Wire Size and Voltage Drop Calculator
Estimate the copper area needed to meet a chosen voltage-drop target for one simple single-phase run, then verify ampacity and installation requirements independently.
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View all saved project estimatesWhat does the wire size electrical calculator calculate?
Enter one-way circuit length, load current, nominal voltage, maximum voltage drop, and a copper resistance constant. The calculator estimates required circular-mil area and rounds up to a common conductor size for voltage drop only. Ampacity, temperature, terminals, bundling, fault current, phase, code rules, and local approval may require a larger conductor.
Single-phase copper area from a voltage-drop target
The factor of two represents the outgoing and return path in a simple single-phase resistance model. K is an entered copper resistance constant. Rounding up to a listed AWG or kcmil area addresses only modeled voltage drop; conductor ampacity and every installation-specific requirement remain separate design checks.
How to measure for an accurate estimate
- Trace the actual conductor route from source to load and record one-way length rather than straight-line building distance.
- Use calculated load current for the circuit and separately identify motor starting, nonlinear, harmonic, continuous, and intermittent behavior.
- Choose a voltage-drop allocation that leaves room for any upstream feeder drop and matches the project design criteria.
- Confirm whether the circuit is single-phase, balanced three-phase, DC, or a distributed-load run because each condition needs an appropriate model.
A 120-volt branch-circuit voltage-drop check
A simple single-phase planning run is 100 feet one way, carries 20 amperes at 120 volts, targets no more than 3% drop, and uses a copper K value of 12.9 ohm-cmil per foot.
The resistance model requires roughly 14,333 circular mils. The next common size in the calculator's list is 8 AWG, with a modeled drop near 2.6%; a qualified electrician must still check ampacity and installation rules.Before you purchase material
- Do not purchase conductor from the voltage-drop result until a qualified electrician verifies ampacity and the adopted code requirements.
- Compare copper and aluminum only with material-specific resistance, termination, size, installation, and product data; the default model is copper.
- Check equipment terminal temperature ratings, insulation listing, wet-location suitability, raceway fill, derating, and available fault current.
- Keep the route, load calculation, upstream voltage drop, selected product, and local approval notes with the final design.
Common questions
Does this result mean the displayed wire size is code compliant?
No. It is the next common area that meets one simplified voltage-drop equation. Code-compliant conductor selection also depends on allowable ampacity, load type, temperature, terminal ratings, insulation, conductor count, installation method, overcurrent protection, fault conditions, and locally adopted requirements.
Why does the formula multiply one-way length by two?
A simple single-phase circuit has an outgoing and return current path, so the resistance model uses twice the entered one-way distance. Balanced three-phase and other systems use different equations; do not force those circuits into this single-phase model.
Can I use this calculator for aluminum conductors?
The default K value and size wording are for copper planning. Aluminum requires an appropriate resistance value plus different product, termination, ampacity, and installation checks. Use manufacturer engineering data and qualified design review rather than changing only one number casually.
Should I enter breaker rating or actual load current?
Use the properly calculated design current for the load and circuit, not an arbitrary breaker rating. Continuous loads, motors, heating, multiple outlets, demand factors, neutral currents, and other conditions can change that current and must be evaluated under the adopted rules.
Sources and reference standards
- NFPA 70 — National Electrical Code developmentOfficial NFPA information for the model electrical code used by adopting jurisdictions; the locally adopted edition and amendments control an installation.
- Southwire — Voltage Drop CalculatorManufacturer engineering calculator that evaluates voltage drop together with conductor size and NEC ampacity constraints.
- Southwire — Building Wire SelectorManufacturer reference emphasizing that final cable selection must account for installation conditions, current codes, a licensed electrician, and the authority having jurisdiction.
See every citation and the calculators that use it in the calculator source index.
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