Cable Voltage Drop Calculator | AWG & mm² Wire Sizing

Cable Voltage Drop Calculator

Introduction

Voltage drop in electrical cables is a critical consideration in electrical system design. When current flows through a conductor, resistance causes a voltage drop along the cable length, reducing the voltage available at the load. This cable voltage drop calculator helps you determine the voltage drop, power loss, and delivered voltage for two-conductor cable systems using either AWG (American Wire Gauge) or metric mm² wire sizes. Proper cable voltage drop calculations ensure safe, efficient electrical installations that meet NEC standards.

How to Calculate Cable Voltage Drop

1. Enter the cable length and select the unit (feet or meters)
2. Select the gauge type (AWG or mm²)
3. Choose the wire gauge from the available options
4. Enter the load current in amperes
5. Enter the supply voltage in volts
6. Results are calculated automatically and displayed instantly

The calculator provides voltage drop, percentage drop, power loss, and delivered voltage, along with a warning if the voltage drop exceeds 3%.

Input Validation

The calculator performs the following checks:

• Cable length must be zero or positive
• Load current must be zero or positive
• Supply voltage must be greater than zero
• Wire gauge must be a valid standard size

If invalid inputs are detected, the calculator returns null results.

Voltage Drop Formula and Calculations

The voltage drop for a two-conductor cable is calculated using:

Voltage Drop Formula: $V_{drop} = 2 \times I \times R \times L$

Where:

• $V_{drop}$ = Voltage drop (volts)
• $I$ = Load current (amperes)
• $R$ = Wire resistance per unit length (ohms per 1000 feet or 1000 meters)
• $L$ = Cable length (feet or meters)
• Factor of 2 accounts for both conductors (positive and negative)

Power Loss Formula: $P_{loss} = I^2 \times R_{total}$

Where:

• $P_{loss}$ = Power loss (watts)
• $I$ = Load current (amperes)
• $R_{total}$ = Total resistance of both conductors

Delivered Voltage: $V_{delivered} = V_{supply} - V_{drop}$

Percentage Drop: $\%_{drop} = \frac{V_{drop}}{V_{supply}} \times 100$

Calculation

The calculator uses standard copper wire resistance values at 75°C:

1. For AWG wire: Resistance values are in ohms per 1000 feet
2. For mm² wire: Resistance values are in ohms per 1000 meters

The calculation steps are:

1. Convert length to appropriate units (feet for AWG, meters for mm²)
2. Calculate resistance per conductor: $R_{conductor} = \frac{R_{per1000}}{1000} \times L$
3. Calculate total resistance: $R_{total} = 2 \times R_{conductor}$
4. Calculate voltage drop: $V_{drop} = I \times R_{total}$
5. Calculate power loss: $P_{loss} = I^2 \times R_{total}$
6. Calculate delivered voltage: $V_{delivered} = V_{supply} - V_{drop}$
7. Calculate percentage drop: $\%_{drop} = \frac{V_{drop}}{V_{supply}} \times 100$

Units and Precision

• AWG wire: Uses feet for length, resistance in ohms per 1000 feet
• mm² wire: Uses meters for length, resistance in ohms per 1000 meters
• Voltage drop: Displayed to 3 decimal places
• Power loss: Displayed to 2 decimal places
• Delivered voltage: Displayed to 2 decimal places
• Percentage drop: Displayed to 2 decimal places

Cable Voltage Drop Applications and Use Cases

The cable voltage drop calculator has various applications in electrical design:

1. Residential Wiring: Ensures proper wire sizing for home electrical circuits to prevent voltage drop issues with appliances and lighting.

2. Commercial Buildings: Helps electricians design distribution systems that maintain voltage levels across long cable runs in large facilities.

3. Solar Panel Systems: Critical for sizing DC cables between solar panels and inverters to maximize energy efficiency.

4. Automotive Applications: Used in vehicle electrical system design to ensure proper voltage delivery to accessories and electronics.

5. Marine Electrical Systems: Helps design boat electrical systems where long cable runs and high currents can cause significant voltage drop.

6. Industrial Equipment: Ensures motors and machinery receive adequate voltage for proper operation and prevents damage from undervoltage conditions.

Alternatives

While this calculator focuses on two-conductor cables, there are related considerations:

1. Three-Phase Systems: Use different formulas accounting for √3 factor and line-to-line voltage.

2. AC vs DC: AC systems may require consideration of reactance in addition to resistance.

3. Conduit Fill: Multiple conductors in conduit may require derating factors.

4. Temperature Derating: Higher ambient temperatures increase resistance and require larger wire sizes.

5. National Electrical Code (NEC): Provides tables and guidelines for maximum voltage drop (typically 3% for branch circuits, 5% total).

