Voltage Drop Calculator
Calculate DC or single-phase AC voltage drop across a run of copper or aluminum wire. Input current, cable length, and AWG; returns drop in volts, percent, and power lost as heat.
How to Use
- Enter current in amps (steady-state, not inrush).
- Enter the one-way length of the cable run. The calculator doubles it automatically for the return path.
- Pick the wire gauge (AWG). Common values: 14 AWG for household branch circuits, 10 AWG for 30 A, 6 AWG for 50 A.
- Enter the supply voltage (12 V DC, 24 V DC, 120/240 V AC, etc.). The percent-drop is relative to this value.
- Check the loss in watts — that is the heat your wire will dissipate.
Show Work
Formulas
History of Voltage Drop
The "War of the Currents" (1886–1893) between Edison's direct-current distribution and the Westinghouse/Tesla alternating-current system turned largely on voltage drop. Edison's DC central stations could not transmit economically beyond a mile or so: wire resistance forced either massive copper cables or unacceptable voltage losses at the customer end. AC, by contrast, could be stepped up to high voltage for transmission (low current, low I²R loss) and stepped back down locally — transformers made long-distance distribution practical at a stroke.
American Wire Gauge (AWG), the sizing standard this calculator uses, was standardized by J. R. Brown & Sharpe in 1857 and adopted nationally in 1893. The logarithmic spacing — every 6 gauges doubles cross-sectional area, every 3 gauges roughly doubles DC resistance — is a 19th-century convention that still governs US wiring today. The NEC (National Electrical Code), first published by the National Fire Protection Association in 1897, codified voltage-drop recommendations as informational notes; 3% branch / 5% total has been the informal target since at least the 1940s.
About This Calculator
Enter steady-state current, one-way cable length, wire gauge, and supply voltage; this calculator returns the voltage drop in volts, the percent drop relative to supply, the voltage actually delivered at the load, and the power lost as heat along the run. Round-trip doubling is automatic — enter the physical distance, not the total conductor length.
Resistance values come from the IEEE/NFPA copper-wire tables (annealed copper, 75 °C). For aluminum, multiply results by 1.6. All math runs in your browser; nothing is sent to a server.
Frequently Asked Questions
Why do I double the length?
Current flows from source to load and back again. One foot of cable run means two feet of conductor. If you enter only one-way length and forget the factor-of-two, your drop is off by half. This calculator does the doubling for you.
What drop is acceptable?
The US National Electrical Code (NEC) recommends ≤3% on a branch circuit and ≤5% total including feeder. Below 3% the load operates at its nameplate voltage; above 5% motors run hot and lights dim noticeably.
Copper vs. aluminum?
Aluminum has about 61% the conductivity of copper — same AWG number means aluminum carries roughly 1.6× the resistance. For the same ampacity you need to step up two sizes (e.g. 6 AWG copper ≈ 4 AWG aluminum).
AC vs. DC drop — are they the same?
For low-frequency residential AC at typical distances, the reactance of household wiring is small enough that the DC resistance formula (R = ρL/A) is within a few percent of the true AC drop. For long runs, high currents, or metallic conduit, factor in reactance using NEC Chapter 9 Table 9.
What about voltage sag on motor starting?
This tool calculates steady-state drop. Motors draw 4–8× nameplate current at start; expect temporary drops that large during inrush. Size the feeder for the starting current if other equipment on the same circuit is sensitive.
Common Use Cases
12 V LED Strip Runs
A 10 A LED strip at 12 V loses 0.6 V over 50 ft of 14 AWG — a 5% drop, exactly at the edge of acceptable. Switch to 12 AWG for the same run and drop falls to 0.4 V (3%).
48 V Solar Array Wiring
From a 48 V / 30 A array to the charge controller, 30 ft of 10 AWG gives about 1 V drop (~2%) — comfortably inside target. Dropping to 12 AWG doubles it.
RV / Automotive 12 V Accessories
Running 15 A back to a rear-mounted inverter on 14 AWG for 20 ft drops nearly 1 V — meaningful on a 12 V system. Upsize to 10 AWG.
Garage Sub-Panel Feeder
50 A sub-panel 80 ft from the main, on 6 AWG copper, drops ~2.4 V on 240 V — 1%. Within NEC recommendation with no upsize needed.
Trolling Motor Battery Cable
60 A trolling motor on 20 ft of 4 AWG drops about 0.5 V from the battery — keeps full thrust. Dropping to 6 AWG costs you roughly 0.8 V and noticeable performance.
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