Voltage drop worksheet
Voltage Drop Volts Calculator
Calculate dropped volts from load current, conductor length, conductor resistance, and circuit type without selecting wire size.
Calculate Dropped Volts
Enter current, one-way length, and conductor resistance per 1,000 ft to estimate voltage drop in volts.
Result
Dropped volts
3.928 V
Uses resistance in ohms per 1,000 ft of conductor.
Result notes. Keep inputs, assumptions, and result together before using this value in project records.
Keep the entered values, assumptions, and result together when adding this calculation to job notes or submittal records. Final installation choices should align with the applicable code edition, equipment listing, manufacturer instructions, local amendments, and AHJ requirements.
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Formula and field context
Calculate dropped volts from load current, conductor length, conductor resistance, and circuit type without selecting wire size.
Formula context
Voltage Drop Chart
Voltage drop is a design-performance check, not a simple wire-size lookup. A 3% target equals 3.6 V on a 120 V circuit, 7.2 V on 240 V, and 14.4 V on 480 V; a 5% target equals 6.0 V, 12.0 V, and 24.0 V. After choosing the target, use the calculator with one-way distance, actual load current, conductor material, conductor size, and phase before changing a conductor size.
Formula
Single phase: Vdrop = 2 x K x I x D / cmil. Three phase: Vdrop = 1.732 x K x I x D / cmil.Variables to keep with the result
- K is the conductor material constant used by the calculator.
- I is the expected load current, not automatically the breaker rating.
- D is one-way circuit distance including real route length.
- cmil is conductor area in circular mils.
Formula and variables
For a DC or single-phase two-wire path, dropped volts can be estimated as Vd = 2 x I x L x R / 1000. For a balanced three-phase path, use Vd = sqrt(3) x I x L x R / 1000. I is load current in amps, L is one-way circuit length in feet, and R is conductor resistance in ohms per 1,000 ft. The result is dropped volts, not percent drop and not wire size.
U.S. field context and example
This worksheet is useful when a conductor resistance value has already been selected from a chart, manufacturer data, or a separate resistance calculation. For example, a 40 A two-wire run with 100 ft one-way length and 0.491 ohm per 1,000 ft has Vd = 2 x 40 x 100 x 0.491 / 1000, or about 3.93 V. After calculating volts, compare the value with the nominal circuit voltage using the voltage-drop percent tool.
Assumptions and limits
This tool assumes a simple resistance-based voltage-drop worksheet. It does not include reactance, power factor, harmonics, starting current, temperature-adjusted resistance, parallel conductors, or conductor selection rules. Use the full voltage-drop calculator when conductor material, conductor size, phase, current, length, and percent target need to be reviewed together.
Common mistakes
Common mistakes include using round-trip length in a formula that already has the two-wire multiplier, applying the two-wire formula to a balanced three-phase feeder, using a resistance value at the wrong conductor temperature, and treating a voltage-drop result as a code approval. Keep the circuit type, length basis, resistance source, and nominal voltage with the result.