Long-Run Voltage Drop Calculator
Calculate voltage drop and select the right wire gauge for long underground or overhead feeder runs
Free voltage-drop calculator for long feeder and branch runs that supply detached shops, barns, pole buildings, and other outbuildings. Enter one-way distance, load current, voltage, phase, and the local motor-start prompt to compare copper and aluminum circular-mil rows, steady-state voltage drop, and voltage at the load. The app treats 2%, 3%, and 5% as local review prompts from NEC informational-note context, not hard pass/fail compliance limits. It does not size ampacity, select conductor insulation, choose OCPD, verify burial or raceway method, approve aluminum terminations, or replace adopted-code, utility, product, AHJ, and qualified electrical review.
Check subpanel load questions before feeder design and AHJ review
Panel Load Study →Check if your feeder can handle specific equipment like welders and compressors
Can I Run This on That? →Check shop equipment load assumptions before service-size and AHJ review
Machine Shop Power Calculator →Cross-check ampacity, conductor type, OCPD, derating, and code/AHJ gaps separately
Wire Sizing Calculator →How It Works
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Enter Run Length
Measure the one-way distance from the source equipment to the load or outbuilding panel. Add installation slack, bends, risers, and vertical sections only when they are part of the actual conductor path.
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Specify Load and Voltage
Enter a source-verified load current and system voltage. Do not use the breaker handle or panel label as a substitute for a load calculation, nameplate data, or adopted-code method.
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Review Local Rows
Compare copper and aluminum local circular-mil rows for steady-state voltage drop and voltage at the load. Treat the row as a review prompt, not final conductor selection.
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Check Motor Prompt Separately
Use the 6x motor-start prompt only as an early calculator. Nameplate LRA, NEMA design, load inertia, starter or VFD behavior, source impedance, and manufacturer limits control motor starting.
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Send to Qualified Review
Verify ampacity, OCPD, grounding, neutral, raceway, burial, derating, equipment listing, permits, utility requirements, AHJ interpretation, and safe-work controls before installation.
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
Assumptions
- Local conductor rows and K values are planning fixtures and require adopted-code/source reconciliation before design use.
- Load is assumed to be at the end of the run, not distributed along the length.
- The calculator does not solve power factor, reactance, harmonics, voltage unbalance, or utility/source impedance.
- Motor starting uses a local 6x current prompt only, not selected motor nameplate LRA or acceleration data.
Limitations
- Does not account for voltage drop in the service entrance, utility transformer, generator, or upstream source.
- Parallel conductor runs and tap conductors are not modeled.
- Does not determine ampacity, conductor insulation, terminal temperature, OCPD, neutral, grounding, burial, conduit fill, derating, or product listing.
- Does not determine motor-start suitability, stalling, tripping, equipment damage, or manufacturer approval.
References
- NFPA-70-2026 - NEC source pointer for voltage-drop informational-note and conductor-data context
- IEEE-3002-7-2018-SOURCE - motor-starting study context
- NEMA-MOTORS-GENERATORS-SOURCE - motor source pointer
- NFPA-70E-2024-SOURCE and OSHA-1910-303-ELECTRICAL-SOURCE - electrical safety and installation source pointers
Frequently Asked Questions
Learn More
Why Your Outbuilding Motor Won't Start
How long wire runs can affect outbuilding motor starting, with voltage measurement, nameplate LRA, source impedance, starter, manufacturer, and qualified-review caveats.
Why Your Well Pump Trips the Breaker
Well pump trips can involve wire length, voltage drop, controls, capacitors, motor current, pump condition, and source/AHJ review before replacement decisions.
Generator Backfeed and Transfer Equipment Source Checks
Backfeed hazard paths, transfer-equipment source checks, interlock review, CO placement, utility coordination, and AHJ boundaries before generator use.
Project Planning: Build a Review List Before Mechanical or Electrical Work
How to organize mechanical and electrical project review lists before work starts. Covers material prompts, calculator seed values, permit questions, tool review, source gaps, and qualified-trade boundaries.
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