History

The American Wire Gauge (AWG) system was established in the United States in 1857, providing a standardized method for specifying wire sizes. The system uses a logarithmic scale where each gauge represents a specific diameter and cross-sectional area.

Voltage drop calculations became increasingly important with the expansion of electrical distribution systems in the late 19th and early 20th centuries. As buildings grew larger and electrical loads increased, engineers needed accurate methods to ensure adequate voltage delivery to loads.

The metric mm² system, used internationally, specifies wire size directly by cross-sectional area in square millimeters, providing a more intuitive measurement system. Both systems are widely used today, with AWG predominant in North America and mm² standard in most other countries.

Modern electrical codes, such as the National Electrical Code (NEC) in the United States and IEC standards internationally, provide specific guidelines for acceptable voltage drop in electrical installations, typically limiting branch circuit voltage drop to 3% and total system voltage drop to 5%.

Examples

Here are code examples to calculate cable voltage drop in different programming languages:

1def calculate_voltage_drop(current, resistance_per_1000, length):
2    """
3    Calculate voltage drop for two-conductor cable.
4
5    Args:
6        current: Load current in amperes
7        resistance_per_1000: Wire resistance in ohms per 1000 units
8        length: Cable length in same units as resistance
9
10    Returns:
11        Dictionary with voltage drop, power loss, etc.
12    """
13    # Calculate resistance for actual length
14    resistance_per_conductor = (resistance_per_1000 / 1000) * length
15    total_resistance = 2 * resistance_per_conductor
16
17    # Calculate voltage drop
18    voltage_drop = current * total_resistance
19
20    # Calculate power loss
21    power_loss = current ** 2 * total_resistance
22
23    return {
24        'voltage_drop': round(voltage_drop, 3),
25        'power_loss': round(power_loss, 2),
26        'total_resistance': total_resistance
27    }
28
29# Example: 100 feet of 12 AWG wire, 15 amps
30awg_12_resistance = 1.93  # ohms per 1000 feet
31result = calculate_voltage_drop(15, awg_12_resistance, 100)
32print(f"Voltage drop: {result['voltage_drop']} V")
33print(f"Power loss: {result['power_loss']} W")
34

1function calculateVoltageDrop(current, resistancePer1000, length) {
2  // Calculate resistance for actual length
3  const resistancePerConductor = (resistancePer1000 / 1000) * length;
4  const totalResistance = 2 * resistancePerConductor;
5
6  // Calculate voltage drop
7  const voltageDrop = current * totalResistance;
8
9  // Calculate power loss
10  const powerLoss = Math.pow(current, 2) * totalResistance;
11
12  return {
13    voltageDrop: parseFloat(voltageDrop.toFixed(3)),
14    powerLoss: parseFloat(powerLoss.toFixed(2)),
15    totalResistance: totalResistance
16  };
17}
18
19// Example: 50 meters of 2.5 mm² wire, 15 amps
20const mm2_2_5_resistance = 7.41; // ohms per 1000 meters
21const result = calculateVoltageDrop(15, mm2_2_5_resistance, 50);
22console.log(`Voltage drop: ${result.voltageDrop} V`);
23console.log(`Power loss: ${result.powerLoss} W`);
24

1public class VoltageDropCalculator {
2    public static class Result {
3        public double voltageDrop;
4        public double powerLoss;
5        public double totalResistance;
6    }
7
8    public static Result calculateVoltageDrop(double current,
9                                             double resistancePer1000,
10                                             double length) {
11        // Calculate resistance for actual length
12        double resistancePerConductor = (resistancePer1000 / 1000.0) * length;
13        double totalResistance = 2 * resistancePerConductor;
14
15        // Calculate voltage drop
16        double voltageDrop = current * totalResistance;
17
18        // Calculate power loss
19        double powerLoss = Math.pow(current, 2) * totalResistance;
20
21        Result result = new Result();
22        result.voltageDrop = Math.round(voltageDrop * 1000.0) / 1000.0;
23        result.powerLoss = Math.round(powerLoss * 100.0) / 100.0;
24        result.totalResistance = totalResistance;
25
26        return result;
27    }
28
29    public static void main(String[] args) {
30        // Example: 100 feet of 12 AWG wire, 20 amps
31        double awg12Resistance = 1.93; // ohms per 1000 feet
32        Result result = calculateVoltageDrop(20, awg12Resistance, 100);
33
34        System.out.printf("Voltage drop: %.3f V%n", result.voltageDrop);
35        System.out.printf("Power loss: %.2f W%n", result.powerLoss);
36    }
37}
38

