Introduction
Voltage drop is the loss of electrical voltage that happens as current flows through a wire or cable. Every conductor has some resistance, and that resistance causes a small amount of voltage to be used up before the electricity reaches its destination. Too much voltage drop can cause lights to dim, motors to run poorly, and equipment to fail. The National Electrical Code (NEC) recommends keeping voltage drop below 3% for branch circuits and 5% for the total system.
This Voltage Drop Calculator helps you quickly figure out how much voltage is lost in your electrical wiring. Just enter your wire size, wire length, current, and voltage, and the tool does the math for you. It works for both single-phase and three-phase circuits. Whether you are an electrician planning a new installation or a student learning about circuit design, this calculator makes it easy to choose the right wire size and keep your system running safely and efficiently. For foundational resistance and current calculations, you may also find our Ohm's Law Calculator helpful.
How to Use Our Voltage Drop Calculator
Enter your circuit details below and the calculator will tell you the voltage drop across your wire, the percentage of voltage lost, the voltage at the load, and the power loss. It also shows whether your circuit passes NEC recommendations.
Application Mode — Choose between Residential, Commercial 60°C, Commercial 75°C, or Commercial 90°C. Residential mode locks the calculator to single-phase with a power factor of 1.0 and no parallel conductors. Commercial modes unlock three-phase, adjustable power factor, conduit types, and parallel conductor options.
Calculation Tab — Pick one of four modes. "NEC Data" uses resistance and reactance values from NEC Chapter 9 Table 9 for the most accurate results. "Estimated" uses simple DC resistance values for a quick estimate. "Custom" lets you type in your own wire impedance. "Reverse" works backward to find the minimum wire size, maximum distance, or to verify the voltage drop for a given setup.
System Voltage (V) — Select the voltage of your electrical system. Options include 120V, 208V, 240V, 277V, 480V, and 600V. If you need to analyze how voltage is distributed across components in a series circuit, try our Voltage Divider Calculator.
Phase — Choose Single Phase (1φ), Three Phase (3φ), or DC. Single-phase and DC use a multiplier of 2 for the round-trip wire length. Three-phase uses the square root of 3 (√3) for line-to-line calculations. Three-phase and DC are only available in commercial modes.
Load Current (A) — Enter the current in amperes that the circuit will carry. This is the amount of electrical current your load draws.
One-Way Distance — Enter the length of wire from the source to the load in one direction. Choose feet or meters from the dropdown. The calculator accounts for the full round-trip length automatically.
Conductor Size (AWG/kcmil) — Select the wire gauge from 14 AWG up to 750 kcmil. Larger wire sizes have lower resistance and produce less voltage drop.
Conductor Material — Choose Copper or Aluminum. Copper has lower resistance than aluminum, so it produces less voltage drop for the same wire size.
Conduit Type — Available in the NEC Data tab. Select PVC, Aluminum, or Steel (EMT/IMC/RMC) conduit. Steel conduit increases impedance due to magnetic effects. Steel conduit is only available in commercial modes.
Power Factor — Available in the NEC Data tab under commercial modes. Enter a value between 0.01 and 1.00. A value of 1.0 means a purely resistive load. Motors and inductive loads typically range from 0.80 to 0.90. This affects the effective impedance used in the calculation.
Parallel Conductors per Phase — Available in commercial modes. Enter the number of conductors run in parallel per phase, from 1 to 12. Running conductors in parallel divides the resistance, which lowers the voltage drop. For help calculating the combined resistance of parallel conductors, see our Parallel Resistor Calculator.
Wire Impedance / Resistance (Custom Tab) — In the Custom tab, enter your own wire impedance or resistance value. Choose the unit from Ω/1000ft, Ω/ft, Ω/km, or Ω/m.
Max Allowable Drop (%) — Set the maximum voltage drop percentage you will allow. The NEC recommends 3% for branch circuits and 5% for combined feeder and branch circuits. The calculator uses this value to show a pass, warning, or fail status. You can use our Percentage Calculator to double-check percentage conversions if needed.
Reverse Calculator — Solve For — In the Reverse tab, choose what you want to find. "Min. Conductor Size" finds the smallest wire that keeps the voltage drop within your limit. "Max. Distance" tells you how far you can run a given wire size before exceeding the limit. "Verify Drop" checks the voltage drop for a specific wire size and distance.
