Voltage Drop Calculation: Formula, Examples, and NEC Guidelines

A voltage drop calculation helps electrical engineers and contractors determine how much voltage is lost as electrical current travels through a conductor from the power source to the connected load.

Every conductor has electrical resistance and, in AC systems, impedance. As current flows through the conductor, the voltage available at the load can be lower than the voltage at the source. Excessive voltage drop can affect the performance of electrical equipment, lighting systems, motors, and other connected loads.

Understanding how to calculate voltage drop is therefore an important part of electrical design and conductor selection. The calculation considers factors such as conductor length, current, conductor size, conductor material, resistance, reactance, circuit configuration, temperature, and power factor.

This guide explains the voltage drop calculation formula, provides single-phase and three-phase examples, and explains how voltage drop relates to conductor sizing and electrical design requirements.

1. What Is Voltage Drop?

Voltage drop is the reduction in voltage that occurs as current flows through the impedance of an electrical conductor or circuit.

In a simple circuit:

Source Voltage − Voltage Drop = Load Voltage

For example, if a circuit starts with 120 V and experiences a 3 V voltage drop:

120 V − 3 V = 117 V

The connected equipment would receive approximately 117 V under the assumed operating conditions.

Voltage drop depends primarily on:

• Conductor length

• Conductor material

• Conductor size

• Load current

• Circuit configuration

• Conductor temperature

• Conductor resistance

• Conductor reactance

• Power factor in AC circuits

The longer the conductor and the greater the current, the more significant the voltage drop can become.

2. Why Is Voltage Drop Important?

Electrical equipment is designed to operate within a specified voltage range. If the voltage delivered to equipment becomes too low, performance can be affected depending on the equipment type and its operating characteristics.

Voltage drop can be particularly important for:

• Motors

• HVAC equipment

• Pumps

• Lighting

• Long branch circuits

• Remote mechanical equipment

• Industrial equipment

• Large commercial buildings

• Site distribution systems

A voltage drop calculation allows the electrical designer to evaluate the voltage available at the load before selecting the final conductor size.

Voltage drop should also be considered together with other electrical design requirements, including conductor ampacity, overcurrent protection, installation conditions, equipment ratings, and applicable codes.

3. Basic Voltage Drop Formula

For a simple single-phase, two-wire circuit where conductor resistance is used as the basis of the calculation, a commonly used formula is:

VD = 2 × L × I × R

Where:

• VD = Voltage drop in volts

• L = One-way conductor length

• I = Load current in amperes

• R = Conductor resistance per unit length

The factor of 2 accounts for the outgoing and returning conductors.

If the resistance is expressed in ohms per 1,000 feet, the formula can be written as:

VD = (2 × L × I × R) ÷ 1000

Where:

• L = One-way length in feet

• R = Resistance in ohms per 1,000 feet

The voltage-drop percentage can then be calculated using:

Voltage Drop (%) = (Voltage Drop ÷ Source Voltage) × 100

This percentage indicates how significant the calculated voltage loss is relative to the nominal system voltage.

4. How to Calculate Voltage Drop Step by Step

A basic voltage drop calculation can be completed using the following process.

Step 1: Determine the System Voltage

Identify the nominal voltage of the circuit.

Examples include:

• 120 V

• 208 V

• 240 V

• 277 V

• 480 V

Step 2: Determine the Circuit Configuration

Identify whether the circuit is:

• Single-phase

• Three-phase

• DC

• Two-wire

• Three-wire

• Four-wire

The appropriate voltage-drop formula depends on the system configuration.

Step 3: Determine Load Current

Identify the current flowing through the circuit under the design condition being evaluated.

Step 4: Determine One-Way Conductor Length

Measure the distance from the source to the load.

For a simple single-phase two-wire calculation, the formula accounts for the return path by using the factor of 2.

Step 5: Determine Conductor Resistance or Impedance

Use appropriate conductor data for:

• Conductor material

• Conductor size

• Operating temperature

• Raceway or installation conditions

• AC resistance and reactance where applicable

Step 6: Calculate Voltage Drop

Apply the appropriate voltage-drop equation.

Step 7: Calculate Voltage Drop Percentage

Divide the voltage drop by the source voltage and multiply by 100.