1' Excel VBA Function for Voltage Drop Calculation
2Function CableVoltageDrop(Current As Double, _
3                         ResistancePer1000 As Double, _
4                         Length As Double) As Double
5    Dim ResistancePerConductor As Double
6    Dim TotalResistance As Double
7
8    ResistancePerConductor = (ResistancePer1000 / 1000) * Length
9    TotalResistance = 2 * ResistancePerConductor
10
11    CableVoltageDrop = Current * TotalResistance
12End Function
13
14' Usage in Excel:
15' =CableVoltageDrop(15, 1.93, 100)
16' Returns voltage drop in volts
17

Numerical Examples

1. Residential 120V Circuit:

   • Cable length: 100 feet
   • Wire gauge: 12 AWG (1.93 Ω/1000 ft)
   • Load current: 15 amperes
   • Supply voltage: 120 volts
   • Results:
     • Voltage drop: 5.79 V (4.83%)
     • Power loss: 86.85 W
     • Delivered voltage: 114.21 V

2. Solar Panel DC Circuit:

   • Cable length: 50 meters
   • Wire gauge: 6 mm² (3.08 Ω/1000 m)
   • Load current: 20 amperes
   • Supply voltage: 48 volts
   • Results:
     • Voltage drop: 6.16 V (12.83%)
     • Power loss: 123.2 W
     • Delivered voltage: 41.84 V

3. Industrial 240V Circuit:

   • Cable length: 200 feet
   • Wire gauge: 6 AWG (0.491 Ω/1000 ft)
   • Load current: 40 amperes
   • Supply voltage: 240 volts
   • Results:
     • Voltage drop: 7.856 V (3.27%)
     • Power loss: 314.24 W
     • Delivered voltage: 232.14 V

4. Low Voltage Drop Example:

   • Cable length: 50 feet
   • Wire gauge: 10 AWG (1.21 Ω/1000 ft)
   • Load current: 10 amperes
   • Supply voltage: 120 volts
   • Results:
     • Voltage drop: 1.21 V (1.01%)
     • Power loss: 12.1 W
     • Delivered voltage: 118.79 V

Frequently Asked Questions (FAQ)

What is voltage drop in electrical cables? Voltage drop is the reduction in voltage that occurs when electrical current flows through a conductor due to the wire's resistance. It results in less voltage being available at the load compared to the supply voltage.

What is an acceptable voltage drop percentage? The National Electrical Code (NEC) recommends limiting voltage drop to 3% for branch circuits and 5% for the total system (feeder plus branch circuit). Voltage drops exceeding these values can cause equipment malfunction and inefficiency.

How do I reduce voltage drop in cables? You can reduce voltage drop by: (1) using a larger wire gauge with lower resistance, (2) shortening the cable length, (3) reducing the load current, or (4) increasing the supply voltage. Using copper instead of aluminum also helps.

What's the difference between AWG and mm² wire sizing? AWG (American Wire Gauge) is predominantly used in North America and uses a logarithmic scale where lower numbers indicate thicker wires. The mm² system, used internationally, directly specifies the wire's cross-sectional area in square millimeters.

Why does the formula multiply by 2 for voltage drop? The factor of 2 accounts for both conductors (positive and negative) in the circuit. Current flows through both wires, so the total resistance and voltage drop includes both the outgoing and return paths.

Does cable voltage drop affect power loss? Yes. Voltage drop causes power loss as heat in the cable, calculated by P = I² × R. Higher voltage drops mean more wasted energy and reduced efficiency, which is especially important in solar and battery systems.

What temperature are the resistance values based on? This calculator uses standard copper wire resistance values at 75°C (167°F), which is the typical rated temperature for most building wire. Higher temperatures increase resistance and voltage drop.

Can I use this calculator for three-phase systems? No, this calculator is designed for single-phase two-conductor systems (one positive and one negative conductor). Three-phase calculations require different formulas that account for the √3 factor and line-to-line voltage.

References

1. "American Wire Gauge." Wikipedia, Wikimedia Foundation, https://en.wikipedia.org/wiki/American_wire_gauge. Accessed 8 Oct. 2025.
2. "Voltage Drop." Wikipedia, Wikimedia Foundation, https://en.wikipedia.org/wiki/Voltage_drop. Accessed 8 Oct. 2025.
3. National Fire Protection Association. "National Electrical Code (NEC)." NFPA 70, 2023 Edition.
4. "Electrical Resistance and Conductance." Wikipedia, Wikimedia Foundation, https://en.wikipedia.org/wiki/Electrical_resistance_and_conductance. Accessed 8 Oct. 2025.