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Voltage Drop Calculator

By · Updated Aug 2026

Check voltage drop, find a voltage-drop-only conductor size, or solve the maximum run length for a target percentage.

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How to Use

  1. Choose the question - Check a selected conductor, find the first configured AWG that meets the drop target, or solve the maximum one-way run length for a selected conductor.
  2. Enter operating current - Use the expected current flowing during operation, not automatically the breaker rating.
  3. Measure one-way distance - Enter source-to-load distance. The calculator applies the return-path factor internally for single-phase/DC and the √3 relationship for balanced three-phase.
  4. Keep ampacity separate - The minimum-wire mode is a voltage-drop screen only. A conductor still has to pass ampacity, termination, adjustment, equipment, and code checks.
  5. Treat 3% and 5% as design guidance - They are common planning targets associated with NEC informational-note guidance, not universal hard limits for every installation.
Quick answer

At 120 V, 20 A, 100 ft one-way, 12 AWG copper is about 6.6% drop in this resistance-based model. The first configured conductor below a 3% target is 8 AWG at about 2.6%. If you keep 12 AWG, the 3% one-way distance is about 45.6 ft. The calculator can solve all three versions directly.

Three Ways to Use Voltage Drop

Check wireAWG + distance→ volts + % dropFind wiredistance + target→ first passing AWGFind distanceAWG + target→ max one-way ftAmpacity remains a separate conductor-sizing requirement.
The same resistance model can solve voltage drop, conductor size for the selected drop target, or maximum run length.

What the Result Includes

Check mode shows percent drop, volts lost, estimated load-end voltage, target status, a larger configured conductor when needed, and the maximum distance the selected wire can run before reaching the target.

Minimum-wire mode starts at the smallest configured AWG and moves upward until the resistance-based voltage-drop result meets the selected target. That answer is not an ampacity recommendation. Use the Wire Size Calculator for a combined ampacity and voltage-drop screen.

Maximum-distance mode rearranges the same equation to solve one-way run length. It is useful when the conductor is already fixed and the design question is how far that conductor can carry the expected current before reaching the chosen drop budget.

Formulas Used

Resistance values are expressed in ohms per 1,000 ft and distance is one-way:

Single-phase / DC 2-wire = VD = 2 × I × R × L ÷ 1000
Balanced three-phase = VD = √3 × I × R × L ÷ 1000
Percent drop = VD% = VD ÷ system voltage × 100
Load-end voltage = V_load = V_source − VD
Maximum distance = L = (V × target% ÷ 100) × 1000 ÷ (factor × I × R)

This is a resistance-only model. It deliberately does not pretend that AC reactance, raceway material, power factor, parallel conductors, or every installation condition is included.

Example: 20 A, 120 V, Copper, 3% Target

WireDrop at 100 ftApprox. max one-way run at 3%
12 AWG6.59%45.6 ft
10 AWG4.14%72.4 ft
8 AWG2.60%115.2 ft
6 AWG1.64%183.1 ft

Resistance-based planning example using the configured conductor resistance data. Ampacity is not evaluated in this table.

Code and Technical Sources

  • NEC branch-circuit voltage-drop guidance appears in an informational note describing 3% at the farthest outlet and 5% total feeder-plus-branch-circuit drop as reasonable design guidance, rather than a universal hard limit for every installation.NFPA 70 development record for 210.19 voltage-drop guidance
  • Voltage drop remains separate from ampacity. NEC Table 310.16 provides conductor ampacity references, while final conductor sizing can also depend on adjustment/correction, terminations, equipment rules, and the adopted code.NFPA 70 Table 310.16 development record

Next Steps

Voltage drop is only one conductor check:

  1. Run the full wire-size screenCombine ampacity, ambient correction, conductor-count adjustment, and optional voltage drop.
  2. Confirm operating currentCombine equipment loads and convert the total to current before checking voltage drop.
  3. Check OCPD planningSplit continuous and noncontinuous current before selecting a standard rating.

Related Calculators

Amp to Wire Size CalculatorQuick ampacity-to-gauge lookup when distance is handled separately.Conduit Fill CalculatorConduit for the circuit.Watts Amps Volts CalculatorConvert watts to amps.Cable Tray Fill CalculatorTray fill for feeders.

Voltage drop is one part of circuit design. Browse the electrical calculator collection.

FAQ

Should I use load amps or breaker amps for voltage drop?
Use the expected operating current when you want a realistic drop estimate. Breaker rating can be higher than actual current and can overstate the expected drop.
Does the minimum-wire mode guarantee the wire is large enough?
No. It finds the first configured conductor that meets the selected voltage-drop target. Ampacity, conductor adjustment/correction, termination ratings, equipment rules, and local code still have to pass separately.
Why are 3% and 5% selectable?
They are common voltage-drop planning targets associated with NEC informational-note guidance. They are not universal hard limits for every circuit or jurisdiction.
Does this model include AC reactance and power factor?
No. It is intentionally a resistance-based planning model. Large conductors, long AC runs, raceway material, power factor, reactance, and parallel-conductor designs can justify a fuller impedance calculation.
Updated Aug 2026 · See our Methodology
Resistance-based planning estimator. The minimum-wire mode addresses voltage drop only and is not an ampacity recommendation. The model does not include every AC impedance, reactance, power-factor, parallel-conductor, raceway, or installation condition. Verify final conductor sizing and the locally adopted electrical code before installed work.