Voltage drop distance worksheet
Max Wire Run Voltage Drop Calculator
Estimate maximum one-way run length from nominal voltage, target voltage drop, current, and conductor resistance.
Calculate Maximum Wire Run
Enter nominal voltage, target percent drop, current, and conductor resistance per 1,000 ft to estimate maximum one-way run length.
Result
Maximum one-way run
183.3 ft
Dropped volts
7.20 V
Result notes
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
Estimate maximum one-way run length from nominal voltage, target voltage drop, current, and conductor resistance.
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
Maximum one-way run length can be rearranged from the voltage-drop formula. First convert the target percent into allowed dropped volts: Vd = nominal voltage x percent / 100. For a DC or single-phase two-wire circuit, L = Vd x 1000 / (2 x I x R). For a balanced three-phase circuit, L = Vd x 1000 / (sqrt(3) x I x R). R is conductor resistance in ohms per 1,000 ft.
U.S. field context and example
This worksheet is useful when a conductor resistance has already been chosen and the question is distance, not conductor selection. For example, a 240 V two-wire circuit with a 3% target allows 7.2 V of drop. At 40 A and 0.491 ohm per 1,000 ft, the maximum one-way run is about 183 ft before that target is reached under the worksheet assumptions.
Assumptions and limits
The result is a planning distance from a simplified resistance formula. It does not include conductor temperature rise, conductor reactance, voltage at motor starting, power factor, harmonics, raceway grouping, terminal limits, or required conductor ampacity. Use the full voltage-drop, wire-size, and ampacity calculators when selecting conductors for an installation.
Common mistakes
Common mistakes include entering round-trip length as the desired output, using the wrong phase multiplier, ignoring the actual conductor material and temperature, and assuming a maximum run estimate approves the conductor. Keep the target percent, nominal voltage, load current, resistance source, and circuit type with the result.