI breaker". Tell the owner where to reset it.
What changes the answer
- Under a driveway or a patio. Table 300.5 has separate rows for one- and two-family driveways and for runs under a slab, and they do not all match the lawn row; take the depth for your method from the depth card before you price the trench.
- A 240 V or larger circuit to a post or a shed. The 12 in GFCI row is gone (it is for 120 V circuits of 20 A or less), and anything feeding a separate building is a feeder or branch circuit with its own disconnect and grounding rules at the far end.
References
- NFPA 70, National Electrical Code, 2023 edition (2020 where numbering moved) as adopted, local amendments checked: 210.8(A), 240.4(D), 300.5 and Table 300.5, 314.23, 406.9(B), 406.12, 110.14(D), Chapter 9 Table 8
- Your state's one-call (811) excavation law
- NFPA 70E, current edition, Article 120; 29 CFR 1910.333(b)(2)
- See related: Underground Wiring Burial Depth Card
- See related: Tamper-Resistant and Weather-Resistant Receptacle Rules