Advanced Free Voltage Drop Calculator Updated 2025

Q: What is the formula for voltage drop?

The formula for voltage drop is V=I×RV = I × R, where V is the voltage drop, I is the current, and R is the resistance. In AC circuits, use V = I × Z, where Z is the impedance. For single-phase circuits, the formula is often written as V_drop = 2 × I × R × L, and for three-phase circuits, it is V_drop = √3 × I × R × L, where L is the length of the conductor.

Q: How to calculate 3 phase voltage drop?

To calculate the 3-phase voltage drop, use this formula: Vdrop=√3×I×R×LVdrop = √3×I×R×L. Where I is the current, R is the resistance per unit length of the conductor, and L is the length of the conductor. This formula describes the line-to-line voltage drop for a balanced three-phase system. For more accurate results, include reactance and power factor if they are significant in your circuit.

Modify the values and click the calculate button to use.

Wire material
Wire size
Material of conduit
Power factor (PF)

Wire material
Wire size

Voltage
Phase
Number of conductors
Distance (one-way)
Load current	Amps

Voltage Drop Calculator: How to Calculate Voltage Drop Easily in 2025 (+ Free Tools & Formula)

Have you ever noticed that when you switch on the lights in a room, the bulbs dim, or the air conditioner runs slower? These issues are often caused by voltage drop, a problem that can compromise safety, efficiency, and the lifespan of your equipment. Voltage drop occurs when the voltage at the end of a circuit is lower than the source voltage due to the resistance of the wires. If left unchecked, it can increase your energy bills, damage your equipment, and even create a fire hazard.

In this article, we’ll show you how to calculate voltage drop using a voltage drop calculator and manual formulas. Whether you’re an electrician, a contractor, or a DIY enthusiast, understanding voltage drop is crucial for every project. Here, you’ll learn how to use a voltage drop calculator, interpret the results, and minimize voltage drop in your installations. So, let’s get started!

What is Voltage Drop and Why Does It Matter?

Voltage drop means that when current flows through a wire, the voltage decreases from the source to the load. This problem is caused by the wire’s material, length, and the amount of current. For example, running a heavy-duty tool on a long extension cord will result in a greater voltage drop.

Why does it matter?

Safety: Excessive voltage drop can burn out motors and increase the risk of fire.
Efficiency: Lower voltage causes devices to operate less efficiently, leading to wasted energy.
Compliance: The NEC and IEC have set strict limits for voltage drop to ensure that all installations are safe and reliable.

Using a voltage drop calculator, you can quickly determine if your installation meets the NEC’s requirements.

Common Causes of Excessive Voltage Drop

Voltage drop depends on several factors, such as:
Wire length: Longer wire = higher resistance = greater voltage drop.
Wire size (gauge): Thinner wires (higher AWG) have higher resistance.
Current (load): Higher current = greater voltage drop.
Conductor material: Copper has lower resistance than aluminum.
Temperature: Wire resistance increases significantly at higher temperatures.

Ignoring any of these factors can exacerbate voltage drop problems and potentially shorten the lifespan of your equipment.

NEC/IEC Voltage Drop Limits and Recommendations (2023–2025)

According to NEC and IEC standards, the voltage drop limits are:

Branch circuits: A 3% voltage drop is recommended for outlets and lighting.
Feeders: A 5% voltage drop is allowed for main supply lines.
Total (branch + feeder): This should not exceed 5%.

Following these limits will ensure your devices operate at the correct voltage and maintain complete safety.

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Understand the Voltage Drop Formula (Single-Phase & Three-Phase)

Use this formula to manually calculate voltage drop:

The Single-Phase Voltage Drop Formula is: $Voltage Drop (V)=2×I×L×R$

Where:

I = Current (in amperes)
L = Wire length (in feet)
R = Resistance per foot (in ohms/ft)

The three-phase voltage drop formula is: $Voltage Drop (V)= √3 ×I×L×R$

$√3≈1.732$ , the other variables remain the same.

This formula is the basis of every voltage drop calculator.

How to Use the Voltage Drop Calculator (Step-by-Step)

Using the voltage drop calculator is very easy:

Enter circuit details: Voltage, current, wire length, wire size (AWG), conductor material.
Select circuit type: Choose single-phase or three-phase according to your needs.
View results: The calculator will display the voltage drop in volts and as a percentage.
Adjust: If the result exceeds NEC limits, increase the wire size or decrease the length.

