Underground Feeder Voltage Drop: Burial Depth, Wire Size, and NEC 300.5 Planning

An underground feeder is a buried branch or feeder circuit that carries power from one building, service equipment, or distribution point to remote equipment such as a garage, barn, pump house, gate operator, pool pad, or detached workshop. It looks simple on a site plan, but the installed route usually adds trench bends, risers, sweeps, building offsets, and future load assumptions that make voltage drop more important than the first ampacity lookup suggests.

This guide is written for electricians laying out trenches, engineers reviewing feeder schedules, and DIY users trying to understand why a legal conductor can still leave a remote panel weak. The code questions and the performance questions are separate. NEC 300.5 tells you about minimum cover for underground wiring methods. NEC 310.16 and 310.12 help with conductor ampacity in common conditions. NEC 215.2(A)(1) and 210.19(A)(1) informational notes point designers toward the familiar 3% feeder or branch and 5% total voltage-drop targets. The buried feeder has to survive all of those checks at the same time.

A recent field-style review illustrates the problem. A detached workshop feeder was drawn as a 150 ft run from a 200A service to a 100A subpanel, but the real trench path measured 184 ft after routing around a driveway, rising at both structures, and avoiding a septic area. The original 1 AWG aluminum choice looked acceptable from a quick ampacity view, yet at a realistic 80A workshop load the voltage drop approached 4% before any branch circuit inside the shop was counted. Moving to 2/0 aluminum brought the feeder drop near 2.5%, leaving useful margin for a compressor, lights, and a future welder circuit.

TL;DR

Underground feeders are sized by ampacity, burial method, voltage drop, and future load margin.
Use NEC 300.5 for cover depth and NEC 310.16 for conductor ampacity before voltage-drop optimization.
A 100A, 240V feeder at 180 ft often needs larger conductors than the ampacity minimum.
Aluminum can be cost-effective underground, but terminations and voltage drop must be checked carefully.

The design baseline in this article is anchored to the National Electrical Code , undergrounding , the International Electrotechnical Commission . Those references matter because code language, conductor physics, and equipment behavior usually fail in the same place: a circuit that was technically legal on paper but poorly optimized for the distance, load, or operating temperature in the field.

“For underground feeders, I measure the trench path, not the straight line on the drawing. A 150 ft idea can become 185 ft of conductor after risers, sweeps, and site obstacles are counted.”
— Hommer Zhao, Technical Director

What Controls an Underground Feeder Design

The first control is ampacity. A feeder conductor must carry the calculated load after correction and adjustment factors are applied. For dwelling feeders, NEC 310.12 may apply in specific cases, while many detached structures and equipment feeders start with NEC 310.16 and the conductor insulation temperature limits of the terminals. That ampacity pass is necessary, but it is only the beginning.

The second control is burial method. Direct-burial cable, PVC conduit, rigid metal conduit, and residential branch-circuit exceptions can have different minimum cover requirements under NEC 300.5. A feeder that changes wiring method between a building wall, trench, and riser must be detailed correctly at every transition. IEC 60364-5-52 uses different language, but the engineering idea is similar: installation method, thermal environment, mechanical protection, and permissible voltage drop must all be checked before the cable is accepted.

The third control is the voltage-drop budget. A 240V feeder feeding a detached garage might look strong at the source, but a 3% feeder drop is already 7.2V. If branch circuits inside the garage add another 2%, the farthest receptacle or motor is operating around a 5% total drop. That may be acceptable for many loads, but it leaves little margin for compressor starting, EV charging, or utility voltage that is already low at peak demand.

Measure the installed one-way length. Include horizontal trench length, vertical risers, sweeps, offsets around obstacles, panel locations, and the route inside each building. Voltage drop follows conductor length, not the shortest line on the site plan.
Confirm burial and protection first. NEC 300.5 cover depth, warning ribbon practice, conduit transitions, and physical protection at risers should be solved before conductor size is finalized.
Separate neutral and equipment grounding decisions. A detached structure feeder normally needs an insulated neutral isolated from the equipment grounding conductor at the subpanel, with grounding electrode work handled under NEC 250.
Run feeder drop at realistic demand. Do not size only at breaker rating or only at a tiny present load. A 100A panel may be expected to serve 60A to 80A of real coincident load later.

Comparison Table: Underground Feeder Choices

These examples show how load, length, voltage, and conductor material change the practical underground feeder decision.

