About Voltage Drop Calculator

What This Calculator Does

Voltage Drop Calculator is a free online tool designed to calculate the voltage loss that occurs when electrical current flows through wire conductors. When you run electricity through any wire, some voltage is inevitably lost due to the wire's electrical resistance. This calculator helps you determine exactly how much voltage will be lost for your specific circuit configuration, and whether your chosen wire size is adequate.

The calculator supports three distinct circuit types, each with its own calculation methodology:

DC (Direct Current) Calculations

For battery systems, solar panel installations, automotive wiring, marine electrical systems, RV power distribution, LED lighting circuits, and any application using direct current. DC calculations consider pure resistive losses without the complexities of alternating current.

Single-Phase AC Calculations

For standard residential circuits (120V/240V), light commercial applications, and typical household branch circuits. Single-phase AC calculations incorporate power factor to account for the phase relationship between voltage and current in AC systems.

Three-Phase AC Calculations

For industrial motors, commercial building power distribution, heavy machinery, and large HVAC equipment. Three-phase calculations use the square root of 3 (approximately 1.732) multiplier that reflects how current returns through the other phases rather than a dedicated neutral conductor.

Beyond basic voltage drop, the calculator also computes end voltage at the load, total power loss in watts, and overall wire resistance. It provides intelligent recommendations when a larger wire gauge would bring your circuit within acceptable limits.

Calculator Features at a Glance

For a detailed head-to-head comparison, read our Southwire calculator comparison article.

The Science Behind It

Every electrical conductor possesses inherent resistance to current flow. This resistance is a physical property of the conductor material and its dimensions. When current passes through this resistance, electrical energy is converted to heat, resulting in a voltage drop along the conductor length. This fundamental relationship is described by Ohm's Law:

Where V is voltage (in volts), I is current (in amperes), and R is resistance (in ohms). The voltage drop formulas used in this calculator extend Ohm's Law to account for practical wiring scenarios:

For DC and Single-Phase AC circuits:

For Three-Phase AC circuits:

The factor of 2 in DC and single-phase formulas accounts for the complete circuit path—current must travel to the load through the "hot" conductor and return through the neutral or ground conductor. Both conductors contribute to total resistance. In three-phase balanced systems, the multiplier changes to √3 (1.732) because return current flows through the other phases rather than requiring a separate return path.

For AC circuits, the power factor (PF) accounts for the phase angle between voltage and current waveforms. Purely resistive loads like heaters have a power factor of 1.0, while inductive loads like motors typically operate at power factors between 0.80 and 0.90. The calculator defaults to 0.85, which represents a typical mixed industrial load.

Wire resistance values in this calculator come from the National Electrical Code (NEC) Chapter 9, Table 8, which provides DC resistance values for uncoated copper and aluminum conductors at 75°C (167°F). These values represent standard operating conditions and are widely accepted throughout the electrical industry.

As the table shows, aluminum consistently has about 1.64x the resistance of copper at any gauge size. This is why aluminum wire must be two sizes larger than copper for equivalent voltage drop performance.

Who This Calculator Is For

This tool serves anyone who needs to verify that their electrical wiring will deliver adequate voltage to connected equipment:

• Licensed Electricians – For quick field calculations when planning circuit runs or verifying existing installations meet code recommendations
• Electrical Engineers – For preliminary design calculations during system planning and specification development
• Solar Installers – For sizing conductors between solar panels, charge controllers, batteries, and inverters where DC voltage drop is critical
• DIY Enthusiasts – For home improvement projects, workshop wiring, and understanding electrical fundamentals
• Marine and RV Owners – For 12V and 24V DC systems where low voltage makes proper wire sizing essential
• Automotive Technicians – For custom wiring projects, audio system installations, and aftermarket electrical accessories
• Students and Educators – For learning electrical principles and verifying textbook calculations
• Facility Managers – For understanding existing electrical infrastructure and planning upgrades

Our Methodology

The calculations in this tool follow established electrical engineering principles that have been standardized across the industry for decades. Here's exactly how the calculator processes your inputs:

1. Wire Resistance Lookup – Based on your selected wire gauge (14 AWG through 4/0 AWG) and material (copper or aluminum), the calculator retrieves the corresponding resistance value in ohms per 1,000 feet from NEC Chapter 9, Table 8.
2. Unit Conversion – If you're using metric units, the wire length in meters is converted to feet (multiplied by 3.281) since the resistance tables are in imperial units.
3. Multiplier Selection – The appropriate circuit multiplier is applied: 2 for DC and single-phase circuits, or √3 (1.732) for three-phase circuits.
4. Voltage Drop Calculation – The formula calculates the voltage drop in volts. For AC circuits, the power factor is included in the calculation.
5. Percentage Calculation – The voltage drop is divided by the source voltage and multiplied by 100 to express the drop as a percentage.
6. Secondary Calculations – End voltage (source minus drop), total wire resistance, and power loss (I²R) are computed.
7. NEC Compliance Check – Results are compared against NEC recommendations: 3% for branch circuits (displayed as "success"), 3-5% (displayed as "warning"), and over 5% (displayed as "error").
8. Wire Size Recommendation – If voltage drop exceeds 3%, the calculator iterates through larger wire gauges to find the smallest size that would achieve compliance.

