Voltage Drop Calculator
Find how many volts a wire run loses between the panel and the equipment, what that is as a percentage, and the longest run that stays within the limit you choose.
Method reviewed October 26, 2026. Version 1.0.0. How it works
This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.
Result
Above your limit
- Voltage drop
- 7.43V
- Voltage drop as a percentage of the source
- 3.1%
10 AWG copper over 100 ft at 30 A drops about 7.4 V (3.1 %), leaving 232.6 V at the load.
- The smallest listed copper size that keeps this run within 3 % is 8 AWG. Voltage drop is only one check: the wire must also be rated for the current (ampacity), the breaker and the terminals under your electrical code.
- Voltage at the load
- 232.6V
- Conductor resistance at this temperature
- 1.239Ω per 1,000 ft
- Heat lost in the wires
- 223W
- Longest one-way run within 3 %
- 97ft
Show the working
- Area of 10 AWGd = 0.005 × 92^((36 − n) ÷ 39) in → 10,383 circular mils
- Resistance at 20 °C = 10.371 Ω·cmil/ft × 1,000 ÷ area × (1 + 2 % stranding)1.0188 Ω per 1,000 ft
- At 75 °C: × (1 + 0.00393 × (75 − 20))1.239 Ω per 1,000 ft
- Drop = 2 × I × R × L ÷ 1,000 (out and back)2 × 30 × 1.239 × 100 ÷ 1,000 = 7.43 V
- Percent drop = drop ÷ source voltage × 1007.43 ÷ 240 × 100 = 3.1 %
How the drop is calculated
Every wire has resistance, and current through it loses voltage in proportion: drop = current × resistance. On a single-phase or DC circuit the current goes out on one wire and back on the other, so the drop is across both. On a balanced three-phase circuit the drop between lines is √3 times the drop along one wire.
The wire’s resistance is not looked up in a code table. It is computed from the conductor standards the National Bureau of Standards (now NIST) published: annealed copper has a resistivity of 10.371 ohm-circular mils per foot at 20 °C, and EC-grade aluminum conductor 17.002. The area of each AWG size follows from the gauge’s definition: No. 0000 is 0.4600 in, No. 36 is 0.0050 in, and the sizes between are a geometric series. Our No. 10 copper result, 0.9988 ohms per 1,000 ft at 20 °C, matches the NBS table.
Temperature and stranding
Resistance rises as wire warms: about 0.39 % per °C for copper and 0.40 % for aluminum. Wire carrying current runs warmer than the room, so the calculator defaults to a conservative 75 °C. Enter 20 °C to see the lowest possible resistance.
Stranded wire has a little more resistance than a solid wire of the same total area, because the strands spiral. The NBS tables put the increase at 2 to 5 %. The default adds 2 %.
What this does not check
Voltage drop is one check among several. The wire must also be rated for the current it carries (its ampacity), for the insulation temperature, and for the breaker and terminals, all under your local electrical code. The calculation also ignores the inductive reactance of AC circuits, which adds to the drop with large conductors.
- Use the result to compare wire sizes and run lengths.
- Check the equipment’s minimum operating voltage on its nameplate or installation manual.
- Have a licensed electrician design and install the circuit.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Conductor
- Copper
- Wire size
- 10 AWG
- Construction
- Stranded
- Stranding allowance
- 2 %
- Circuit
- Single-phase or DC
- Source voltage
- 240 V
- Load current
- 30 A
- One-way length of the run
- 100 ft
- Conductor temperature
- 167 °F
- Your design limit for voltage drop
- 3 %
Result
- Voltage drop
- 7.43 V
- Voltage drop as a percentage of the source
- 3.1 %
- Voltage at the load
- 232.6 V
- Conductor resistance at this temperature
- 1.239 Ω per 1,000 ft
- Heat lost in the wires
- 223 W
- Longest one-way run within 3 %
- 97 ft
10 AWG copper over 100 ft at 30 A drops about 7.4 V (3.1 %), leaving 232.6 V at the load.
Assumptions
- DC resistance from the NBS wire-table constants: copper 10.371 Ω·cmil/ft and aluminum (EC-H19) 17.002 Ω·cmil/ft at 20 °C, corrected linearly with temperature (0.00393 and 0.00403 per °C).
- AWG areas are computed from the AWG definition; kcmil sizes are 1,000 circular mils per kcmil.
- Single-phase and DC: the current flows out and back, so the drop is across two conductors. Balanced three-phase: the line-to-line drop is √3 times the drop in one conductor.
Limitations
- Uses resistance only. Inductive reactance and a low power factor add to the drop on AC circuits, most noticeably with large conductors; treat the result as a minimum for those.
- Voltage drop is not a wire-sizing method on its own: ampacity, insulation rating, terminations and overcurrent protection come from your electrical code. Wiring work must be done by a qualified electrician.
Frequently asked questions
What length do I enter?
The one-way distance from the panel to the equipment, following the route the cable takes. The calculator adds the return path itself.
Why does aluminum wire need to be bigger?
Aluminum conducts about 61 % as well as copper, so for the same voltage drop it needs about 1.6 times the area — roughly two AWG sizes larger.
What happens if voltage drop is too high?
Motors and compressors draw more current and run hotter at low voltage, and may struggle to start. Lights dim when large loads start. The fix is a larger wire, a shorter run, or a higher supply voltage, chosen by an electrician.
Sources
- Copper Wire Tables (NBS Handbook 100, 1966) — National Bureau of Standards (now NIST), U.S. Department of Commerce. International Annealed Copper Standard: 10.371 ohm-circular mil/ft at 20 °C; temperature coefficient 0.00393 per °C at 20 °C; AWG diameters in geometric progression from No. 0000 = 0.4600 in to No. 36 = 0.0050 in in 39 steps (ratio = 39th root of 92); No. 10 solid = 0.9988 Ω per 1,000 ft at 20 °C; stranded conductors 2 to 5 % above the equivalent solid rod.
- Aluminum Wire Tables (NBS Handbook 109, 1972) — National Bureau of Standards (now NIST) with The Aluminum Association. EC-H19 (1350) aluminum conductor: 61.0 % IACS, 17.002 ohm-circular mil/ft at 20 °C; constant-mass temperature coefficient of resistance 0.00403 per °C at 20 °C.
- University Physics Volume 2, §9.4 Ohm’s Law — OpenStax, Rice University (CC BY 4.0). V = IR; electrical power P = IV.
- Determining Electric Motor Load and Efficiency — U.S. DOE. Equation 1: three-phase input kW = V × I × PF × √3 ÷ 1,000 (worked example 469.7 V, 37 A, PF 0.763 → 22.9 kW); Equation 2: full-load input kW = hp × 0.7457 ÷ efficiency.
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