Scenario	Load Used	One-Way Length	Conductor Compared	Approx. Drop	Design Reading
Detached garage lighting and tools	240V / 50A	95 ft	6 AWG Cu	2.5%	Usable if branch circuits stay short
Detached workshop subpanel	240V / 80A	184 ft	1 AWG Al	3.8%	Too much feeder loss for future loads
Detached workshop subpanel	240V / 80A	184 ft	2/0 Al	2.5%	Better balance of cost and performance
Barn feeder with small loads	240V / 40A	260 ft	2 AWG Al	3.3%	May need larger wire or load limits
Well pump building feeder	240V / 30A running	320 ft	4 AWG Cu	2.6%	Startup voltage still needs review
Remote EV-ready garage	240V / 60A continuous	140 ft	2 AWG Cu	2.4%	Plan around NEC 625 continuous load

“NEC 300.5 protects the underground wiring method, but it does not tell the compressor in a detached shop whether it will see 240V, 232V, or 228V under load.”
— Hommer Zhao, Technical Director

Example 1: 100A Detached Workshop Feeder at 184 ft

Assume a 240V feeder to a detached workshop with a 100A panel. The calculated realistic load is 80A because the shop may run lighting, dust collection, a 5 hp compressor, and receptacle loads together. The installed route is 184 ft one way after trench offsets and risers are included. With 1 AWG aluminum, the feeder voltage drop can approach 3.8% at 80A. That means the workshop panel could see about 231V before any branch-circuit drop is included.

Changing to 2/0 aluminum reduces the feeder drop to roughly 2.5%, so the workshop panel stays near 234V under the same load. The material cost rises, but the trench, conduit, permits, labor, and restoration are already the expensive part of the job. When the trench is open, this comparison is usually worth doing before the conductor order is placed.

Example 2: Long Barn Feeder with Modest Present Load

A barn feeder may start with a few lights, a receptacle circuit, and a door opener, but later gain heat tape, a small welder, water equipment, or ventilation. If the one-way distance is 260 ft and the load used for planning is 40A at 240V, 2 AWG aluminum may land around 3.3% feeder drop. That might be workable for simple loads, but it does not leave much room for long branch circuits inside the barn.

The better engineering conversation is not only copper versus aluminum. It is whether the service point, feeder voltage, route, and future load should be changed. Sometimes upsizing one conductor size is enough. On larger rural sites, it may be better to place distribution equipment closer to the load or plan a higher-voltage feeder where the installation rules allow it.

Common Underground Feeder Mistakes

Using straight-line distance

A drawing that says 150 ft can become 180 ft to 200 ft once the trench avoids utilities, landscaping, driveways, and building entry points.

Treating burial depth as the whole code review

NEC 300.5 cover is only one part. Ampacity, grounding, neutral isolation, warning identification, physical protection, and voltage drop still matter.

Forgetting future continuous loads

A garage that only has lights today may later add a 48A EV charger, which changes both NEC 625 continuous-load sizing and voltage-drop expectations.

A Practical Underground Feeder Workflow

Use this sequence before ordering wire or closing the trench.

1. Calculate the load and likely future additions. Separate present load, calculated load, and realistic future loads such as EV charging, compressors, pumps, or heating equipment.
2. Lock the wiring method and cover requirement. Use NEC 300.5 and the chosen cable or raceway method to confirm cover depth, riser protection, and trench details.
3. Check ampacity and terminal temperature limits. Confirm conductor material, insulation, equipment terminals, and any derating before using voltage drop to choose a larger size.
4. Compare voltage drop at realistic current. Run at the expected load, not only at the breaker handle, then decide whether copper, aluminum, upsizing, or a route change is most practical.
5. Document the result before inspection. Record voltage, current, one-way length, conductor material, conductor size, and the calculated percent drop so the decision is easy to defend later.

FAQ

How much voltage drop is acceptable for an underground feeder?

A practical target is about 3% or less on the feeder and about 5% total feeder plus branch-circuit drop, matching the common NEC 215.2(A)(1) and 210.19(A)(1) informational-note guidance.

Does NEC 300.5 tell me what wire size to use?

No. NEC 300.5 addresses underground cover and wiring-method conditions. Wire size still comes from calculated load, ampacity tables such as NEC 310.16, terminal ratings, and voltage-drop checks.

Should I use copper or aluminum for a long underground feeder?

Both can work. Aluminum is often cost-effective for 100A to 200A feeders, but a long 180 ft to 300 ft route may need upsizing, proper terminations, and antioxidant or torque practices required by the equipment instructions.

Do detached garage feeders need four wires?

In modern NEC practice, a detached structure feeder usually uses ungrounded conductors, an insulated neutral if needed, and an equipment grounding conductor, with neutral isolated from ground at the subpanel and grounding electrodes installed as required by NEC 250.

How do I calculate underground feeder voltage drop?

Use voltage, load current, conductor material, conductor size, and one-way route length. For a 240V single-phase feeder, the current travels out and back, so the calculator accounts for the full circuit path.

When should I upsize an underground feeder?

Upsize when feeder drop is near 3%, the total path may exceed 5%, the route is longer than about 150 ft, the load includes motors or EV charging, or future loads are likely after the trench is backfilled.

Planning an Underground Feeder?

If the trench route is long, the load may grow, or the conductor choice is close on voltage drop, use the contact page before backfill. A feeder review is cheaper while the route is still visible.

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