Step 8: Compare the Result With Project Requirements

The result should be evaluated against:

• Applicable code provisions

• Project specifications

• Equipment requirements

• Utility requirements where applicable

• Design criteria established for the project

5. Single-Phase Voltage Drop Example

Suppose a 120 V single-phase circuit has:

• Load current = 20 A

• One-way conductor length = 100 ft

• Conductor resistance = 0.001 Ω/ft

Using:

VD = 2 × L × I × R

VD = 2 × 100 × 20 × 0.001

VD = 4 V

The voltage at the load would therefore be:

120 V − 4 V = 116 V

Now calculate the voltage-drop percentage:

Voltage Drop (%) = (4 ÷ 120) × 100

Voltage Drop (%) = 3.33%

Therefore, under the assumptions in this example, the circuit experiences a calculated voltage drop of 4 V, or approximately 3.33%.

This example demonstrates why conductor resistance, circuit length, and load current are important inputs in voltage-drop calculations.

6. Three-Phase Voltage Drop Formula

Three-phase circuits use a different relationship because of the phase arrangement.

For a simplified balanced three-phase calculation based on conductor resistance:

VD = √3 × L × I × R

Where:

• VD = Voltage drop in volts

• L = One-way circuit length

• I = Line current in amperes

• R = Conductor resistance per unit length

For AC circuits where reactance and power factor are significant, a more complete impedance-based approach can be used:

VD = √3 × I × L × (R cosφ + X sinφ)

Where:

• R = Conductor resistance per unit length

• X = Conductor reactance per unit length

• cosφ = Power factor

• sinφ = Reactive component of the power factor

The appropriate equation depends on the electrical system, conductor data, load characteristics, and required level of calculation accuracy.

For AC cable calculations, both resistance and reactance can contribute to impedance, and power factor affects the voltage-drop calculation.

7. Three-Phase Voltage Drop Example

Consider a three-phase circuit with:

• System voltage = 480 V

• Current = 40 A

• One-way length = 200 ft

• Resistance = 0.0005 Ω/ft

Using the simplified formula:

VD = √3 × L × I × R

VD = 1.732 × 200 × 40 × 0.0005

VD ≈ 6.93 V

The voltage-drop percentage is:

Voltage Drop (%) = (6.93 ÷ 480) × 100

Voltage Drop (%) ≈ 1.44%

Therefore, based on the assumed conductor resistance and simplified calculation, the circuit has an estimated voltage drop of approximately 6.93 V, or 1.44%.

For a more detailed AC calculation, conductor impedance and power factor should be considered where applicable.

8. Factors That Affect Voltage Drop

Several factors influence the amount of voltage drop in an electrical circuit.

Conductor Length

The longer the conductor, the greater the potential voltage drop for a given conductor size and current.

Long runs can therefore require additional attention during electrical design.

This is particularly relevant for:

• Large buildings

• Long branch circuits

• Site distribution

• Parking-area equipment

• Remote mechanical equipment

• Industrial facilities

• Large campus projects

Conductor Size

Larger conductors generally have lower resistance.

If voltage drop is higher than the project’s design criterion, increasing conductor size may reduce the voltage drop.

However, conductor sizing should also account for ampacity, installation conditions, terminations, overcurrent protection, and applicable code requirements.

Load Current

Voltage drop increases as current increases when other variables remain constant.

A circuit carrying a higher current will generally experience greater voltage drop than an otherwise identical circuit carrying a lower current.

Conductor Material

Copper and aluminum have different electrical resistivity characteristics.

Consequently, conductor material affects resistance and therefore affects voltage-drop calculations.

Conductor Temperature

Conductor resistance changes with temperature.

Voltage-drop calculations should therefore use appropriate conductor data and operating assumptions for the design condition.

Power Factor

Power factor becomes particularly important in AC circuits with inductive or other reactive loads.

Loads such as motors can have characteristics that make an impedance-based calculation more appropriate than a resistance-only calculation.

Conductor Reactance

AC conductor reactance can contribute to total circuit impedance.

For longer circuits or applications where reactance is significant, the voltage-drop calculation should account for both resistance and reactance.

Southwire’s electrical installation guidance also distinguishes AC voltage-drop calculations involving resistance, reactance, and power factor from simpler DC or basic single-phase calculations.

9. Voltage Drop and Conductor Sizing

Voltage-drop calculations are closely connected with conductor sizing.

