How to Calculate Voltage Drop Over Long Wiring Runs

How to Calculate Voltage Drop Over Long Wiring Runs


Voltage drop is an important consideration when running long wiring lengths. Excessive voltage drop can cause problems like reduced equipment performance, overheating of conductors, and nuisance tripping of circuit breakers or blown fuses. As an electrical engineer, I need to be able to accurately calculate expected voltage drop to ensure proper system operation.

In this article, I will provide a step-by-step guide on how to calculate voltage drop over long wiring runs. I will cover the basic formulas, discuss the key factors that affect voltage drop, and provide examples to demonstrate the voltage drop calculation process. Properly accounting for voltage drop is crucial for any electrical design, so being able to accurately determine expected voltage losses is an essential skill for engineers.

What is Voltage Drop?

Voltage drop refers to the reduction in voltage that occurs as power flows through a conductor. It is essentially the loss of voltage between the power source and the electrical load. This voltage reduction happens due to the electrical resistance of the conductors.

As current flows through a conductor, the resistance causes some energy to be lost in the form of heat. This energy loss causes a slight drop in voltage along the length of the conductor. The longer the conductor, the more resistance it has and the larger the voltage drop.

Excessive voltage drop is undesirable because it can lead to:

By calculating the expected voltage drop for a particular wiring run, I can predict if these issues might occur and take steps to avoid them through wiring sizing or other methods.

Basic Voltage Drop Formula

The basic formula for calculating voltage drop is:

Voltage Drop = (2 × Resistance of Conductor × Load Current)


This simple formula gives me the voltage drop over the entire length of the conductor, accounting for the out and return paths. The (2 × Resistance × Current) portion covers the total voltage drop over the complete circuit loop.

To determine the resistance of the conductor, I need to know:

The resistance is calculated as:

Resistance (Ω) = (Conductor Length × Material Resistivity) / Conductor Area

I'll demonstrate how to use these formulas with some examples in the next sections. But first, let's look at the key factors that affect voltage drop.

Key Factors Affecting Voltage Drop

The amount of voltage drop depends on several important factors:

As you can see, the conductor length, size, and load current have the biggest impact on voltage drop. Understanding this allows me to minimize voltage drop through proper conductor and overcurrent device sizing.

Voltage Drop Examples

Let's walk through some examples to demonstrate calculating voltage drop using the basic formula:

Example 1

A 10 AWG copper conductor feeds a 200A load drawing 20A of current, over a 75ft circuit length. Calculate the voltage drop if the power system is 120V.

First, calculate the conductor resistance:

Resistance = (Length x Resistivity) / Area
= (75ft x 10.4 Ω-cmil/ft) / 10,380 cmil
= 0.072 Ω

Now use the voltage drop formula:

Voltage Drop = (2 x Resistance x Current)
= (2 x 0.072Ω x 20A)
= **2.88V**

For a 120V system, this 2.88V drop represents 2.4% loss. This is an acceptable voltage drop.

Example 2

A residential bathroom circuit uses #14 AWG copper wiring to feed a 20A load at 120V. If the total circuit length is 65ft, what is the expected voltage drop?

The resistance is:

Resistance = (Length x Resistivity) / Area
= (65ft x 10.4 Ω-cmil/ft) / 4,110 cmil
= 0.166 Ω

The voltage drop is:

Voltage Drop = (2 x Resistance x Current)
= (2 x 0.166Ω x 20A)
= **6.64V**

For a 120V circuit, this represents a 5.5% drop. This exceeds the recommended 3% limit, so a larger conductor size is required.

Recommended Voltage Drop Limits

For proper system performance, voltage drop should be limited to:

For low voltage systems like 12V or 24V DC, even smaller drop limits may be required, sometimes 1% or less.

As the examples show, calculating the expected voltage drop during design allows corrective actions to be taken early to avoid problems.

Tips for Minimizing Voltage Drop

Here are some tips to minimize voltage drop in electrical systems:

Carefully calculating and limiting voltage drop is crucial for maintaining proper equipment operation and power quality in electrical systems.


Voltage drop is a critical electrical design consideration, especially for long wiring runs. In this article, I've explained the key factors that contribute to voltage drop and demonstrated how to use the basic formulas to calculate expected voltage loss in different scenarios.

The examples illustrate how conductor length, size, and load current impact the drop. By limiting drop to recommended values, issues like overheating, poor performance, and nuisance tripping can be avoided. Proper calculation and minimizing of voltage drop ensures electrical systems will operate safely and efficiently.