Underground Feeder Voltage Drop and Wire Size Planning
Underground feeders are expensive to rework because the trench, conduit, sweeps, pull points, and terminations are usually committed before the first load is energized. A feeder that passes ampacity can still deliver poor voltage at the far end, especially where pumps, compressors, welders, or gate motors start under load.
Treat the voltage drop calculation as a trench-planning decision, not only a wire-size decision. For NEC projects, check NEC 300.5 for cover, NEC 300.17 and Chapter 9 for raceway fill, NEC 310.15 for ampacity adjustments, NEC 225 for outside feeders, and the design notes in NEC 210.19(A)(1) and 215.2(A)(1). For IEC work, coordinate conductor sizing, grouping, installation method, and voltage-drop limits under IEC 60364-5-52. National Electrical Code and IEC references are included as planning anchors.
Field Workflow Before the Trench Is Closed
- Use the actual one-way route length, including vertical risers, sweeps, panel offsets, and service loops. A 180 ft straight-line run can easily become 205 ft of conductor.
- Size from load current first, then run voltage drop at the expected continuous or coincident load. For motors, also check starting current where dimming, nuisance trips, or low-torque starts matter.
- If voltage drop upsizes the conductors, re-check raceway fill, pulling tension, bend count, equipment lug range, and box volume before ordering material.
- Separate design targets from enforceable code. The NEC 3% branch and 5% total voltage-drop notes are design guidance, while burial depth, ampacity, raceway fill, grounding, and equipment rating rules remain enforceable.
Code and Standard Checkpoints
- NEC 300.5: burial cover depends on wiring method, circuit voltage, GFCI protection, and location. Do not let a voltage-drop upsizing decision create a conduit or cover conflict.
- NEC Chapter 9 Table 1 and NEC 300.17: raceway fill is still limited after upsizing. More than two conductors generally uses the 40% fill limit.
- NEC 310.15: ambient temperature, more than three current-carrying conductors, and rooftop or grouped runs can reduce ampacity before voltage drop is considered.
- NEC 210.19(A)(1) informational note, NEC 215.2(A)(1) informational note, and IEC 60364-5-52: many designs target about 3% on one segment and about 5% total, then tighten the target for motors, electronics, or long utilization equipment leads.
Underground Feeder Planning Examples
These examples use rounded planning values. Always verify the final conductor, insulation, raceway, grounding, and overcurrent protection with the applicable code edition and local AHJ.
| Load | Run | Voltage-drop result | Planning decision |
|---|---|---|---|
| Detached garage panel, 60A at 240V | 205 ft one-way copper feeder | 6 AWG is near 3.6%; 4 AWG is near 2.3% | Upsize to 4 AWG if the garage has compressor, welder, or EV charging load, then re-check conduit fill |
| Irrigation pump, 32A at 240V | 310 ft one-way aluminum feeder | 2 AWG aluminum can exceed 4%; 1/0 aluminum can bring the run closer to 2.6% | Motor starting torque favors the larger conductor even when ampacity alone looks acceptable |
| Gate operator, 8A at 120V | 420 ft one-way copper circuit | 10 AWG can still land above 6%; 6 AWG is often needed for a stable 120V load | Low current does not guarantee low voltage drop when the route is very long |
| IEC site shed, 40A at 400/230V | 85 m buried cable route | 10 mm2 may be around 2.7%; 16 mm2 is closer to 1.7% | Coordinate the cable size with IEC 60364-5-52 installation method and grouping factor |
| Landscape transformer feeder, 20A at 120V | 150 ft to transformer plus low-voltage secondary | Primary feeder may be acceptable while 12V secondary drop dominates | Calculate both sides; moving the transformer can beat oversized secondary cable |
Worked Examples with Specific Numbers
60A detached garage feeder at 205 ft
A homeowner wants a 60A, 240V feeder to a detached garage. A basic ampacity check may point to 6 AWG copper, but at 60A and 205 ft one-way the voltage drop is roughly 3.6%. Moving to 4 AWG copper lowers the drop to about 2.3%. That is a practical difference if the garage will run a 24A EV charger and a 15A compressor at the same time. After upsizing, NEC Chapter 9 raceway fill and the panel lug range must be checked before the trench is backfilled.
32A irrigation pump at 310 ft
A farm pump is 310 ft from the service equipment and starts against pressure. With aluminum conductors, a size that passes steady-state ampacity can still leave the motor with poor starting voltage. If 2 AWG aluminum is above a 4% design target and 1/0 aluminum is around 2.6%, the larger feeder may prevent nuisance trips and low starting torque. NEC 430 motor rules, NEC 300.5 cover, and equipment grounding conductor sizing still need separate review.
420 ft gate operator branch circuit
An 8A, 120V gate operator looks small until the route length is included. At 420 ft one-way, 10 AWG copper can still exceed 6% voltage drop. A 6 AWG branch may be needed for a stable controller and motor start, but the installer must confirm termination ratings or splice down at the equipment using an enclosure with enough volume under NEC 314.16.
Pre-Backfill Checklist
- Confirm the real one-way conductor length, not the site-plan straight-line distance.
- Run voltage drop at realistic coincident load, not only breaker size.
- Check motor starting voltage where pumps, compressors, gates, and HVAC equipment are involved.
- After upsizing, re-check conduit fill, pull points, bends, box fill, lugs, and grounding conductor size.
- Document NEC/IEC assumptions on the worksheet so the AHJ, engineer, electrician, and owner are reviewing the same design target.
Use the Calculator Before Ordering Conduit and Cable
Enter the one-way route length, conductor material, system voltage, and load current in the voltage drop calculator, then verify the chosen conductor against the wire-size and conduit-fill tools before trench work starts.
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