The resistance values used represent conductors at 75°C operating temperature, which is the standard rating for most modern wire insulation types (THWN, THHN, XHHW). This aligns with how professional electricians and engineers perform these calculations in practice.

How to Use the Calculator

Follow these steps to calculate voltage drop for your circuit:

Step 1: Select Your Circuit Type

Use the dropdown menu at the top to choose between DC (Direct Current), Single-Phase AC, or Three-Phase AC. This selection determines which formula the calculator uses and which input fields are displayed. AC circuit types will reveal additional fields for power factor and conduit type.

Step 2: Choose Your Unit System

Toggle between Imperial (feet) and Metric (meters) units using the switch at the top right. The calculator automatically converts values when you switch systems.

Step 3: Set Electrical Parameters

In the left column, configure your circuit's electrical characteristics:

• Source Voltage – The voltage at your power source (5V to 600V range). Common values: 12V for automotive/marine, 120V/240V for residential, 208V/480V for commercial/industrial.
• Load Current – The current draw of your equipment in amperes (0.5A to 400A range). This should be the actual or expected operating current, not the circuit breaker rating.
• Wire Length – The one-way distance from the power source to the load. The calculator automatically accounts for the return path in its formulas.
• Power Factor (AC only) – For motors and inductive loads, use 0.80-0.90. For resistive loads like heaters, use 1.0. The default of 0.85 works for most mixed loads.

Step 4: Configure Wire Selection

In the right column, specify your conductor:

• Wire Gauge – Select from 14 AWG (smallest) to 4/0 AWG (largest). The dropdown shows each gauge's cross-sectional area in mm² and maximum ampacity for reference.
• Wire Material – Choose copper (lower resistance, higher cost) or aluminum (higher resistance, lower cost, lighter weight).
• Conduit Type (AC only) – Select PVC, steel, or aluminum conduit. This affects AC impedance calculations.

Step 5: Calculate and Review Results

Click "Calculate Voltage Drop" to see your results. The display shows voltage drop in volts and percentage, color-coded to indicate compliance with NEC recommendations. Below that, you'll find end voltage at the load, power loss in watts, and total wire resistance. If your voltage drop exceeds 3%, the calculator recommends an appropriate wire size upgrade.

Why We Built This

We created this calculator to provide a genuinely useful tool without the frustrations common to many online calculators:

• Completely Free – No premium tiers, no feature restrictions, no payment required. Every calculation and feature is available to everyone.
• No Account Required – Use the calculator immediately without signing up, verifying email, or creating passwords. Your time is valuable.
• Privacy Focused – All calculations happen entirely in your web browser using JavaScript. Your circuit data is never transmitted to any server. We don't collect, store, or analyze your calculations.
• No Data Storage – When you close the browser tab, your session ends completely. We have no database of user calculations because we never receive them in the first place.
• Mobile Friendly – The calculator works on phones and tablets, so you can use it on the job site or in the field.
• Shareable Results – The share feature creates a URL with your parameters encoded, allowing you to bookmark specific calculations or send them to colleagues—all without creating an account.

Accurate voltage drop calculations are essential for safe, efficient electrical installations. Making this tool freely available helps ensure that more circuits are designed properly, reducing energy waste and equipment problems.

Limitations and Disclaimer

While this calculator uses industry-standard formulas and NEC-referenced resistance values, please understand its limitations:

• Estimates, Not Measurements – Calculated values are theoretical estimates based on standard conditions. Actual voltage drop in installed wiring may differ due to temperature variations, conductor quality, connection resistance, and installation factors.
• Standard Conditions Assumed – Wire resistance values are for 75°C conductor temperature. Higher temperatures increase resistance; lower temperatures decrease it. Ambient temperature, conduit fill, and load duration all affect actual conductor temperature.
• Connections Not Included – The calculator accounts only for conductor resistance, not voltage drop across terminals, splices, breakers, switches, or other connection points. In some installations, connection resistance can be significant.
• Simplified AC Model – AC calculations use DC resistance values adjusted by power factor. This approach, while common in the industry, doesn't fully account for skin effect, proximity effect, or reactive impedance in steel conduit at higher frequencies.
• Not a Substitute for Professional Judgment – This tool aids in planning and verification but doesn't replace the expertise of a licensed electrician or electrical engineer who can assess your complete installation.

Important: Electrical work can be dangerous and is regulated by law in most jurisdictions. Always comply with the National Electrical Code, local amendments, and permitting requirements. Have electrical installations inspected by qualified authorities. Consult a licensed electrician for any work you're not qualified to perform yourself.

The operators of this website provide this calculator as-is for informational and educational purposes. We assume no liability for the use or misuse of calculation results. Users are responsible for verifying results and ensuring their electrical installations meet all applicable codes and safety standards.

Learn More About Voltage Drop

Explore our comprehensive library of voltage drop guides, tutorials, and reference materials:

Contact Us

Have questions about the calculator, found an error, or want to suggest improvements? We welcome your feedback.

Email: info@voltagedropcalculator.org

You can also visit our contact page for more information.