Voltage drop — the reduction in voltage as electrical current flows through a conductor — is a critical consideration in electrical system design that directly affects equipment performance, safety, and energy efficiency. The National Electrical Code (NEC) recommends maximum voltage drop of 3% for branch circuits and 5% for combined feeder and branch circuits, though these are recommendations, not requirements. Excessive voltage drop causes motors to overheat and lose torque, lighting to dim, sensitive electronics to malfunction, and overall energy waste as power dissipates as heat in wiring. Our voltage drop calculator computes the voltage drop for any circuit based on conductor size (AWG), length, material (copper or aluminum), load current, and voltage system (single-phase or three-phase). It helps electricians and engineers select the correct wire gauge, verify code compliance, and optimize wire sizing for long-run circuits where standard minimum sizing would cause excessive drop.
Voltage drop formula and variables
The voltage drop formula VD = (2 × K × I × L) / CM calculates drop in volts, where K is the resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C), I is load current in amps, L is one-way circuit length in feet, and CM is the conductor cross-sectional area in circular mils. For three-phase circuits, replace the factor 2 with 1.732. A 20-amp circuit using 12 AWG copper (6530 CM) over 100 feet: VD = (2 × 12.9 × 20 × 100) / 6530 = 7.9V, which is 6.6% on a 120V circuit — exceeding the 3% recommendation. Upsizing to 10 AWG (10380 CM) drops this to 5.0V (4.1%), and 8 AWG (16510 CM) achieves 3.1V (2.6%). Long runs almost always require upsizing beyond the minimum ampacity-rated conductor.
Wire gauge selection for long circuits
NEC ampacity tables (Table 310.16) specify minimum wire sizes based on current carrying capacity, but voltage drop often demands larger conductors for runs exceeding 50-100 feet. Common scenarios requiring upsizing: garage subpanels (typical 50-100 foot runs need 4 AWG or larger for 60A service), well pumps (200-500 foot runs may need 6 AWG for a 10A 240V pump), outdoor lighting circuits (long landscape lighting runs), and agricultural buildings. A practical sizing table for 120V, 20A circuits with 3% max drop: up to 50 feet use 12 AWG, 50-80 feet use 10 AWG, 80-120 feet use 8 AWG, and 120-200 feet use 6 AWG. For 240V circuits, the same wire sizes support double the distance because the percentage drop is halved by the higher voltage.
Impact on motor performance and energy costs
Motors are particularly sensitive to voltage drop. A motor receiving 5% below rated voltage draws approximately 5% more current (to maintain power output), increasing winding temperature by 10-15% and reducing insulation life. At 10% voltage drop, motor torque decreases by approximately 19% (torque varies as voltage squared), potentially causing failure to start under load. Variable frequency drives (VFDs) are somewhat more tolerant but may fault on low voltage input. From an energy cost perspective, voltage drop represents pure waste — power dissipated as heat in conductors equals I²R. A 100-foot run of 12 AWG carrying 20A continuously wastes approximately 158 watts (7.9V × 20A), costing $140 per year at $0.12/kWh. Upgrading to 10 AWG reduces this waste by 37%, with the wire cost difference typically paying back within 2-3 years through energy savings.