Smallest modeled size meeting the selected drop target with the entered circuit assumptions.
Voltage Drop Calculator
Voltage Drop Calculator
Estimate voltage loss, compare conductor sizes, and coordinate the result with an ampacity minimum. New here? Read the quick guide.
Voltage Drop Calculation
Report details
Circuit inputs and voltage-drop result
- Project
- —
- Circuit
- —
- Prepared by
- —
- Revision
- —
- System / source
- AC single-phase · 120 V
- Load
- 15 amps · 15 A calculated
- Run length
- 100 ft one way
- Conductor
- 12 AWG Copper
- Parallel conductors
- 1 per phase
- Power factor / reactance
- 0.9 · 0.08 mΩ/m
- Conductor temperature
- 75°C
- Maximum target
- 3%
Calculated decision
- Voltage lost
- 5.63 V
- Calculated drop
- 4.69%
- Estimated load voltage
- 114.37 V
- Target status
- REVISE
- Maximum one-way length
- 63.96 ft
- Recommended size
- 10 AWG
Project and report information
Circuit inputs
Circuit and load
Calculated load current15 A
Advanced conductor and AC assumptions
The reactance presets are sensitivity scenarios, not installation-specific guarantees. Use manufacturer or project impedance data when available. Parallel conductors are modeled as identical, equal-length paths sharing current equally.
Circuit preview
Source-to-load voltage
Source120 V
100 ft one wayCopper 12 AWG
Estimated load114.37 V
Calculated result
Voltage-drop details
Voltage lost5.63 V
Percentage drop4.69%
Estimated load voltage114.37 V
Selected target3% — REVISE
Approximate maximum length before the selected drop target is exceeded.
Nearby conductor comparison
Comparison uses the first run's length and the same material, load, temperature, parallel-path, and impedance assumptions.
| Size | Drop | Load voltage | Target |
|---|---|---|---|
| 14 AWG | 7.44% | 111.07 V | REVISE |
| 12 AWG | 4.69% | 114.37 V | REVISE |
| 10 AWG | 2.96% | 116.45 V | PASS |
| 8 AWG | 1.87% | 117.75 V | PASS |
| 6 AWG | 1.19% | 118.58 V | PASS |
Governing calculation
2 × current × one-way length × effective impedance
- 75°C effective resistance
- 6.8001 Ω/km
- Effective modeled impedance
- 6.155 Ω/km
- Circuit multiplier
- 2
- Parallel conductors
- 1 per phase/polarity
Voltage drop is only one conductor-sizing check
The recommended size is based only on this voltage-drop model. It does not verify ampacity, continuous-load sizing, temperature correction, conductor count, terminal ratings, overcurrent protection, fault current, or local code requirements. The final conductor must satisfy every applicable requirement.
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Calculation guidance
Use the result as one part of conductor selection
Enter the one-way source-to-load distance. The calculator defaults to conductor resistance at 75°C and, for AC circuits, combines resistance and reactance using the entered power factor. Temperature and parallel-path assumptions are available under Advanced options. Review the complete formulas and source notes on the methodology page.
Voltage-drop FAQ
Common calculation questions
Should I enter one-way length or total circuit length?
Enter the one-way distance from the source to the load. The calculator applies the return-path factor for DC two-wire and single-phase circuits, or the square-root-of-three factor for balanced three-phase circuits.
Does the recommended conductor size verify ampacity?
No. It is the smallest modeled conductor that meets the selected voltage-drop target only. Ampacity, continuous-load sizing, temperature correction, conductor count, terminal ratings, overcurrent protection, fault current, and local requirements must be checked separately.
Why are power factor and reactance included?
AC voltage drop depends on both resistance and reactance. Power factor changes how those components contribute. The advanced inputs let you replace the planning assumptions when project-specific impedance data is available.
Is 3% voltage drop a mandatory limit?
Not universally. Three percent is a common design target, and NEC informational notes discuss 3% branch-circuit and 5% combined feeder-plus-branch performance. Informational notes are explanatory, so confirm the adopted code, project criteria, and equipment requirements.
Can I save or share the calculation?
Yes. Print a branded report or save it as PDF, download a branded CSV, or copy a URL that restores the calculator inputs.
Can I calculate a feeder and branch circuit together?
Yes. Select Feeder plus branch circuit, then enter each segment's one-way length and conductor size. The calculator shows the feeder drop, branch drop, combined drop, and estimated voltage at the final load.
Can I enter watts, kilowatts, VA, or kVA?
Yes. Choose the load unit and the calculator converts the entered load to current using circuit voltage, phase, and power factor as applicable. The derived amperes remain visible beside the inputs.