What Is Voltage Drop?
Voltage drop is the loss of electrical voltage that happens as current flows through a wire from the power source to the load (like a light, outlet, or motor). Every wire has some resistance, and that resistance causes a small amount of voltage to be used up along the way. The longer the wire or the smaller the wire size, the more voltage is lost before it reaches the device that needs it.
Why Does Voltage Drop Matter?
If too much voltage is lost in the wires, the equipment at the end of the circuit won't get enough power to work correctly. Lights may dim, motors may overheat or run slowly, and sensitive electronics can malfunction. Excess voltage drop also wastes energy as heat in the wires, which raises your electricity costs and can create safety hazards. To understand how much that wasted energy might cost you, check out our Electricity Cost Calculator.
The National Electrical Code (NEC) recommends keeping voltage drop to no more than 3% for branch circuits and no more than 5% for the combined feeder and branch circuit. These are not hard requirements in the code, but they are widely accepted guidelines that electricians and engineers follow to ensure safe, efficient wiring.
How Is Voltage Drop Calculated?
The basic formula for voltage drop in a single-phase circuit is:
Voltage Drop = 2 × Current × Resistance × Distance ÷ 1000
For three-phase circuits, the multiplier changes from 2 to √3 (about 1.732). The "2" in the single-phase formula accounts for the round-trip distance the current must travel — out to the load and back to the source. The resistance value depends on the wire size (AWG or kcmil), the conductor material (copper or aluminum), and sometimes the type of conduit used. If you want to explore the fundamental relationship between voltage, current, and resistance, our Ohm's Law Calculator is a great companion tool.
Key Factors That Affect Voltage Drop
- Wire size: Larger wires (lower AWG numbers or higher kcmil values) have less resistance and reduce voltage drop.
- Wire length: Longer wire runs cause more voltage drop. This is a common issue in large homes, farms, outbuildings, and commercial facilities.
- Load current: Higher current draws increase voltage drop proportionally.
- Conductor material: Copper has lower resistance than aluminum, so copper wires produce less voltage drop for the same size.
- Conduit type: In AC circuits, steel conduit increases the effective impedance of the wire due to magnetic effects, while PVC and aluminum conduit do not.
- Power factor: In AC systems, the power factor affects how resistance and reactance combine. A purely resistive load (power factor of 1.0) is typical in residential settings, while motors and industrial equipment often have power factors between 0.80 and 0.90.
NEC Table 9 and Effective Impedance
For accurate AC voltage drop calculations, professionals use data from NEC Chapter 9, Table 9. This table lists the resistance (R) and reactance (X) of wires per 1,000 feet for different conductor sizes, materials, and conduit types. The effective impedance is calculated as:
Zeffective = R × cos(θ) + X × sin(θ)
Here, θ is the power factor angle. This formula gives a more precise voltage drop value than using resistance alone, especially for larger wire sizes where reactance becomes significant. Understanding power calculations is also important when sizing circuits — our Power Calculator can help with that.
Choosing the Right Wire Size
When planning a circuit, you need to pick a wire size that keeps the voltage drop within acceptable limits while also meeting the ampacity (current-carrying capacity) requirements of the NEC. A wire might be large enough to safely carry the current but still too small to prevent excessive voltage drop over a long distance. Always check both ampacity and voltage drop before finalizing your wire selection. If you're sizing a backup power system alongside your wiring, our Generator Sizing Calculator can help determine the right generator capacity for your loads.
Residential vs. Commercial Applications
Residential circuits are almost always single-phase with a power factor of 1.0 and run in PVC conduit or cable assemblies like Romex (NM cable). Commercial and industrial systems often involve three-phase power, lower power factors, steel conduit, and parallel conductors per phase. These differences change the voltage drop calculation, which is why this tool offers separate modes for each application type. For commercial and industrial settings, you may also want to evaluate overall equipment performance with our OEE Calculator.
Important: This calculator is an educational and estimation tool. Always consult a licensed electrician or engineer for actual electrical design work. Local codes and specific installation conditions may require adjustments beyond what any calculator can provide.