This allows you to quickly and easily perform your compliance checks.

Manual Calculation Example: 120V Single-Phase Circuit

Assume you have a 120V circuit that is 100 ft long, carries 15A of current, and uses 12 AWG copper wire.

The resistance of 12 AWG copper is 1.588 ohms/1000 ft.
$R=1.588/1000=0.001588 ohms/ft$
Voltage drop: $2×15×100×0.001588=4.76V$
Percentage: $(4.76/120)×100=3.97%$

This is more than 3%, so the wire size will need to be increased.

Manual Calculation Example: 480V Three-Phase Circuit

480V three-phase circuit, 200 ft long, 50A, and 1 AWG copper:

Resistance of 1 AWG copper: 0.154 ohms/1000 ft
$R=0.154/1000=0.000154 ohms/ft$
Voltage Drop: $1.732×50×200×0.000154=2.67V$
Percentage: $(2.67/480)×100=0.56%$

This is within NEC limits.

Cable size, length, material (copper vs. aluminum), and temperature all affect voltage drop.

AWG	Copper (Ohms/1000 ft)	Aluminum (Ohms/1000 ft)	Voltage Drop (100 ft, 15A) Copper	Voltage Drop (100 ft, 15A) Aluminum
14	2.525	4.084	7.58V	12.25V
12	1.588	2.572	4.76V	7.72V
10	0.999	1.618	3.00V	4.85V
8	0.628	1.018	1.88V	3.05V
6	0.395	0.641	1.19V	1.92V

Copper has a lower voltage drop than aluminum. Therefore, its resistance increases as the temperature rises.

Tips for Reducing Voltage Drop in Real-World Installations

Use larger wire sizes: This reduces resistance.
Keep wire runs short: Minimize the distance between the panel and the load.
Balance the load: Distribute the current evenly across the phases.
Choose copper: Use copper instead of aluminum whenever possible.
Monitor temperature: Avoid placing wires in hot areas.

These tips can help you save energy, extend equipment lifespan, and easily maintain compliance.

What is the difference between Voltage Drop and Voltage Drop Percentage?

Voltage drop: The actual decrease in voltage (e.g., 4.76V).
Voltage drop percentage: The percentage relative to the supply voltage (e.g., 3.97%).

Both are important; the voltage drop shows the absolute loss in volts, while the percentage helps in comparing voltage drops across different voltage levels.

Understanding voltage drop is crucial for every electrical installation. By using a voltage drop calculator, following NEC limits, and applying practical tips, you can ensure safety, efficiency, and compliance. Always calculate voltage drop before finalizing the design to avoid equipment failure or fire hazards. Try the free voltage drop calculators provided above and optimize your next project effectively.

Frequently Asked Questions

What is the formula for voltage drop?

The formula for voltage drop is $V = I × R$ , where V is the voltage drop, I is the current, and R is the resistance. In AC circuits, use V = I × Z, where Z is the impedance. For single-phase circuits, the formula is often written as V_drop = 2 × I × R × L, and for three-phase circuits, it is V_drop = √3 × I × R × L, where L is the length of the conductor.

What is the voltage drop per 100 feet?

The voltage drop per 100 feet depends on the wire gauge, current, and material (typically copper or aluminum). For common household wiring (e.g., 12-gauge copper wire carrying 15 amperes of current), the voltage drop is approximately 1.5 volts per 100 feet.

What is the voltage drop across a 4 ohm resistor?

The voltage drop across a 4-ohm resistor depends on the current flowing through it, which is calculated using Ohm’s law: $V=I×R$ , where $V$ is the voltage, $I$ is the current, and $R$ is the resistance. For example, if a current of 2 amperes flows through the resistor, the voltage drop will be
2 × 4 = 8 volts.

What is the 5% voltage drop rule?

The 5% voltage drop rule is a guideline stating that the total voltage drop in an electrical installation, across both the feeder and branch circuits, should not exceed 5% of the supply voltage under full load conditions. This ensures that equipment receives the correct voltage for optimal performance and efficiency, but it is not a safety requirement; rather, it is a recommendation for performance.

How to calculate 3 phase voltage drop?

To calculate the 3-phase voltage drop, use this formula: $Vdrop = √3×I×R×L$ . Where I is the current, R is the resistance per unit length of the conductor, and L is the length of the conductor. This formula describes the line-to-line voltage drop for a balanced three-phase system. For more accurate results, include reactance and power factor if they are significant in your circuit.