Voltage drop is an important consideration when designing or troubleshooting an electrical circuit.
Whenever current flows through a conductor, the conductor has some resistance or impedance. As a result, the voltage measured at the load can be lower than the voltage supplied by the source.
For short circuits with appropriate conductor sizes, the voltage drop may be small. For longer wire runs, higher currents, or smaller conductors, the voltage loss can become more significant.
A voltage drop calculator can make these calculations much faster by allowing you to enter the circuit information and calculate the estimated voltage drop.
What Is Voltage Drop?
Voltage drop is the reduction in voltage that occurs as electrical current travels through a conductor.
For example, a circuit may start with a 120 V supply, but the load may receive less voltage because of the resistance of the conductors.
The amount of voltage drop depends on several factors, including:
- Circuit current
- Conductor resistance
- Conductor size
- Circuit length
- Conductor material
- Circuit configuration
Understanding these factors is useful when selecting wire and checking whether a circuit is suitable for a particular application.
Why Does Voltage Drop Matter?
Excessive voltage drop can affect electrical equipment and circuit performance.
Depending on the application, excessive voltage loss may result in:
- Lower voltage at the load
- Reduced equipment performance
- Motor starting problems
- Additional heating in conductors
- Poor performance of lighting or other loads
This is why voltage drop should be considered when designing circuits with long conductor runs or significant current.
Basic Voltage Drop Formula
A basic voltage-drop calculation can be expressed as:
Voltage Drop = Current × Resistance
or:
Vd = I × R
Where:
- Vd = voltage drop
- I = circuit current
- R = conductor resistance
For practical electrical calculations, the conductor length and conductor size must also be considered because they affect the total resistance of the circuit.
Wire Size and Voltage Drop
Wire size has a direct relationship with voltage drop.
A larger conductor generally has lower resistance than a smaller conductor of the same material and length. As conductor size increases, voltage loss generally decreases for the same current and distance.
For example, a circuit carrying a relatively high current over a long distance may require a larger conductor than a short circuit carrying the same current.
This is one reason why choosing wire solely based on ampacity may not provide the complete picture.
Circuit Length Matters
Voltage drop increases as conductor length increases.
A short 10-foot run and a 200-foot run carrying the same current do not have the same voltage-drop characteristics.
When calculating voltage drop, it is important to account for the actual circuit path rather than simply looking at the distance from the power source to the load.
In many circuit calculations, current travels out to the load and returns to the source, so the effective conductor length depends on the circuit configuration.
Single-Phase and Three-Phase Circuits
Voltage-drop calculations depend on the type of electrical circuit.
A basic single-phase circuit and a three-phase circuit use different calculation approaches.
For that reason, a voltage-drop calculator should account for the circuit type rather than treating every circuit as identical.
When entering values into a calculator, make sure you select the appropriate configuration for the circuit you are evaluating.
Copper vs. Aluminum Conductors
Conductor material also affects voltage drop.
Copper and aluminum have different electrical resistance characteristics, so the conductor material needs to be included when performing a voltage-drop calculation.
Two conductors with the same nominal size and length can produce different voltage-drop results depending on the material used.
A Simple Example
Suppose a circuit has:
- A known supply voltage
- A known load current
- A specific conductor size
- A specific conductor material
- A defined circuit length
The voltage-drop calculation uses these values to determine how much voltage is lost between the source and the load.
For example, assume a circuit has a calculated voltage drop of 4 volts on a 120-volt supply.
The approximate voltage available at the load would be:
120 V − 4 V = 116 V
The percentage voltage drop would then be:
(4 ÷ 120) × 100 = 3.33%
The exact calculation for a real installation depends on the conductor characteristics and circuit configuration.
Use a Voltage Drop Calculator
Manual calculations are useful when learning how voltage drop works, but a calculator can make repeated calculations easier.
You can use the Voltage Drop Calculator to quickly work through voltage-drop calculations using the required circuit inputs.
A calculator is especially useful when comparing different conductor sizes or evaluating longer circuit runs.
Voltage Drop and Conduit Sizing Are Different
Voltage drop should not be confused with conduit fill.
Conduit fill deals with the physical space occupied by conductors inside a raceway.
Voltage drop deals with the loss of electrical potential as current travels through the conductors.
They are different calculations, but both can matter when designing an electrical installation.
For conduit calculations, you can use the Conduit Fill Tool to check conductor capacity within a raceway.
You can also read:
How to Calculate Conduit Fill Step by Step
Voltage Drop vs. Ampacity
Another common mistake is treating voltage drop and ampacity as the same thing.
They are not.
Ampacity relates to how much current a conductor is permitted to carry under the applicable conditions.
Voltage drop describes the amount of voltage lost along the circuit.
A conductor can have sufficient ampacity for a circuit while still producing a voltage drop that needs to be considered because of the length of the run.
For another electrical calculation, see the Wire Ampacity Calculator.
Where Voltage Drop Calculations Are Especially Useful
Voltage-drop calculations can be particularly useful for applications involving longer conductor runs.
Examples include:
- Detached buildings
- Workshops
- Long feeder circuits
- Outdoor equipment
- Pumps
- Motors
- HVAC equipment
- EV charging circuits
- Solar installations
The longer the conductor run and the greater the current, the more important it becomes to evaluate voltage loss as part of the design process.
Common Voltage Drop Calculation Mistakes
Using the wrong circuit length
Make sure the length entered into the calculation matches the circuit configuration being evaluated.
Forgetting conductor material
Copper and aluminum do not have identical resistance characteristics.
Ignoring the load current
Voltage drop depends on current, so the actual circuit load needs to be considered.
Using the wrong circuit type
Single-phase and three-phase calculations can use different formulas and assumptions.
Confusing voltage drop with ampacity
A wire may be adequate from an ampacity perspective while still requiring additional consideration for voltage drop.
Final Thoughts
Voltage drop is an important part of electrical circuit planning, particularly when dealing with long wire runs or higher-current loads.
The basic concept is simple: current flowing through conductor resistance results in a loss of voltage. Conductor size, material, circuit length, current, and circuit configuration all influence the result.
Using a voltage-drop calculator can make the calculation process faster and easier, especially when comparing different conductor options.
For a quick calculation, visit the Voltage Drop Calculator.
You can also explore the other electrical tools available at ConduitFillTool.com.