If calculated voltage drop is excessive, an electrical designer may evaluate options such as:

• Increasing conductor size

• Reducing circuit length

• Increasing distribution voltage where appropriate

• Adjusting circuit configuration

• Relocating equipment

• Reviewing the connected load

• Revising feeder arrangements

• Evaluating alternative distribution configurations

Increasing conductor size is only one possible solution.

The selected conductor must also satisfy requirements related to:

• Ampacity

• Temperature limitations

• Installation conditions

• Conduit or raceway conditions

• Overcurrent protection

• Termination ratings

• Equipment requirements

• Applicable electrical codes

Southwire’s voltage-drop calculator, for example, evaluates conductor size using both voltage-drop criteria and NEC ampacity considerations rather than treating voltage drop as the only conductor-sizing variable.

10. Voltage Drop Recommendations and NEC Guidelines

Voltage-drop requirements should be evaluated according to the applicable electrical code edition, project specifications, AHJ requirements, utility requirements, and equipment requirements.

For ordinary branch circuits, the NEC’s informational guidance commonly associated with Article 210 recommends sizing conductors to limit voltage drop to 3% at the farthest outlet, with the combined voltage drop of feeder and branch-circuit conductors to the farthest outlet not exceeding 5% for reasonable efficiency of operation. These are informational-note recommendations rather than a universal mandatory 3% limit for every ordinary circuit.

A practical design review may therefore consider:

• Branch-circuit voltage drop

• Feeder voltage drop

• Combined feeder and branch-circuit voltage drop

• Equipment voltage requirements

• Project specifications

• Local amendments

• AHJ requirements

• Special equipment requirements

Some installations can have requirements that differ from the commonly cited 3%/5% design guidance. For example, certain jurisdictions and special applications may establish specific voltage-drop requirements.

Therefore, designers should always verify the adopted code edition and project-specific requirements rather than treating 3% or 5% as universal limits.

11. What’s Typically Included in a Voltage-Drop Calculation?

A voltage-drop calculation generally requires the following information:

• System voltage

• Phase configuration

• Load current

• One-way conductor length

• Conductor material

• Conductor size

• Conductor resistance

• Conductor reactance, where applicable

• Power factor, where applicable

• Circuit configuration

• Operating temperature assumptions

• Applicable code requirements

• Project specifications

• Equipment voltage requirements

For larger projects, electrical engineers may perform voltage-drop calculations for:

• Feeders

• Branch circuits

• Motor circuits

• HVAC equipment

• Pumps

• Emergency systems

• Long-distance electrical distribution

• Large equipment connections

• Critical electrical loads

The exact scope depends on the project’s electrical design requirements.

12. Voltage Drop Calculation for Long Electrical Runs

Long electrical runs require particular attention because conductor impedance increases the voltage loss between the source and load.

Examples include:

• Parking-lot lighting

• Site lighting

• Remote HVAC equipment

• Pumps

• Detached buildings

• Industrial equipment

• Large commercial facilities

• Campus distribution systems

When a circuit is unusually long, the designer may need to evaluate conductor size, operating current, system voltage, installation conditions, and equipment requirements together.

A higher distribution voltage can sometimes reduce current for a given power level, which can affect voltage-drop performance. However, changing system voltage is a system-design decision and must be evaluated against equipment ratings, distribution architecture, code requirements, and project objectives.

13. Voltage Drop for Motor Loads

Motor circuits can require additional consideration because motors may have significant starting current and reactive characteristics.

A voltage-drop calculation for a motor circuit may need to distinguish between:

• Normal operating conditions

• Starting conditions

• Motor full-load current

• Starting current

• Power factor

• Conductor impedance

• Equipment voltage requirements

A circuit that performs adequately under steady-state conditions may need additional evaluation during motor starting if the starting current produces a significant temporary voltage drop.

For industrial and commercial power systems, voltage considerations include maintaining satisfactory voltage at utilization equipment under operating conditions. IEEE 3001.3-2024 addresses voltage considerations for industrial and commercial power systems.

14. Common Voltage Drop Calculation Mistakes

Several errors can produce inaccurate voltage-drop calculations.

Using the Wrong Circuit Length

A common mistake is using the wrong length or failing to understand whether the selected formula expects one-way or total circuit length.

