Voltage Drop Calculator
Voltage drop across a wire run from length, current, AWG, and material
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What this calculator computes
Voltage drop is the loss of electrical potential along a current-carrying conductor caused by the conductor's resistance, expressed as a percentage of the source voltage. Every wire has resistance proportional to its length and inversely proportional to its cross-sectional area, so a long run of small-gauge wire carrying significant current can lose a meaningful fraction of the source voltage before the load receives it. The US National Electrical Code (NEC) recommends a maximum 3% drop on branch circuits and 5% combined on feeders and branches together for normal operation, with safety-critical circuits often held to tighter margins. This calculator computes drop from four inputs: the one-way wire length in feet (the calculator doubles it internally for the round-trip current path), the current in amperes, the wire size in American Wire Gauge (AWG) or millimetres-squared cross-section, and the conductor material (copper or aluminium, with aluminium having about 60% higher resistance per gauge). Common applications include sizing conductors for EV-charger circuits where 40 A draws over 50–100 ft can push drop above the NEC limit, planning long runs to outbuildings or detached garages, and verifying that low-voltage landscape-lighting circuits will not dim the bulbs at the end of the run. The calculation also drives the choice between copper and aluminium conductors on long feeders, where aluminium's lower cost competes against its lower conductivity per AWG.
Calculator
The formula
Formula
V_drop = 2 × L × I × R_per_unit_length
Worked example
When to use this calculator
Use this calculator when you are sizing conductors for any new electrical installation, verifying an existing circuit's compliance with NEC voltage-drop limits, or planning a long run where wire-size economics matter. The most common scenarios are EV charger circuits, sub-panel feeders, detached-garage and outbuilding circuits, low-voltage lighting installations, and audio-amplifier speaker runs where drop affects damping factor. The calculator is most useful at the design stage before wire is purchased, because upsizing one AWG step typically halves the drop while raising material cost by about 25–40%, and the trade-off is worth running through deliberately. It does not substitute for an electrician's full load-calculation analysis or for a permit-required NEC review on services and feeders.
Common input mistakes
- Entering one-way distance and forgetting that current flows out and back, so the resistance affects the loop length. The calculator handles this internally by multiplying the entered length by 2, but manual hand-calculations using a single-distance formula will under-estimate drop by half.
- Using copper-AWG resistance values for aluminium conductors. Aluminium has about 61% of copper's conductivity, so an aluminium conductor at the same AWG has about 1.6× the resistance per foot. Mixing the two material conductivities into a single calculation produces wildly wrong drop figures.
Frequently asked questions
What is the maximum voltage drop allowed by the NEC?
The National Electrical Code recommends a maximum 3% drop on branch circuits (the run from the breaker to the final load) and 5% combined drop across feeders and branches together. These are recommendations rather than hard requirements in most editions of the code, but most jurisdictions enforce them through the local building inspector and utility-coordination requirements. Safety-critical and life-safety circuits often hold tighter margins, with 2% being a common design target.
How does voltage drop differ for single-phase vs three-phase circuits?
Single-phase calculations use the round-trip distance (out-and-back through hot and neutral) and the multiplier of 2 in the formula. Three-phase balanced calculations use a single-line distance with a multiplier of √3 (about 1.732) instead of 2, because the three-phase return path is shared across the phases and the math reduces to the line-to-neutral voltage. The calculator on this page handles single-phase residential and small commercial circuits; three-phase applications need a dedicated three-phase calculator.
Why does aluminium wire need to be larger than copper for the same circuit?
Aluminium has about 61% of copper's conductivity per cross-sectional area, so an aluminium conductor at the same AWG has about 1.6× the resistance per foot of a copper one. To carry the same current with the same voltage drop, aluminium needs roughly two AWG steps larger than copper. A 100 A circuit that uses 3 AWG copper typically uses 1 AWG aluminium, and the cost-vs-weight trade-off is what drives utilities and large-feeder applications toward aluminium.
Does temperature affect voltage drop?
Yes — copper and aluminium conductors both have positive temperature coefficients, with resistance rising about 0.4% per degree Celsius above 20°C. A wire running at its full ampacity in a hot attic at 40°C carries about 8% more resistance than the same wire at 20°C, producing proportionally higher voltage drop. NEC ampacity tables include temperature derating factors that account for this in conductor sizing.
What if my voltage drop is over the NEC limit?
Increase the wire size by one or more AWG steps until the calculated drop falls within the 3% (branch) or 5% (combined) target. Each step up in AWG roughly halves the resistance per foot, so a 6 AWG run that calculates at 4% drop typically falls to 2% on 4 AWG. Alternatively, reduce the run length by relocating the panel or sub-panel closer to the load, or split the load across multiple parallel circuits at smaller AWG.