Calculation details
Planning estimate only, not financial, tax, or legal advice. Verify assumptions and current rules before making decisions.
This tool runs in your browser. Your calculator inputs and results are never transmitted to us or to ad/affiliate partners. Ads and sponsored links may set third-party cookies to serve and measure them, but they never receive your calculation values. If you explicitly save a scenario, its permitted fields stay in local browser storage until you clear them. See our Privacy Policy.
Use this result
Share the current inputs or ask ChatGPT to explain the calculation in context.
What does this calculator estimate?
A voltage drop calculator sizes wire runs. Enter the voltage, current, length, and gauge to see the drop in volts and percent.
- Drop = 2 × L × R × I ÷ 1000
- 12 AWG at 100 ft, 10 A → 3.18 V
- Keep drop under 3–5%
Why voltage drop matters
Every wire run loses voltage to resistance — too much drop makes motors overheat, lights dim, and equipment malfunction. The calculation uses the wire's resistance per 1,000 ft, doubled for the round trip, times the current.
Limitations to watch for
The tool covers copper at common gauges — aluminum and other materials differ. Longer runs and higher currents drop more; the fix is a larger gauge or shorter run. Code requirements (NEC) set the limits for most installations.
How to use it in practice
Enter the supply voltage, expected amps, one-way run length, and planned gauge. If the drop exceeds 3–5%, step up a gauge or two and re-check. Verify against local code before installing.
['Enter the supply voltage and current.', 'Enter the one-way length and wire gauge.', 'Read the drop in volts and percent.']
What voltage drop is
Voltage drop is the reduction in voltage along a conductor caused by its resistance: Vdrop = 2 × length × resistance × current ÷ 1,000 for single-phase circuits. A 100-foot run of 12 AWG at 10 amps drops about 3.2 volts — about 2.6 percent on a 120-volt circuit.
Why it matters
Undersized wire drops voltage, which dims lights, slows motors, and can damage equipment at the end of long runs. The National Electrical Code recommends keeping drop under 3 percent for branch circuits and 5 percent total. The calculator applies the standard thresholds.
Wire gauge and length trade-offs
Doubling the wire gauge (smaller number) roughly halves resistance; doubling the length doubles the drop. A 200-foot run that needs under 3 percent drop may require a gauge two sizes larger than a 50-foot run. The calculator shows the drop at any gauge and length.
AC vs. DC and three-phase
The 2× factor covers the round trip in single-phase; DC uses the same form; three-phase uses a √3 factor. The calculator models the single-phase and DC cases with selectable voltage, which covers most residential and automotive applications.
A worked example
A 120-volt circuit, 15 amps, 100 feet of 12 AWG (1.588 Ω/1000 ft): drop = 2 × 100 × 1.588 × 15 ÷ 1,000 = 4.76 volts — 3.97 percent, over the 3 percent branch limit. Moving up to 10 AWG cuts the drop to about 2.5 percent. The calculator shows both.
How this calculator works
Formula
Voltage drop = 2 × one-way length × resistance per 1,000 ft × amps ÷ 1,000. The tool uses standard wire-gauge resistances and reports the drop as volts and % of supply.
Worked example
120 V, 10 A, 100 ft of 12 AWG (1.588 Ω/1000 ft): drop ≈ 3.18 V = 2.6%.
Assumptions to verify
- Copper conductors at standard resistances.
- The run is the one-way length (doubled internally).
- Ambient temperature and load power factor are typical.
Frequently asked questions
How do I calculate voltage drop?
2 × length × resistance × amps ÷ 1000: 2 × 100 × 1.588 × 10 ÷ 1000 = 3.18 V.
What is an acceptable drop?
Under 3% for most circuits; 5% is the NEC limit for feeders in many cases.
How do I fix too much drop?
Use a larger gauge wire, shorten the run, or reduce the load.
Why is the length doubled?
Current travels out and back — the total conductor length is two times the run.
Does wire material matter?
Yes — aluminum has higher resistance than copper; the tool models copper.
What happens if drop is too high?
Lights dim, motors overheat and run inefficiently, and equipment can malfunction.
How do I find the amps?
Divide watts by voltage (see the watts-to-amps calculator), or use the device's rating.
What is voltage drop?
The voltage lost along a wire from resistance — it grows with length and current.
How do I calculate it?
2 × length × resistance per 1000 ft × current ÷ 1000 (single-phase).
What is the maximum recommended drop?
Under 3% for branch circuits, 5% total including feeders.
How do I fix excess drop?
Use a larger gauge wire, shorten the run, or raise the supply voltage.
Cite this tool
BoringToolsKit. “Voltage Drop Calculator.” boringtoolskit.com/voltage-drop-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
Privacy: Inputs and results stay in this browser. Any future sponsored recommendation or advertisement will be clearly labeled and kept separate from the calculation.