⚡ Voltage Drop Calculator
Calculate the voltage loss in copper or aluminum wire. Enter current, wire length, cross-section area, and source voltage.
Voltage drop occurs because a conductor has electrical resistance. Voltage drop along a circuit increases with higher current, longer conductor runs, and smaller conductor cross-sectional area.
Formula recap: Vdrop = I * R, where R = rho * (2L) / A. Here rho is the resistivity of the conductor (ohm-m), L is the one-way length in metres, A is the cross-sectional area in square metres, and the factor 2 accounts for the round trip (out-and-back) path.
Units & conversions: Wire area is commonly given in mm^2. Convert to m^2 by multiplying mm^2 * 1e-6. Typical resistivities (ohm-m): copper = 1.72e-8, aluminium = 2.82e-8.
Practical guidance: NEC recommends keeping voltage drop under 3% for branch circuits and 5% total (feeder + branch) where possible. If calculated % drop is high, consider: increasing conductor size (larger mm²), shortening run length, or increasing supply voltage (if appropriate).
Example: A 12 A load over 25 m one-way using 2.5 mm^2 copper: convert area to m^2 (2.5e-6 m^2), compute resistance R = 1.72e-8 * (2*25) / 2.5e-6 ? 0.0003447 ohm, then Vdrop = 12 * 0.0003447 ? 0.00414 V (very small). For long runs or higher currents the drop grows proportionally.
AC vs DC & other effects: For AC at mains frequencies (50/60 Hz) the DC resistivity formula is usually sufficient for short runs. At high frequencies skin effect and proximity effect can increase effective resistance. Also consider connections, terminations, and temperature (resistance increases with temperature).
Wire sizing tip: Doubling conductor cross-sectional area roughly halves the resistance and voltage drop. Common residential wire sizing (approximate): 1.5 mm² (~15 A), 2.5 mm² (~2025 A), 4 mm² (~32 A), 6 mm² (~40 A). Always follow local electrical codes and consult an electrician for installations.
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Voltage Drop: Why Wire Size Matters
Voltage drop is the reduction in voltage that occurs as electricity travels through a wire due to the wire's resistance. All conductors have resistance, and that resistance causes a portion of the voltage to be "lost" as heat before reaching the load. The formula is: Voltage Drop = Current (A) × Wire Resistance (Ω).
NEC Allowable Voltage Drop Limits
The National Electrical Code (NEC) recommends keeping voltage drop below 3% for branch circuits and below 5% total for the combined feeder and branch circuit. Excessive voltage drop causes: motors running hot and burning out early, lights flickering or dimming, electronics malfunctioning, and reduced efficiency in all connected equipment.
How Wire Gauge Affects Voltage Drop
Thicker wire (lower AWG number) has less resistance per foot. Common resistance values: 14 AWG = 2.525 Ω/1000 ft, 12 AWG = 1.588 Ω/1000 ft, 10 AWG = 0.999 Ω/1000 ft, 8 AWG = 0.628 Ω/1000 ft. For long runs — subpanels, outbuildings, EV chargers — always upsize the wire gauge. A 100-foot run of 12 AWG carrying 20A loses about 3.4V on a 120V circuit (2.8%) — right at the limit. The same run in 10 AWG loses only 2.1V (1.8%).
Why Voltage Drop Matters in Electrical Systems
Voltage drop is the reduction in voltage along a conductor caused by its resistance. As current flows through a wire, it encounters resistance and loses voltage: V_drop = I × R. Excessive voltage drop causes motors to run hot and wear out faster, lights to dim, electronics to malfunction, and GFCI/AFCI breakers to nuisance-trip. The NEC (National Electrical Code) recommends total voltage drop not exceed 3% for branch circuits and 5% for the combined feeder and branch circuit. On a 120V circuit, 3% drop = 3.6V maximum. Long wire runs — common in large homes, shop buildings, or outdoor installations — are the most common cause of problematic voltage drop.
How to Calculate Voltage Drop for Wiring Runs
For single-phase AC circuits: V_drop = 2 × K × I × L ÷ CM, where K is the resistivity constant (12.9 for copper, 21.2 for aluminum in circular-mil units), I is current in amps, L is one-way length in feet, and CM is the wire's cross-sectional area in circular mils. A simpler approximation: for copper wire, voltage drop ≈ (2 × 0.0167 × I × L) ÷ wire_CM. Wire gauge determines CM: #14 AWG = 4,110 CM; #12 AWG = 6,530 CM; #10 AWG = 10,380 CM; #8 AWG = 16,510 CM; #6 AWG = 26,240 CM. To reduce voltage drop, either increase wire gauge (larger CM) or use a sub-panel to shorten the run.
Practical Wire Sizing for Long Runs
The NEC minimum wire gauge for 20A circuits is #12 AWG — but for long runs this may be undersized. A 20A circuit running 150 feet to a workshop: V_drop = 2 × 0.0167 × 20 × 150 ÷ 6,530 = 1.54V (1.28% — acceptable). The same circuit at 250 feet: 2.56V drop (2.13% — borderline). At 350 feet: 3.59V drop (2.99% — at the limit). To keep under 3% at 350 feet, upsize to #10 AWG (10,380 CM): V_drop = 2 × 0.0167 × 20 × 350 ÷ 10,380 = 2.26V (1.88% — acceptable). General rule: for runs over 100 feet at 120V or 200 feet at 240V, calculate voltage drop and consider upsizing one or two gauge sizes.
