1.405
V
1.17
%
118.59
V
28.1
W
0.0703
Ω
1.405
V
1.17
%
118.59
V
28.1
W
0.0703
Ω
The voltage drop calculator determines how much voltage is lost across a wire run given the conductor size, length, and current. Voltage drop is an inevitable consequence of conductor resistance — every real wire has nonzero resistance, and when current flows through it, a fraction of the supply voltage is consumed heating the wire rather than powering the load. Calculating this drop is essential for ensuring loads receive adequate voltage to operate correctly.
The voltage drop formula for DC and single-phase AC circuits is: Vd = 2 × I × R × L / 1000, where I is the current in amperes, R is the resistance in ohms per 1000 feet (Ω/kft), and L is the one-way length in feet. The factor of 2 accounts for both the supply and return conductors. For three-phase circuits, the factor becomes √3 instead of 2.
The National Electrical Code (NEC) informational notes recommend that voltage drop not exceed 3% for branch circuits and feeders individually, with a combined limit of 5%. While these are not mandatory requirements under the NEC, they represent best engineering practice and are often codified in local amendments and utility interconnection standards. The IEC (used in most countries outside North America) specifies similar limits: 3% for lighting circuits, 5% for power circuits.
Excessive voltage drop causes numerous practical problems. Motors running at reduced voltage draw higher current (increasing their temperature and shortening insulation life), produce less torque, and are more susceptible to stalling under load. LED drivers and switching power supplies may operate outside their input voltage range. Battery chargers may charge incompletely. Sensitive electronic equipment may malfunction or report undervoltage alarms.
For long outdoor runs, solar PV systems, landscape lighting, EV charging stations, and subpanel feeders, voltage drop is often the governing factor in wire sizing rather than ampacity alone. A 200-foot run at 120V supplying a 15A load at 3% maximum drop requires AWG 10 copper instead of the ampacity-limited AWG 14, nearly tripling the wire cost but ensuring proper voltage delivery.
This calculator uses AWG-based resistance values derived from the copper/aluminum resistivity and circular mil cross-sectional areas. Input values include current, one-way wire length, AWG gauge, supply voltage, and conductor material (copper or aluminum).
The conductor cross-sectional area in circular mils (CM) is calculated from the AWG number: CM = 211600 × 0.5^(AWG/4.312). Resistance per 1000 feet: R/kft = ρ / CM × 1000, where ρ = 10.8 (copper) or 17.0 (aluminum) in Ω·CM/ft. Total round-trip resistance: R_total = 2 × R/kft × L / 1000. Voltage drop: Vd = I × R_total. Power loss: P = I² × R_total.
Voltage drop below 2% is excellent. 2-3% is acceptable for most applications. 3-5% may cause issues with sensitive loads or motors. Above 5% is generally unacceptable. If drop exceeds the limit, use a larger AWG wire (lower number), shorter runs, or a higher supply voltage. Power loss in the wire directly represents wasted energy — on a circuit running continuously, even 10W of wire loss costs several dollars per month in electricity.
Inputs
Results
AWG 6 copper over 80 ft at 60A on 240V gives 1.3% voltage drop — well within NEC 3% recommendation.
Inputs
Results
AWG 12 over 150 ft at 8A gives 5.98% drop — exceeding the 3% limit. Upgrade to AWG 8 copper to bring drop below 3%.
Voltage drop is the reduction in voltage across a conductor due to its resistance. It matters because loads receive less voltage than the supply, potentially causing malfunction, overheating, and shortened equipment life.
The resistive voltage drop (IR drop) is the same for DC and AC at low frequencies. At higher frequencies, skin effect increases AC resistance, and inductive/capacitive reactance add phase angle. For 60 Hz power circuits, the resistive formula is sufficiently accurate.
NEC informational notes recommend maximum 3% drop for branch circuits and 3% for feeders (5% combined). These are recommendations, not code requirements, but are widely adopted as minimum standards.
Current flows from the source to the load and back. The total conductor length carrying current is twice the one-way distance, so the resistance (and voltage drop) doubles.
Options include: increasing supply voltage (e.g., use 240V instead of 120V — cuts drop by half for same current), reducing the run length by locating panels closer to loads, or reducing the load current.
For balanced three-phase circuits, the voltage drop formula uses √3 instead of 2: Vd = √3 × I × R × L / 1000. Three-phase systems have lower voltage drop per unit of power transmitted compared to single-phase.
Conductor resistance increases with temperature: R(T) = R₂₀ × (1 + α(T - 20)), where α = 0.00393/°C for copper. At 75°C (NEC standard for wire insulation rating), copper resistance is about 22% higher than at 20°C. This calculator uses 75°C values typical for NEC calculations.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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