Underground electrical wiring is a common need for Aiken property owners. Whether you are running power to a detached garage, workshop, barn, pool house, landscape lighting system, or an outdoor kitchen, the wiring between buildings or between the main panel and an outdoor feature almost always needs to go underground. Overhead wiring is an option in some situations, but underground runs are cleaner, safer from storm damage, and required by code in many circumstances.
Underground wiring involves more planning and more stringent code requirements than indoor wiring. Burial depths, conduit types, wire types, waterproofing, and permitting all come into play. Getting any of these wrong can result in a failed inspection, a safety hazard, or expensive rework. Here is what you need to know.
When Underground Wiring Is Needed
The most common scenarios that call for underground wiring in the Aiken area include running power from the main house to a detached garage, workshop, or barn, feeding a sub-panel in an accessory building, powering landscape lighting or irrigation systems, connecting an outdoor kitchen or pool area to the main electrical panel, installing a well pump circuit from the house to the well head, and running circuits for gate operators, security cameras, or driveway lighting at the property boundary.
In each of these cases, the electrical circuit must cross open ground between the power source and the endpoint. The NEC and local codes in Aiken County specify exactly how this must be done, based on the type of wire, the type of conduit (if used), and the burial depth.
Burial Depth Requirements
Burial depth is one of the most important code requirements for underground wiring, and it varies based on the wiring method used. The NEC (Article 300.5) specifies minimum cover (the distance from the top of the wire or conduit to the finished grade) for several common configurations.
Direct-burial cable (UF-B) without conduit: 24 inches minimum cover. This is the deepest requirement because the cable has no physical protection from digging, ground movement, or rodent damage other than its own insulation and sheath.
Rigid metal conduit (RMC) or intermediate metal conduit (IMC): 6 inches minimum cover. Metal conduit provides the most physical protection, which is why the burial depth requirement is the shallowest.
PVC Schedule 80 conduit: 18 inches minimum cover. Schedule 80 PVC is stronger than Schedule 40 and is required for above-ground stub-ups where the conduit exits the ground, because it resists impact damage better.
PVC Schedule 40 conduit: 18 inches minimum cover. Schedule 40 is acceptable for buried sections but must transition to Schedule 80 or rigid metal where the conduit emerges from the ground and is subject to physical damage.
Under a concrete slab or driveway: The depth requirements are reduced when the wiring runs under a minimum 4-inch concrete slab. RMC requires zero additional cover beyond the slab, and PVC conduit requires 4 inches of cover below the slab.
These are minimum depths. In practice, digging a few inches deeper than the minimum provides an additional safety margin and costs very little extra. Many electricians in the Aiken area routinely bury conduit at 24 inches regardless of the conduit type, simply as a best practice to keep the wiring well below the reach of typical landscaping activities.
Types of Underground Wire and Cable
Not all electrical wire is rated for underground use. Using the wrong wire type underground will fail inspection and can create a serious safety hazard as the wire's insulation degrades from moisture exposure.
UF-B (Underground Feeder) cable is designed for direct burial without conduit. The conductors are encased in a solid, waterproof thermoplastic sheath rather than the loose paper-wrapped sheath used in standard NM-B (Romex) cable. UF-B is available in the same sizes as NM-B (14/2, 12/2, 10/2, 10/3, 8/3, 6/3, etc.) and can be buried directly in the ground at the required depth. UF-B is most commonly used for shorter runs and smaller circuits, such as landscape lighting circuits, shed power feeds, and similar applications.
THWN/THWN-2 are individual insulated conductors rated for wet locations, including underground use in conduit. THWN wire is the standard choice for pulling through underground conduit runs. You run individual THWN conductors (typically black, red, white, and green) through the conduit, which allows you to use larger wire sizes that would be difficult or impossible to obtain in UF-B cable. For larger feeder circuits (50-amp, 100-amp, and above), individual conductors in conduit are the standard method.
XHHW-2 is another individual conductor type rated for wet locations that offers slightly better temperature ratings than THWN. It is commonly used for larger feeder circuits where the wire will operate at higher temperatures.
Standard NM-B (Romex) cable is not rated for underground use, not even inside conduit. NM-B cable's paper-wrapped sheath absorbs moisture, which degrades the insulation over time. Never use NM-B underground. Similarly, standard THHN wire (without the W rating for wet locations) is not appropriate for underground conduit where water can accumulate.
Conduit Types for Underground Runs
When running individual conductors underground (rather than direct-burial UF-B cable), you need conduit to protect and house the wires. The most common conduit types for underground residential work in the Aiken area are as follows.
Schedule 40 PVC is the most commonly used conduit for underground residential runs. It is lightweight, inexpensive, easy to work with, and resistant to corrosion and moisture. PVC conduit sections are joined with solvent cement (PVC glue), creating watertight joints. Schedule 40 is acceptable for buried portions of the run but must transition to Schedule 80 PVC or rigid metal conduit at the point where the conduit exits the ground, because above-ground sections are subject to physical damage that Schedule 40 cannot adequately resist.