Forgetting the Return Path

For a basic single-phase two-wire calculation, the outgoing and returning conductors must be accounted for.

Using Incorrect Resistance Data

Resistance values must correspond to the appropriate conductor material, size, and calculation conditions.

Ignoring Reactance

For applicable AC circuits, resistance alone may not provide an adequate representation of conductor impedance.

Using Incorrect Load Current

The current used in the calculation should correspond to the appropriate design operating condition.

Ignoring Power Factor

Power factor can affect voltage drop in AC circuits where the reactance component is significant.

Treating Voltage Drop as the Only Sizing Criterion

A conductor may satisfy a voltage-drop target while failing to satisfy another electrical design requirement.

Applying 3% or 5% Without Checking the Project

The commonly cited 3%/5% values should be evaluated in context. The adopted code, local requirements, project specifications, and equipment requirements should be reviewed.

15. Voltage Drop Calculation vs. Ampacity Calculation

Voltage drop and ampacity are related to conductor selection, but they address different design considerations.

Ampacity

Ampacity evaluates the amount of current a conductor can safely carry under specified installation conditions.

Voltage Drop

Voltage-drop analysis evaluates the reduction in voltage between the source and load.

A conductor can have sufficient ampacity but still produce excessive voltage drop on a long circuit.

Conversely, increasing conductor size to reduce voltage drop does not eliminate the need to verify all other conductor-sizing requirements.

Therefore, a complete electrical design should consider both.

16. How to Reduce Voltage Drop

When a calculated voltage drop exceeds the project’s design criterion, several design options may be evaluated.

Increase Conductor Size

A larger conductor generally has lower resistance and can reduce voltage drop.

Reduce Circuit Length

Moving equipment closer to the electrical source can reduce conductor length and therefore reduce voltage drop.

Increase System Voltage

Where technically appropriate, a higher distribution voltage can reduce current for a given power level.

Review the Distribution Arrangement

The designer may evaluate feeder and branch-circuit arrangements to determine whether the electrical distribution can be optimized.

Review Load Requirements

Confirm that the connected load and design current used in the calculation accurately represent the project requirements.

Consider Equipment Requirements

Some equipment has specific voltage requirements that should be considered when establishing the allowable voltage at the load.

The appropriate solution depends on the electrical system and project constraints.

17. Voltage Drop Calculation Checklist

Before completing a voltage-drop calculation, verify:

• System voltage

• Single-phase or three-phase configuration

• Load current

• One-way conductor length

• Conductor material

• Conductor size

• Resistance value

• Reactance value, where applicable

• Power factor, where applicable

• Circuit configuration

• Operating temperature assumptions

• Applicable code requirements

• Project specifications

• Equipment voltage requirements

• Calculated voltage drop

• Voltage-drop percentage

• Ampacity

• Overcurrent protection

• Final conductor selection

18. How Electrical Engineers Use Voltage Drop Calculations

Voltage-drop calculations are commonly incorporated into electrical design as part of evaluating power distribution and conductor selection.

The engineer may review:

1. Electrical load information

2. System voltage

3. Feeder and branch-circuit configuration

4. Conductor size and material

5. Circuit length

6. Resistance and impedance

7. Load characteristics

8. Voltage-drop percentage

9. Equipment requirements

10. Applicable code and project criteria

The results can then be used to determine whether the proposed electrical distribution arrangement is appropriate or whether conductor sizing or another design parameter should be revised.

19. When Is a Voltage Drop Calculation Needed?

Voltage-drop analysis can be particularly useful for:

• Long electrical circuits

• Large commercial buildings

• Industrial facilities

• Large residential developments

• Remote mechanical equipment

• Motor circuits

• Site distribution

• Parking-area equipment

• Emergency systems

• High-current feeders

• Circuits with sensitive equipment

The need for a detailed calculation depends on the project design criteria, equipment requirements, circuit characteristics, and applicable regulations.

How Daymark Engineers Supports Electrical Design

Daymark Engineers provides electrical engineering as part of its MEP design services. Its electrical engineering scope includes electrical calculations and room design, lighting systems, receptacle layouts, power distribution, emergency electrical systems, electrical riser diagrams, and generator and transformer sizing.

Daymark also lists electrical engineering as one of its MEP-FP engineering disciplines and provides engineering services across building projects.

