Voltage Drop Calculator

Calculate voltage drop, power loss, and wire sizing for electrical circuits.

Determine the voltage drop across electrical conductors, calculate power loss, and ensure your electrical installations meet safety standards with accurate calculations.

Examples

Click on any example to load it into the calculator.

Residential Outlet Circuit

Residential Outlet Circuit

A typical 15A residential circuit with 120V supply and 50m wire run.

Current: 15 A

Voltage: 120 V

Length: 50 m

Resistance: 1.83 Ω/km

Power Factor: 1.0

Industrial Motor Circuit

Industrial Motor Circuit

A 30A industrial motor circuit with 480V supply and 100m wire run.

Current: 30 A

Voltage: 480 V

Length: 100 m

Resistance: 0.727 Ω/km

Power Factor: 0.85

Solar Panel DC Circuit

Solar Panel DC Circuit

A 10A solar panel circuit with 24V DC supply and 25m wire run.

Current: 10 A

Voltage: 24 V

Length: 25 m

Resistance: 3.67 Ω/km

Power Factor: 1.0

LED Lighting Circuit

LED Lighting Circuit

A 5A LED lighting circuit with 12V supply and 20m wire run.

Current: 5 A

Voltage: 12 V

Length: 20 m

Resistance: 7.34 Ω/km

Power Factor: 0.95

Other Titles
Understanding Voltage Drop Calculator: A Comprehensive Guide
Master the principles of electrical voltage drop calculations and learn how to design safe, efficient electrical circuits that meet code requirements and ensure reliable operation of your electrical equipment.

What is Voltage Drop?

  • Basic Concepts
  • Why It Matters
  • Electrical Safety Standards
Voltage drop is the reduction in voltage that occurs when electrical current flows through a conductor (wire). This phenomenon is caused by the inherent resistance of the wire material, which converts some of the electrical energy into heat. The voltage drop is directly proportional to the current flowing through the wire and the resistance of the wire itself. Understanding voltage drop is crucial for designing electrical systems that operate efficiently and safely.
The Physics Behind Voltage Drop
According to Ohm's Law, voltage drop (Vdrop) equals current (I) multiplied by resistance (R): Vdrop = I × R. In electrical circuits, the resistance of a wire depends on its material, cross-sectional area, length, and temperature. Copper and aluminum are the most common conductor materials, with copper having lower resistance than aluminum for the same size wire. The longer the wire run and the smaller the wire size, the higher the resistance and consequently the greater the voltage drop.
Why Voltage Drop Calculations Are Critical
Excessive voltage drop can cause several problems in electrical systems. Equipment may not operate properly, motors may overheat, lights may dim, and sensitive electronic devices may malfunction. Most electrical codes specify maximum allowable voltage drop percentages (typically 3-5% for branch circuits and 1-2% for feeders) to ensure reliable operation. Calculating voltage drop helps electricians and engineers select appropriate wire sizes and design circuits that meet these requirements.
AC vs DC Voltage Drop
For DC circuits, voltage drop calculations are straightforward using Ohm's Law. However, for AC circuits, additional factors come into play. The power factor (cos φ) affects the voltage drop calculation, and for long runs, the inductive reactance of the wire may also be significant. The calculator accounts for power factor in AC calculations, providing more accurate results for real-world applications.

Common Voltage Drop Limits:

  • Branch Circuits (Lighting, Outlets): 3% maximum voltage drop
  • Feeder Circuits (Main Distribution): 1-2% maximum voltage drop
  • Motor Circuits: 3-5% maximum voltage drop depending on motor type
  • Control Circuits: 2-3% maximum voltage drop for reliable operation

Step-by-Step Guide to Using the Calculator

  • Gathering Circuit Information
  • Inputting Data
  • Interpreting Results
Using the voltage drop calculator requires accurate information about your electrical circuit. Follow these steps to obtain reliable results that will help you make informed decisions about wire sizing and circuit design.
1. Determine Circuit Parameters
Start by identifying the current (amperage) that your load will draw. This can be found on equipment nameplates, calculated from power ratings (I = P/V), or measured with an ammeter. Next, note the supply voltage - this is typically 120V, 240V, or 480V for AC systems, or 12V, 24V, or 48V for DC systems. Measure or estimate the total wire length from the power source to the load and back (for single-phase circuits, this is twice the one-way distance).
2. Select Wire Characteristics
Choose the appropriate wire resistance based on the wire material and size you plan to use. Copper wire has lower resistance than aluminum, and larger wire sizes have lower resistance per unit length. Common wire resistance values can be found in electrical engineering handbooks or wire manufacturer specifications. For AC circuits, determine the power factor of your load - resistive loads like heaters have a power factor of 1.0, while inductive loads like motors typically have power factors between 0.8 and 0.95.
3. Enter Data and Calculate
Input all the gathered information into the calculator. Double-check your values, especially the current and wire length, as these have the greatest impact on voltage drop. Click 'Calculate' to see the results. The calculator will show you the voltage drop in volts, the percentage drop relative to the source voltage, the power loss in watts, and the actual voltage available at the load.
4. Evaluate and Optimize
Compare the calculated voltage drop percentage against the applicable code requirements. If the voltage drop exceeds the limit, consider using a larger wire size, reducing the wire length, or using a higher voltage system. The power loss calculation helps you understand the energy efficiency of your circuit and the potential for heat generation in the wires.

