Voltage Drop Calculator

To calculate voltage drop, use the formula: V_drop = (2 × I × R × L) / 1000 for single-phase AC or DC circuits, where I is the load current in amperes, R is the conductor resistance in Ω/1000 ft, and L is the one-way distance in feet. For three-phase systems, replace the factor 2 with √3 (≈ 1.732). Percentage voltage drop = (V_drop / V_source) × 100. Our free online voltage drop calculator automates this entire process — just enter your source voltage, current, wire gauge, and circuit length to get an instant result.

Voltage drop per 100 ft depends on conductor size and current. Using NEC Table 8 copper values at 75°C: 14 AWG (3.14 Ω/kft) carries 15 A → 0.942 V per 100 ft (one-way). 12 AWG (1.98 Ω/kft) at 20 A → 0.792 V per 100 ft. 10 AWG (1.24 Ω/kft) at 30 A → 0.744 V per 100 ft. Multiply these single-direction values by 2 for the full round-trip circuit. Use our voltage drop calculator to instantly compute the voltage drop per 100 ft for any wire size, current, and conductor material.

For a 240 V single-phase circuit the formula is identical: V_drop = (2 × I × R × L) / 1000. The percentage drop = (V_drop / 240) × 100. Example — 50 A load, 12 AWG copper, 75 ft run: V_drop = (2 × 50 × 1.98 × 75) / 1000 = 14.85 V → 6.2%, which exceeds the NEC 3% guideline. You would need to upsize the wire. This 240 V voltage drop calculation is handled automatically by our AC single-phase voltage drop calculator.

NEC 210.19(A) and 215.2(A) recommend ≤ 3% voltage drop for branch circuits and ≤ 3% for feeders, with a combined total of ≤ 5% from the service panel to the final load. Use the 3% threshold when designing branch circuits supplying sensitive equipment (motors, variable-frequency drives, LED drivers, medical devices). Use the 5% combined threshold when sizing feeders — the 5% represents the maximum allowable cumulative loss across the entire distribution path. For critical or industrial loads, many engineers use 2% as the design target. Our voltage drop calculator lets you set any target threshold between 0.1% and 20%.

A 4% voltage drop on a branch circuit exceeds the NEC recommended limit of 3% but falls within the combined 5% feeder-plus-branch budget. Whether it is acceptable depends on the load type. For general lighting and receptacle circuits the NFPA 70 handbook notes these limits are recommendations, not hard code violations. However, for motors a sustained 4% drop can increase operating temperature and reduce torque, shortening motor life. For sensitive electronics a 4% drop may cause instability. In practice most licensed electricians and the Southwire voltage drop calculator methodology both use 3% as the design standard for branch circuits.

The NEC does not set a mandatory maximum voltage drop; instead NEC 210.19(A) Informational Note recommends ≤ 3% for 3-phase branch circuits and ≤ 5% total (feeder + branch). The 3-phase voltage drop formula is: V_drop = (√3 × I × Z_e × L) / (1000 × sets), where Z_e is the effective impedance from NEC Table 9. For a 480 V three-phase motor circuit, 3% = 14.4 V maximum allowable drop. Many industrial standards (ANSI C84.1) specify that utilization voltage must remain within ±5% of nominal under full load. Use our 3 phase voltage drop calculator to verify compliance for any conductor size and distance.

The three-phase voltage drop formula is: V_drop = (√3 × I × Z_e × L) / (1000 × sets). Where: √3 ≈ 1.732 accounts for line-to-neutral geometry in a balanced three-phase system; I = line current in amperes; Z_e = effective AC impedance in Ω/1000 ft (from NEC Chapter 9 Table 9, calculated as R·cos φ + X_L·sin φ); L = one-way conductor length in feet; sets = number of parallel conductor sets. Percentage drop = (V_drop / V_line-to-line) × 100. Our 3 phase voltage drop calculator applies this exact formula using NEC Table 9 impedance data with your selected conduit type and power factor.

