Loading…
Calculate voltage drop over wire runs. Use for wiring; for sizing use wire-gauge-calculator. Input: length/load/AWG. Returns drop percent. 100% client-side, files never leave browser.
Voltage Drop Calculator is loading — interactive editor will appear here. Use examples below to try instantly once loaded.
Press ? for keyboard shortcuts
Computes the voltage drop along a wire run for 12V, 24V, 120V, or 240V AC systems, single- or three-phase. Enter the current, the one-way length, the AWG, and the wire material (copper or aluminum); see the drop in volts and percent, plus a pass/fail against the recommended limit (3% for 12V/24V, 3% for 120V/240V single-phase, 5% for 240V three-phase).
Voltage drop is the silent killer of low-voltage systems. A wire that's 'thick enough' for the current can still cause motors to stall and LEDs to dim if the run is long. The recommended limit (typically 3% for low-voltage and 5% for three-phase mains) is what the NEC and most equipment manuals use. This calculator checks that limit instantly.
This tool is part of the DevStackIO platform — a collection of free online developer tools from DevStackIO. Browse more free developer resources on DevStackIO.
Solar installers sizing battery and panel cables, marine and RV electricians, off-grid and battery-backed-up electricians, audio / PA / lighting installers running long cable runs, and three-phase commercial electricians verifying feeder sizes.
Pick the system: 12V / 24V / 120V single / 240V single / 240V three-phase
Pick the wire material: copper or aluminum
Enter the load current in amps
Enter the one-way run length in feet (round-trip is computed automatically)
Pick the wire gauge from the AWG dropdown
Read the drop in volts and percent; green = within limit, red = too high
12V, 15A, 50ft, AWG 12 copper → 1.18V drop, 9.84% — exceeds 3%, use AWG 812V, 15A, 50ft, AWG 8 copper → 0.47V drop, 3.94% — still over 3%, use AWG 6120V, 15A, 100ft, AWG 14 copper → 2.40V drop, 2.00% — within 3%240V three-phase, 50A, 200ft, AWG 6 aluminum → 9.49V drop, 3.95% — within 5%24V, 10A, 30ft, AWG 14 copper → 1.13V drop, 4.71% — exceeds 3%, use AWG 12What's the recommended voltage-drop limit?
3% for low-voltage (12V/24V) and 120V/240V single-phase branch circuits, 5% total (feeder + branch) for 240V three-phase. Below 3% is 'excellent'; 3-5% is 'acceptable'; over 5% means equipment may not work correctly.
Why does the formula double the length?
Current flows out on one conductor and back on the other. The total wire the current travels is L + L, so we use 2L. For three-phase, the formula is √3 × I × R × L (line-to-line voltage, line current).
Does the wire temperature matter?
Yes. The values shown are at 20°C. A wire in a hot attic (say 40°C) has ~1.08× the resistance. The calculator doesn't model this; for hot-environment runs, derate by 10-15% or use a heavier gauge.
What gauge do I need for a 100ft run at 12V / 30A?
Use the calculator with current=30, length=100, gauge=8 copper. The drop is ~6.28V (way too high). Try AWG 4: drop ~2.5V (4.2%, still over 3%). Try AWG 2: drop ~1.6V (2.7%, OK). So AWG 2 copper for 100ft at 30A, 12V.
Why does the calculator say 'over limit' but the equipment still works?
The recommended limits are conservative. Many loads (especially resistive ones like heaters) tolerate 5-10% drop without complaint. Sensitive electronics (CPUs, sensors, LED drivers) need the full 3%.
Authoritative specifications, standards, and in-depth reading for Voltage Drop Calculator.
Voltage drop
2.382 V
% of nominal
1.99%
End voltage
118 V
Within the 3% recommended limit for 120V systems.
Round-trip length (out + back) is used. Single-phase: V_drop = 2 × I × R × L. Three-phase: V_drop = √3 × I × R × L. All math runs in your browser.
Convert AWG gauge to mm and diameter. Use for sizing wire; for drop use voltage-drop-calculator. Input: AWG number. Returns specs. 100% client-side, files never leave browser.
Total resistance for any list of resistors in series or parallel.
Solve voltage, current, resistance, and power from any two knowns (V=IR, P=VI).
Count characters, words, lines, and bytes. Use for quick length checks; for full stats use word-counter. Input: text ≤10MB. Returns counts. 100% client-side.
Add or subtract days and diff dates. Use for durations; for workdays use business-days-calculator. Input: dates. Returns difference. 100% client-side, files never leave browser.
Calculate exact age from birthdate. Use for years/months/days; for differences use date-calculator. Input: date. Returns age. 100% client-side, files never leave browser.
Count words, characters, and sentences. Get reading time and Flesch score as you type.
Sort lines alphabetically and remove duplicates. Use when cleaning lists; for comparing two texts use text-diff-visual. Input: text lines ≤10MB. Returns sorted unique lines. 100% client-side.