Running power to a detached garage, a shed, a yard light, or a garden water feature starts with a buried circuit, and the cable must be rated for outdoor exposure and direct burial. Underground feeder cable, better known as UF cable, is the standard choice. You size it the same way you size indoor household circuits: begin with the total load, or electrical demand, of every device the circuit will feed.
Buried runs are usually much longer than interior runs, and every foot of wire adds resistance. Past roughly 100 feet, the wire size usually increases to hold voltage drop inside acceptable limits. Voltage drop is the loss of voltage caused by the natural resistance of the conductors, and ignoring it leaves pumps, motors, and lights starved for power. The same planning discipline that went into a deep underground parking garage applies at every scale: buried electrical work works only when the conductor matches both the load and the run length.
This walkthrough covers the three numbers that control sizing: the load in amperes, conductor ampacity, and voltage drop over distance. It also covers cable types, burial depths, and the code checks that keep an underground circuit legal for decades.
Understanding Voltage Drop in Buried Cable
Every conductor resists the flow of electricity, and that resistance converts some voltage into heat. The result is voltage drop: the voltage at the far end of the circuit is lower than at the panel. On a long underground run the numbers add up quickly. A 120-volt circuit that loses 3 percent at the load end delivers only about 116 volts, enough to make motors run hot and lights dim.
What Voltage Drop Does to Equipment
Motors feel the loss first. Pumps, garage door openers, and power tools draw more current when voltage sags, which heats the windings and shortens their life. Electronic controls and chargers can drop out when voltage falls below their operating window. Even heaters and incandescent lamps produce less output: a 5 percent drop cuts light output by roughly 10 percent, so a yard light on a long run looks permanently dim.
Recommended Voltage Drop Limits
The National Electrical Code does not mandate a voltage drop percentage for branch circuits, but standard practice holds branches to 3 percent and feeders to 2 percent, keeping the combined drop at 5 percent or less from the service point to the farthest outlet.
| Circuit segment | Recommended maximum | Why the limit matters |
|---|---|---|
| Feeder | 2 percent | Protects every branch circuit downstream |
| Branch circuit | 3 percent | Keeps motors and electronics stable |
| Feeder plus branch | 5 percent | Total allowed drop from panel to load |
| Sensitive equipment | 1 to 2 percent | Precision controls and well pumps |
The same gauge-versus-length tradeoff shows up above ground on job sites. The rules behind jobsite extension cord selection, where wire gauge, length, and cold weather performance decide whether a tool receives full voltage, mirror what happens inside a buried cable: longer runs demand thicker conductors.
Estimating Voltage Drop With a Formula
For planning, voltage drop equals two times the one-way length in feet times the current in amperes times conductor resistance in ohms per thousand feet, divided by 1,000. Doubling the length accounts for the round trip to the load and back. Resistance tables come from any wire manufacturer, and most sizing software does the arithmetic automatically.
Calculating Load and Applying Ampacity Rules
Sizing starts with the load. Add up the wattage of every device the circuit will serve, then divide by the circuit voltage to get amperes. A 1,500-watt heater on a 120-volt circuit draws 12.5 amps, and a 3,600-watt pump on a 240-volt circuit draws 15 amps. That result is the starting point, not the number you use directly.
Continuous and Non-Continuous Loads
A continuous load runs for three hours or more at a stretch, and underground circuits serve plenty of them: sump pumps, well pumps, landscape lighting on a timer, and electric heat. The NEC treats these loads at 125 percent, so a 20-amp circuit with continuous load is limited to 16 amps and a 30-amp circuit to 24 amps. Non-continuous loads use the full rating.
The 80 Percent Rule
The 125 percent factor and the 80 percent rule are two views of the same limit. A breaker is rated for continuous duty at 80 percent of its trip rating, so 16 amps is the ceiling on a 20-amp breaker. For mixed loads, size the continuous portion first, then add the occasional portion on top.
Ampacity and Temperature Ratings
Ampacity is the maximum current a conductor can carry without exceeding its temperature rating. UF cable is normally rated at 60 degrees Celsius at the termination, which is why 12 AWG UF tops out at 20 amps and 10 AWG at 30 amps. Bundling, burial in insulation, and hot attics all apply derating factors that lower the allowable current.
Where the trench meets concrete work, coordination matters. A panel pad, a light pole footing, or a pier for a structure all contain rebar, and reinforcing steel follows its own splice rules: the distinction between lap length and development length controls how far bars must overlap to transfer stress. Reading the electrical and structural drawings together prevents conflicts that surface after the concrete is poured.
Selecting Cable Type and Burial Depth
UF cable is the workhorse for direct burial. Its conductors sit inside a solid, water-resistant jacket that can be buried without conduit, making it the least expensive way to run power across a yard. For runs under a driveway, a patio, or a sidewalk, schedule 40 PVC conduit with THWN-2 conductors gives you a protected, replaceable raceway.