There is no NEC-mandated minimum voltage drop — theoretically zero voltage drop is ideal. In practice, conductor resistance always causes some loss. Practical minimums depend on economics and conductivity: a very large conductor (e.g., 500 kcmil copper) for a 20 A, 25 ft run would yield a voltage drop well below 0.1%, but is grossly oversized. Engineers typically size wire to achieve 1–3% drop at full load. The voltage drop calculator formula gives V_drop approaching zero as conductor size increases toward infinity or run length approaches zero.

For a 24 VDC circuit the voltage drop is especially critical because the absolute voltage tolerance is tight. At 3% of 24 V, the maximum allowable drop is only 0.72 V. The DC voltage drop formula is: V_drop = (2 × I × R × L) / 1000. Example — 10 A load, 18 AWG copper (7.77 Ω/kft), 30 ft run: V_drop = (2 × 10 × 7.77 × 30) / 1000 = 4.66 V — a crippling 19.4% drop. You would need to use 12 AWG (1.98 Ω/kft) to achieve V_drop = (2 × 10 × 1.98 × 30) / 1000 = 1.19 V (4.95%). For 24VDC control circuits many engineers target ≤ 2% drop. The same logic applies to 12V systems used in camper/RV wiring. Use our DC voltage drop calculator and set the voltage to 24 to size wires for any 24VDC run.

An online voltage drop calculator works by applying NEC Chapter 9 Table 8 (DC resistance) and Table 9 (AC impedance) values to the standard voltage drop formulas. You input: source voltage, load current (amps), one-way distance, wire gauge (AWG or kcmil or mm²), conductor material (copper or aluminum), circuit type (DC, single-phase AC, or three-phase AC), conduit type, and power factor. The calculator looks up the conductor's resistance per 1000 ft, computes effective impedance Z_e for AC circuits, applies the appropriate formula, and returns the voltage drop in volts and as a percentage of source voltage. It also compares the result against your target threshold (usually 3%) to give a PASS / FAIL status, and recommends the minimum wire size needed to meet that threshold.

Step-by-step voltage drop calculation: (1) Identify circuit type — DC, single-phase AC, or 3-phase AC. (2) Look up conductor resistance R (Ω/1000 ft) from NEC Table 8 for DC, or resistance R and reactance X_L from Table 9 for AC. (3) For AC circuits, calculate effective impedance: Z_e = R·cos(φ) + X_L·sin(φ), where φ = arccos(power factor). (4) Apply the formula: V_drop = (K × I × Z_e × L) / (1000 × sets), where K = 2 for single-phase/DC or √3 for three-phase; I = amps; L = one-way distance (ft); sets = parallel conductors. (5) Calculate percentage: % Drop = (V_drop / V_source) × 100. (6) Compare against NEC recommendations: ≤ 3% for branch circuits, ≤ 5% combined. The voltage drop calculator formula reference table on this page lists all NEC resistance values for copper and aluminum conductors in both Ω/kft and Ω/km (for metric mm² cable sizing).

To drop voltage with a resistor, use Ohm's Law: V = I × R, where V is the voltage drop across the resistor, I is the current through the circuit (in amperes), and R is the resistance in ohms. Rearranging: R = V_drop / I. Example — you need to drop 5 V at 0.1 A: R = 5 / 0.1 = 50 Ω; power dissipated P = V × I = 5 × 0.1 = 0.5 W (select a 1 W resistor for safety margin). For a resistor voltage divider with two resistors R1 and R2: V_out = V_in × R2 / (R1 + R2). Unlike a wire voltage drop calculator which models conductor loss you want to minimize, a resistor drop is an intentional circuit design. The Southwire voltage drop calculator and similar tools focus on conductor losses; for resistor divider design use the Ohm's Law approach above combined with the power rating formula P = I²R to avoid overheating the component.