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