For projects requiring voltage-drop analysis, the calculation should be developed using the project’s actual electrical distribution information, conductor data, load characteristics, applicable code requirements, and equipment criteria.

Final Takeaway

A voltage drop calculation helps electrical engineers and contractors determine how much voltage is lost between the electrical source and connected equipment.

The calculation depends on factors such as conductor length, current, conductor size, conductor material, resistance, reactance, temperature, circuit configuration, and power factor.

For simple circuits, basic resistance-based formulas can provide a useful estimate. More complex AC systems may require conductor impedance and power factor to be considered.

Voltage drop should never be evaluated in isolation. Conductor ampacity, overcurrent protection, installation conditions, equipment ratings, applicable electrical codes, project specifications, and local requirements should all be considered when selecting conductors and designing electrical distribution systems.

A properly evaluated electrical distribution system can provide the required voltage to connected equipment while supporting reliable and efficient building operation.

FAQs About Voltage Drop Calculation

What is voltage drop?

Voltage drop is the reduction in electrical voltage that occurs as current flows through the resistance and impedance of a conductor or electrical circuit.

How do you calculate voltage drop?

For a basic single-phase two-wire circuit, a commonly used formula is:

VD = 2 × L × I × R

The resulting voltage drop can then be converted to a percentage using:

Voltage Drop (%) = (VD ÷ Source Voltage) × 100

What is the voltage drop formula for a three-phase circuit?

A commonly used simplified three-phase formula is:

VD = √3 × L × I × R

For AC systems where reactance and power factor are significant, an impedance-based formula may be more appropriate.

What is the difference between voltage drop and voltage-drop percentage?

Voltage drop is the actual voltage lost, expressed in volts. Voltage-drop percentage expresses that loss relative to the source voltage.

For example, a 4 V drop on a 120 V circuit equals approximately 3.33%.

Does conductor size affect voltage drop?

Yes. Larger conductors generally have lower resistance, which can reduce voltage drop when other circuit conditions remain the same.

Does conductor length affect voltage drop?

Yes. As conductor length increases, voltage drop generally increases for the same conductor, current, and operating conditions.

Does current affect voltage drop?

Yes. Voltage drop generally increases as load current increases when other variables remain constant.

What is the NEC voltage-drop recommendation?

The NEC informational guidance commonly associated with branch circuits recommends 3% maximum voltage drop at the farthest outlet and 5% maximum combined feeder and branch-circuit voltage drop for reasonable efficiency of operation. These figures should not automatically be treated as universal mandatory limits; the applicable code edition, project requirements, AHJ requirements, and equipment requirements should be reviewed.

Is 3% voltage drop a mandatory NEC requirement?

Not generally for ordinary branch circuits. The commonly cited 3% and 5% values appear in NEC informational notes, which provide explanatory guidance rather than universal mandatory requirements. Certain specific applications or local requirements can impose different or mandatory voltage-drop limits.

Can a conductor meet ampacity requirements but still have excessive voltage drop?

Yes. Ampacity and voltage drop address different design considerations. A conductor can have sufficient ampacity while producing excessive voltage drop on a long circuit.

How can voltage drop be reduced?

Possible design approaches include increasing conductor size, reducing circuit length, reviewing the distribution voltage, adjusting the distribution arrangement, or evaluating the connected load.

Does power factor affect voltage drop?

Yes. Power factor affects the relationship between the resistance and reactance components of voltage drop in AC circuits. It becomes more relevant when the circuit has significant reactance or reactive loads.

Does temperature affect voltage drop?

Yes. Conductor resistance changes with temperature, so the temperature assumption used for a calculation can affect the result.

Do motor circuits require special voltage-drop consideration?

They can. Motor circuits may need evaluation under both normal operating and starting conditions because motor starting current can be substantially higher than normal operating current.

What information is needed for a voltage-drop calculation?

Typical inputs include system voltage, phase configuration, load current, conductor length, conductor material, conductor size, resistance, reactance where applicable, power factor, operating conditions, and project-specific voltage-drop criteria.

When should a voltage-drop calculation be performed?

Voltage-drop analysis is particularly relevant for long circuits, high-current feeders, motor circuits, remote equipment, large commercial and industrial projects, and other applications where maintaining adequate load voltage is important.