UF Cable vs Conduit-Rated Conductors
- UF cable: direct burial without conduit, simplest for open trench runs
- THWN-2 in PVC conduit: protected and replaceable, required under driveways and slabs
- Rigid metal conduit: maximum mechanical protection near structures and exposed locations
Conduit runs need planning for future pulls: install a pull string, use sweeping bends, and keep the total of 90-degree bends under 360 degrees per pull. Clean cuts matter when you pull conductors. Cable stripping tools for underground distribution line work remove the outer jacket without nicking the insulation, and that care pays off: a scored conductor is a future ground fault.
Minimum Burial Depths
Code minimums depend on the wiring method; the table lists common depths for residential circuits. Local amendments can be deeper, so confirm with the authority having jurisdiction before you dig.
| Wiring method | Minimum cover | Typical use |
|---|---|---|
| Direct-burial UF cable | 24 inches | Yard runs and landscape lighting |
| Rigid metal conduit | 6 inches | Short exposed transitions |
| Schedule 40 PVC conduit | 18 inches | General buried runs and under driveways |
| Rigid nonmetallic conduit | 18 inches | High-traffic and commercial areas |
Picking Wire Gauge and Overcurrent Protection
Once the load and distance are known, the gauge falls out of two tables: ampacity and voltage drop. The ampacity table sets the floor; the voltage drop calculation raises it on long runs.
Ampacity at a Glance
For copper conductors terminated at 60 degrees Celsius, the common residential sizes and breaker limits are below. They match the standard ratings for each gauge.
| AWG copper | Maximum breaker | Typical circuit use |
|---|---|---|
| 14 AWG | 15 amps | Lighting and general receptacles |
| 12 AWG | 20 amps | Small pumps, outdoor outlets, kitchen circuits |
| 10 AWG | 30 amps | Water heaters, 240-volt tools, garage feeders |
| 8 AWG | 40 amps | Ranges and moderate-distance subpanels |
| 6 AWG | 55 amps | Large subpanels and long feeders |
Step-by-Step Sizing Sequence
- Total the connected load in watts and divide by the voltage to get amperes
- Apply the 125 percent factor to any continuous load
- Choose the smallest gauge whose ampacity covers the result
- Measure the one-way run length and calculate the voltage drop
- Increase the gauge if the drop exceeds 3 percent for a branch circuit
- Select the breaker to protect the final gauge, not the bare load number
Termination quality is part of sizing. Loose lugs, corroded grounds, and under-torqued connections create heat that no wire gauge can overcome, and a cramped panel or buried junction box makes tool selection part of the job. The choice between stubby vs extended ratchets affects how much swing you have when tightening lugs in tight enclosures, so keep both in the tool bag.
Worked Examples for Long Underground Runs
Two examples show how the rules combine, assuming copper conductors and a 3 percent branch-circuit target.
Example 1: 120-Volt, 15-Amp Circuit at 200 Feet
A yard light and a small outlet at a shed draw about 8 amps on a 15-amp circuit. At 200 feet, 14 AWG copper loses roughly 5.7 percent, well over the target. Stepping up to 12 AWG lowers the loss to about 3.6 percent, still slightly over. Moving to 10 AWG brings the drop to roughly 2.2 percent, which passes with room to spare. On long runs, distance sets the gauge as much as load does.
Example 2: 240-Volt, 30-Amp Circuit at 150 Feet
A 240-volt circuit feeding a workshop welder draws about 24 amps. At 150 feet, 10 AWG copper loses roughly 3.4 percent, just over the 3 percent branch target. Moving to 8 AWG cuts the loss to about 2.1 percent and leaves headroom for a future 40-amp upgrade. The same 150-foot run at 120 volts would need 6 AWG to stay inside the limits.
Building a Termination Station
Plan the far end before you dig. A clean work surface for splicing, testing, and labeling saves time, and the dimensions you choose matter. Guidance on how to size a mobile workbench, including length, depth, height, and worktop choices, transfers directly to a station built for electrical terminations.
Code Checks, Inspections, and Safe Installation Practices
An underground circuit touches the ground, the structure, and the panel, so it collects more code requirements than an interior run.
Permits and Inspections
Most jurisdictions require a permit for buried circuits. The inspector checks trench depth before backfill, verifies the cable type, and confirms that splices sit in accessible boxes rather than in the ground. Schedule the inspection before covering the trench; backfilling early means digging it back up.
Safety During Installation
- Call the utility locator before digging any trench
- De-energize the circuit at the panel and lock out the breaker
- Keep splices out of the ground and use approved junction boxes
- Backfill with sand or stone around direct-burial cable to protect the jacket
- Test continuity and polarity before restoring power
Where the cable enters the building, the foundation transition deserves the same attention as the wall itself. A structure with an underground basement wall depends on watertight details at every penetration, and a sealed, correctly sleeved cable entry keeps groundwater from following the conductors in.