Voltage Drop Calculator
Analyze circuit voltage drop, percentage line loss, power dissipation, and load terminal voltage for AC and DC systems incorporating cable resistance and inductive reactance.
Circuit & Conductor Inputs
Voltage Drop Estimation
Calculated drop in volts, percentage, and load voltage
7.72 V 6.43%
Voltage at Load 112.28 V
Resistance 0.193 Ω
Circuit Power Loss 154.4 W
NEC 3% Target 3.60 V (Fail)
AC Effective Impedance Formula
Unlike simple DC formulas, precise AC voltage drop calculations must incorporate conductor resistance (R) and inductive reactance (X) influenced by conduit material (magnetic steel vs non-magnetic PVC).
Formula:
V_drop = (Phase_Factor × I × L × [R·cosθ + X·sinθ]) / 1000 | % Drop = (V_drop / V_supply) × 100% Variables & Parameters
V_drop (3Ø) √3 × I × L × (R cosθ + X sinθ) / 1000
V_drop (1Ø) 2 × I × L × (R cosθ + X sinθ) / 1000
R AC resistance per 1000 ft at 75°C (NEC Ch. 9 Table 9)
X AC inductive reactance per 1000 ft (NEC Ch. 9 Table 9)
cosθ (PF) Load power factor (Active power kW / Apparent power kVA)
Frequently Asked Questions
How do you calculate A voltage drop?
Voltage drop is calculated using Ohm’s Law and AC impedance. For Single-Phase AC or DC: Vdrop = (2 × I × L × R) / 1,000. For Three-Phase AC: Vdrop = (1.732 × I × L × Z) / 1,000, where I is current in Amperes, L is one-way length in feet, R is conductor resistance, and Z = R·cos(θ) + X·sin(θ) is effective AC impedance from NEC Chapter 9 Table 9 accounting for load power factor and conduit reactance.
How to check voltage drop?
To check voltage drop in an active installation: (1) Measure the line voltage at the power source/panel using a calibrated true-RMS multimeter under normal load. (2) Measure the voltage at the furthest operating equipment or load receptacle. (3) Subtract load voltage from source voltage to find voltage drop in Volts (Vdrop = Vsource - Vload). (4) Calculate percentage drop: % Drop = (Vdrop / Vsource) × 100%. If % Drop exceeds 3% on branch circuits or 5% for the total feeder and branch system, upsize the conductor gauge.
How do you calculate AC vs DC voltage drop and wire size?
For a dc voltage drop calculator or resistor voltage drop calculator, calculate V_drop = (2 × I × L × R) / 1,000 based on pure DC loop resistance. For a wire voltage drop calculator or voltage drop calculator wire size in AC single-phase and 3-phase circuits, calculate effective AC impedance using NEC Chapter 9 Table 9 resistance and reactance values: V_drop = √3 × I × L × (R cosθ + X sinθ) / 1,000.
How does this compare to the Southwire voltage drop calculator?
Our voltage drop calculator southwire alternative computes exact voltage loss, terminal voltage, and percentage drop across AC (120V, 208V, 240V, 277V, 480V) and DC (12V, 24V, 48V, 120V) circuits, with automatic steel vs PVC conduit reactance adjustment matching and surpassing the Southwire voltage drop calculator.
What is the NEC recommendation for maximum voltage drop?
NEC 210.19(A) Informational Note 4 recommends that voltage drop should not exceed 3% on branch circuits, and the combined drop on both feeders and branch circuits should not exceed 5% total for maximum operating efficiency.