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Long-Run Voltage Drop Calculator - Wire Sizing for Outbuildings, Barns & Detached Shops

Calculate voltage drop and select the right wire gauge for long underground or overhead feeder runs

Free voltage drop calculator built specifically for the long feeder runs that supply detached shops, barns, pole buildings, and other outbuildings. Unlike generic wire sizing charts, this tool handles the real-world distances common in rural and semi-rural properties - 100 to 1,000+ feet from the main panel to the outbuilding. Enter your run length, load in amps, voltage (120V, 240V single-phase, or 208/480V three-phase), and conductor material (copper or aluminum) to get the voltage drop percentage, actual voltage delivered at the load, and the minimum wire gauge that meets NEC recommendations. The NEC recommends no more than 3% voltage drop on branch circuits and 5% total from service entrance to final outlet. On a 300-foot run to a shop panel, undersized wire that "passes" at the panel can still deliver 6-8% drop at the furthest outlet - causing dim lights, motor overheating, and welder performance problems. This calculator shows exactly where you stand and what wire size fixes it.

Pro Tip: Aluminum feeder cable is the standard choice for long outbuilding runs, and for good reason. On a 200-foot, 100-amp, 240V run, copper requires 1/0 AWG at about $4.50/ft - $1,800 for the wire alone. Aluminum 1/0 handles the same load with slightly higher drop but costs about $1.80/ft - saving $540. Go up one size to 2/0 aluminum for comparable drop to copper 1/0 and you're still saving $300+. Use anti-oxidant compound on all aluminum terminations and torque to spec.

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Long-Run Voltage Drop Calculator
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How It Works

  1. Enter Run Length

    Measure the distance from your main panel (or meter base) to the outbuilding subpanel. For underground runs, add 10-15% for slack, bends, and riser height. For overhead runs, account for drip loops and pole height.

  2. Specify Load and Voltage

    Enter the total expected load in amps and your system voltage. For a shop subpanel, use the calculated load from your panel schedule - not the main breaker size. A 100A subpanel may only carry 65A of actual load.

  3. Select Conductor Material

    Choose copper or aluminum. Aluminum is standard for feeder runs over 50 feet due to cost savings. The calculator automatically applies the correct resistivity for each material.

  4. Review Voltage Drop Results

    See the voltage drop in volts and percentage, actual voltage delivered, and whether you meet the NEC 3% feeder / 5% total recommendation. If you exceed 3%, the calculator shows the next wire size up that brings you into compliance.

  5. Factor in Future Load Growth

    Add 20-25% to your current load to account for future equipment. It is far cheaper to install one size larger wire during construction than to re-pull cable later.

Built For

  • Homeowners wiring a detached garage or workshop 150+ feet from the house
  • Farmers running power to a barn, grain bin, or irrigation pump house
  • Contractors sizing underground feeder for a new pole building
  • Electricians verifying that an existing feeder can handle added shop loads
  • Property owners planning an overhead service run to a remote outbuilding

Frequently Asked Questions

The NEC does not set a hard maximum but recommends no more than 3% voltage drop on branch circuits or feeders, and no more than 5% total from the service entrance to the final outlet. Most inspectors treat the 3% feeder recommendation seriously for long runs. At 240V, 3% is 7.2 volts - meaning the load end should see at least 232.8V under full load.
It depends entirely on the load. For a 60A, 240V subpanel on a 200-foot run: 6 AWG copper gives about 3.1% drop (marginal), 4 AWG copper gives 1.9% (comfortable). In aluminum: 4 AWG gives about 3.1%, 2 AWG gives 1.9%. For a 100A panel at 200 feet: 1/0 copper or 2/0 aluminum keeps you under 3%.
Yes. Aluminum feeder cable (AA-8000 series alloy) is the industry standard for feeder runs and has been used reliably for decades. The problems associated with aluminum wiring in the 1960s-70s involved branch circuit wiring with the old AA-1350 alloy. Modern aluminum feeder cable with proper anti-oxidant compound, correct connectors rated for aluminum, and proper torque values is completely safe and code-compliant.
If you run any combination of welder, air compressor, dust collector, and power tools, go with 100A minimum. If you have a large welder (over 200A), plasma cutter, or plan to add a car lift or electric vehicle charger, go 200A. The wire cost difference is modest compared to the labor of pulling cable, especially in a buried conduit. Future-proofing with 200A is almost always worth the extra $200-400 in wire cost.
Disclaimer: This calculator provides voltage drop estimates based on NEC Chapter 9 conductor resistance values at 75°C. Actual voltage drop may vary based on conductor temperature, conduit fill, ambient conditions, and power factor. All electrical work must comply with the National Electrical Code and local amendments. Hire a licensed electrician for installation.

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