Voltage DropCalculator
Calculate electrical voltage drop, percentage cable loss, and end-of-line voltage for single-phase and three-phase AC/DC circuits.
Circuit Specifications
Voltage Loss & Compliance
Voltage Drop Calculator - Free NEC Wire Sizing & Cable Loss Estimator: Calculate AC/DC circuit voltage drop, percentage loss, and NEC 3% compliance. Fast, free, and secure client-side execution in your browser.
How to Use the Voltage Drop Calculator
Select Circuit Phase & Voltage
Choose Single-Phase (120V / 240V residential / DC) or Three-Phase (208V / 480V industrial AC).
Enter Circuit Amperage
Input continuous load current in Amperes drawn by connected appliances or equipment.
Specify Distance & Gauge
Enter one-way run length and select conductor AWG size and metal material (Copper vs Aluminum).
Verify NEC 3% Compliance
Inspect calculated voltage loss against National Electrical Code standards (≤ 3% for branch circuits).
Electrical Voltage Drop: National Electrical Code (NEC) Standards and Conductor Sizing
In electrical engineering and building construction, voltage drop describes the inevitable reduction in electrical potential along the length of a conductor. All electrical wires possess finite intrinsic resistance dictated by their material composition, cross-sectional area, temperature, and length. When current (I) flows through this resistance (R), Ohm’s Law dictates an unavoidable loss of electrical voltage (ΔV = I × R), dissipated harmlessly or destructively as heat.
The National Electrical Code (NEC), published by the NFPA, provides clear informational notes (NEC Section 210.19(A) and 215.2(A)(1)) regarding acceptable voltage drop limits. The code recommends that the voltage drop on branch circuits should not exceed 3% of the nominal voltage. Furthermore, the combined voltage drop across the feeder and branch circuit combined should not exceed 5% to ensure reasonable efficiency and safe operation of connected electrical loads.
Conductor metal selection significantly impacts voltage drop performance. Copper features substantially lower electrical resistivity (approximately 1.68 × 10^-8 Ω·m) than aluminum (approximately 2.82 × 10^-8 Ω·m). Consequently, when using aluminum conductors to service equivalent distances and amperages, electricians must typically increase the conductor size by one to two AWG gauges (e.g., using #2 AWG aluminum in place of #4 AWG copper) to maintain equivalent resistance.
Failing to account for voltage drop across long wire runs produces severe operational consequences. Motors running on under-voltage draw excess current, leading to winding overheating and premature burnout. Sensitive electronic microprocessors and LED power supplies can reset, flicker, or malfunction when end-of-line voltage drops below manufacturer minimum operational thresholds.
Key Advantages & Analytical Value
Single & 3-Phase Support
Calculates both residential 1-phase 2-wire circuits and industrial 3-phase √3 networks.
NEC 3% Benchmark Alert
Automated compliance color badge flags marginal or excessive voltage loss instantly.
Copper & Aluminum Tables
Integrates NEC Chapter 9 Table 8 conductor resistance values at 75°C.
I²R Heat Dissipation
Calculates exact wattage lost as resistive heat along the entire cable run.
Confidential Facility Sizing
Evaluate electrical blueprints and industrial plant circuits privately in your browser.
Instant Live Math
Real-time calculation updates immediately as gauges, distances, or loads change.
Frequently Asked Questions
The NEC recommends a maximum 3% voltage drop on branch circuits, and a maximum 5% total voltage drop for feeder and branch circuits combined.
Excessive voltage drop causes motors to overheat and draw excess current, lights to flicker and dim, electronic controls to malfunction, and wasted electrical energy as cable heat.
In single-phase circuits, current travels out through the hot conductor and returns through the neutral (a round-trip distance of 2L). In balanced three-phase circuits, phase currents offset each other at 120-degree angles, resulting in a √3 (1.732) multiplier.
The most effective solution is to upsize the wire gauge (e.g., changing from 12 AWG to 10 AWG or 8 AWG), which decreases conductor resistance and lowers voltage drop.
Yes. As conductor temperature rises, electrical resistance increases, leading to higher voltage drop. Calculations typically assume a conservative operating temperature of 75°C.