Wire Resistance Values (Ω/km at 20°C):

  • Copper 2.5mm² (14 AWG): 7.34 Ω/km
  • Copper 4mm² (12 AWG): 4.61 Ω/km
  • Copper 6mm² (10 AWG): 3.08 Ω/km
  • Copper 10mm² (8 AWG): 1.83 Ω/km
  • Aluminum 16mm² (6 AWG): 1.91 Ω/km

Real-World Applications and Design Considerations

  • Residential Wiring
  • Industrial Applications
  • Renewable Energy Systems
Voltage drop calculations are essential in various electrical applications, from simple residential wiring to complex industrial systems. Understanding how to apply these calculations in real-world scenarios helps ensure safe, efficient, and code-compliant electrical installations.
Residential Electrical Design
In residential wiring, voltage drop calculations help ensure that all outlets and lighting circuits provide adequate voltage to connected devices. Long wire runs to detached garages, workshops, or outdoor lighting often require larger wire sizes to maintain acceptable voltage levels. The calculator helps homeowners and electricians determine if existing circuits can handle additional loads or if upgrades are necessary.
Industrial and Commercial Applications
Industrial facilities often have extensive electrical distribution systems with long wire runs and high current loads. Voltage drop calculations are critical for motor circuits, where excessive voltage drop can cause motors to overheat and fail prematurely. The calculator helps engineers design efficient distribution systems that minimize energy losses and ensure reliable operation of industrial equipment.
Renewable Energy Systems
Solar photovoltaic systems and wind turbines often operate at lower voltages (12V, 24V, or 48V DC) and require careful voltage drop calculations. Since these systems typically have long wire runs from the energy source to the load or battery bank, even small voltage drops can significantly impact system efficiency. The calculator helps designers optimize wire sizing to maximize energy harvest and system performance.

Common Misconceptions and Design Errors

  • Wire Size Myths
  • Distance Considerations
  • Code Compliance
Many electrical installations suffer from voltage drop problems due to common misconceptions and design errors. Understanding these pitfalls helps avoid costly mistakes and ensures reliable electrical system operation.
Myth: Any Wire Size Will Work
One of the most common mistakes is selecting wire size based solely on current-carrying capacity without considering voltage drop. While a wire may safely carry the required current, it may create excessive voltage drop over long distances. The calculator helps identify when larger wire sizes are needed to maintain acceptable voltage levels, even when the current rating is adequate.
Ignoring Power Factor in AC Circuits
In AC circuits, the power factor significantly affects voltage drop calculations. Inductive loads like motors and transformers have power factors less than 1.0, which increases the effective voltage drop. Many designers overlook this factor, leading to under-sized wires and poor system performance. The calculator accounts for power factor to provide accurate results for AC applications.
Underestimating Wire Length
Accurate wire length measurement is crucial for voltage drop calculations. Many installations fail to account for the actual routing of wires through conduits, around obstacles, and through junction boxes. The total wire length should include all bends and routing, not just the straight-line distance. This is especially important in complex installations where wire runs may be significantly longer than expected.

Design Tips:

  • Always measure actual wire length, not straight-line distance
  • Consider future load increases when sizing wires
  • Use the calculator to compare different wire sizes and materials
  • Account for temperature effects on wire resistance in hot environments

Mathematical Derivation and Advanced Calculations

  • Ohm's Law Application
  • AC Circuit Analysis
  • Temperature Effects
The voltage drop calculator uses fundamental electrical engineering principles to provide accurate results. Understanding the mathematical basis helps users interpret results and make informed design decisions.
Basic Voltage Drop Formula
The fundamental voltage drop formula is derived from Ohm's Law: Vdrop = I × R. For a wire with resistance per unit length (Rperkm) and total length (L), the total resistance is R = Rperkm × L / 1000. Therefore, Vdrop = I × Rperkm × L / 1000. This formula applies to DC circuits and resistive AC loads with unity power factor.
AC Circuit Considerations
For AC circuits with non-unity power factor, the voltage drop calculation becomes more complex. The effective voltage drop is V_drop = I × R × cos(φ), where cos(φ) is the power factor. This accounts for the phase relationship between voltage and current in AC circuits. For long wire runs, inductive reactance may also contribute to voltage drop, but this effect is typically small for most practical applications.
Temperature Effects on Resistance
Wire resistance increases with temperature according to the formula R(T) = R₀ × [1 + α(T - T₀)], where α is the temperature coefficient of resistance, T is the operating temperature, and T₀ is the reference temperature (usually 20°C). For copper, α ≈ 0.00393/°C, and for aluminum, α ≈ 0.00403/°C. The calculator uses standard resistance values at 20°C, but users should consider temperature effects in high-temperature environments.
Three-Phase Systems
In three-phase systems, voltage drop calculations are similar to single-phase systems, but the line-to-neutral voltage drop is typically calculated. The total power loss in a three-phase system is three times the single-phase power loss for balanced loads. The calculator can be used for three-phase systems by using the line-to-neutral voltage and the actual wire length.

Advanced Calculation Example:

  • For a 20A load at 240V, 100m wire run, 2.5mm² copper wire:
  • Resistance: R = 7.34 Ω/km × 100m / 1000 = 0.734 Ω
  • Voltage Drop: V_drop = 20A × 0.734 Ω = 14.68V
  • Percentage Drop: %Drop = (14.68V / 240V) × 100 = 6.12%
  • Power Loss: P_loss = (20A)² × 0.734 Ω = 293.6W