Schedule 80 PVC has thicker walls than Schedule 40, providing greater impact resistance and crush resistance. It is required for above-ground stub-ups and is sometimes used for the entire run when extra protection is desired. Schedule 80 uses the same outside diameter as Schedule 40, so fittings are interchangeable, but the thicker walls reduce the inside diameter, which slightly reduces the number and size of conductors that can be pulled through.
Rigid metal conduit (RMC) is galvanized steel conduit that provides the strongest physical protection. RMC allows the shallowest burial depth (6 inches) and is the best choice when the underground run crosses a high-traffic area, runs under a gravel driveway, or passes through rocky soil where PVC could be damaged during backfilling. RMC is significantly heavier and more expensive than PVC, and the joints are threaded rather than glued, making installation slower. It is typically used for the above-ground sections and transitions rather than the entire underground run.
Rigid nonmetallic conduit (RNMC) and HDPE conduit are occasionally used for longer runs, particularly for utility-scale work or runs exceeding 100 feet. For typical residential underground runs in Aiken, Schedule 40 PVC is the most practical and cost-effective choice.
Trenching: Methods and Considerations
The trench is where most of the labor and cost in an underground wiring project goes. Trenching methods depend on the length of the run, the soil conditions, and what lies between the starting and ending points.
For short runs (under 50 feet) in open ground, hand-digging with a trenching shovel is often the most practical method. A trenching shovel has a narrow, pointed blade designed to cut a narrow, deep trench efficiently. For longer runs or harder soil, a walk-behind trencher or a small excavator significantly reduces the labor time. In Aiken's clay-heavy soil, which is common throughout much of the CSRA, a trencher is particularly valuable because hand-digging in hard-packed red clay is extremely labor-intensive.
Before any digging begins, you must call 811 to have underground utilities located and marked. This is not optional. It is South Carolina law (SC Code 58-36-10 et seq.) and a basic safety requirement. Utility locators will mark the approximate location of buried gas lines, water lines, sewer lines, cable TV lines, phone lines, and existing electrical lines within a few business days of your request. Digging without calling 811 can result in hitting a gas line (life-threatening), a water main (expensive flooding), or a fiber optic cable (very expensive repair), and you are legally and financially liable for the damage.
The trench should be dug to the required depth plus a few inches for a sand or gravel bed at the bottom. A 2- to 3-inch bed of sand at the bottom of the trench cushions the conduit and prevents rocks in the soil from pressing against it. After the conduit is laid, another layer of sand over the top provides additional protection before the trench is backfilled with native soil.
Running Under Driveways and Sidewalks
When the underground wiring route crosses a driveway, sidewalk, or patio, you have two basic options: trench across by cutting the concrete, or bore underneath without disturbing the surface.
Boring is the preferred method because it does not damage the hardscape. A horizontal bore uses a water jet, auger, or pneumatic tool to push conduit underneath the concrete from one side to the other. For short crossings (10 to 15 feet), boring can be done quickly by an experienced electrician with the right equipment. For longer crossings or crossings under heavily reinforced concrete, a directional boring contractor may be needed.
If the concrete is already cracked, heaved, or scheduled for replacement, cutting and patching may be more practical than boring. A concrete saw cuts a narrow channel across the surface, the conduit is laid in the channel, and the channel is filled with new concrete. The patch is visible but functional.
Permits and Inspections
Underground electrical wiring in Aiken County requires an electrical permit and an inspection before the trench is backfilled. The inspector needs to verify the conduit type, burial depth, wire type, and connections before the trench is covered. If you backfill the trench before the inspection, the inspector may require you to re-excavate to verify compliance, which means doing the trenching work twice.
The inspection also covers the termination points, the sub-panel or disconnect at the receiving end, the grounding, and the circuit breaker sizing at the main panel. A licensed electrician handles the permit application and inspection coordination as part of the project.
Common Underground Wiring Projects and Costs
Here are typical cost ranges for common underground wiring projects in the Aiken area.
Power to a detached garage (60-amp sub-panel, 50 to 100 feet): $1,500 to $3,500. This includes trenching, conduit, wire, sub-panel, breakers, and permit.
Power to a workshop or barn (100-amp sub-panel, 75 to 200 feet): $2,500 to $5,000. Longer runs and larger wire increase material costs.
Landscape lighting circuit (20-amp, 50 to 150 feet): $800 to $2,000. Landscape lighting often uses multiple shorter conduit runs from a central junction point.
Pool equipment circuit (50 to 60 amp, 30 to 80 feet): $1,200 to $2,500. Pool circuits have additional code requirements for bonding and GFCI protection.
Well pump circuit (30 to 50 amp, 100 to 500 feet): $1,500 to $4,000. Long runs to well heads require careful wire sizing to manage voltage drop.
Next Steps
If you need underground electrical wiring for any project on your Aiken property, Unity Power & Light handles the entire process from design through inspection. We determine the correct wire size, conduit type, and burial depth for your specific application, handle the trenching and conduit installation, pull and terminate the conductors, install the sub-panel or equipment connections at the receiving end, and coordinate the permit and inspection. Contact us for a site evaluation and quote.