What is Voltage Drop?
Voltage drop is the reduction in electrical potential (voltage) that occurs as current flows through the resistance of conductors in an electrical circuit. Every conductor, regardless of material or size, possesses some amount of resistance that impedes current flow and converts electrical energy into heat.
When electricity travels from the power source to the load through wiring, a portion of the voltage is "lost" or "dropped" across the resistance of the conductors. This means the voltage available at the load is less than the voltage at the source, potentially affecting equipment performance and system efficiency.
Key Concept
Voltage drop is not electricity being "used up" - it's electrical potential being converted to heat in the wire due to resistance. The current remains constant throughout a series circuit, but voltage decreases along the conductor length.
Why Voltage Drop Matters
Equipment Performance
Most electrical equipment is designed to operate within a specific voltage range. Excessive voltage drop can cause:
- Motors: Reduced starting torque, overheating, decreased efficiency, and shortened lifespan
- Lighting: Dimmer illumination, color temperature shifts, and premature bulb failure
- Electronics: Malfunction, data errors, or complete failure to operate
- Heating elements: Reduced heat output proportional to voltage squared (P = V²/R)
Energy Efficiency
Voltage drop represents wasted energy converted to heat in conductors. In large facilities with long wire runs and high currents, this can result in significant energy costs over time. Reducing voltage drop improves overall system efficiency and reduces operating expenses.
Code Compliance
The National Electrical Code (NEC) provides recommendations for maximum acceptable voltage drop to ensure safe and reliable electrical system operation. While not strictly mandatory in most cases, following these guidelines is considered best practice and may be required by local authorities.
NEC Voltage Drop Requirements
The National Electrical Code Article 210.19(A) and 215.2(A) recommend the following voltage drop limits:
| Circuit Type | Maximum Voltage Drop | Notes |
|---|---|---|
| Branch Circuits | 3% | From panel to furthest outlet |
| Feeder Circuits | 2% | From service to panel |
| Combined (Feeder + Branch) | 5% | Total from service to load |
Important Note
These are recommendations, not requirements. However, exceeding these limits may result in equipment malfunction, code violations at the local level, and warranty issues. Critical equipment and sensitive electronics often require even tighter voltage regulation.
Factors Affecting Voltage Drop
1. Current (Amperage)
Voltage drop is directly proportional to current. Doubling the current doubles the voltage drop. This is why heavily loaded circuits experience more significant voltage drop than lightly loaded ones.
2. Distance
Longer conductor runs result in greater voltage drop because current must flow through more resistance. Voltage drop increases linearly with distance - doubling the wire length doubles the voltage drop.
3. Wire Size (Cross-Sectional Area)
Larger conductors have lower resistance per unit length. Increasing wire gauge (smaller AWG number means larger wire) reduces resistance and voltage drop. The relationship is inverse - doubling the cross-sectional area halves the resistance.
4. Conductor Material
Different materials exhibit different resistivity:
- Copper: Lower resistance, better conductivity, higher cost. Resistivity ≈ 1.724 × 10⁻⁸ Ω·m
- Aluminum: Higher resistance than copper, lighter weight, lower cost. Resistivity ≈ 2.826 × 10⁻⁸ Ω·m
For equivalent performance, aluminum conductors must be approximately 1.6 times larger in cross-sectional area compared to copper.
5. Temperature
Conductor resistance increases with temperature. The NEC uses 75°C as the standard temperature for resistance calculations. In practice, conductors may operate at different temperatures depending on ambient conditions, load, and bundling effects.
The Voltage Drop Formula
The standard formula for calculating voltage drop in single-phase and DC circuits is:
Where:
- VD = Voltage drop in volts
- K = Resistance constant (12.9 for copper, 21.2 for aluminum at 75°C)
- I = Current in amperes
- L = One-way distance in feet
- CM = Conductor cross-sectional area in circular mils
The factor of 2 accounts for the fact that current must travel through both the supply and return conductors, effectively doubling the circuit length.
Alternative Formula Using Resistance
Voltage drop can also be calculated using the conductor's resistance:
Where R is the resistance of one conductor for the given length, typically expressed in ohms per 1000 feet in wire tables.
Voltage Drop in Three-Phase Systems
For three-phase systems, the voltage drop calculation differs slightly:
The factor of √3 (approximately 1.732) replaces the factor of 2 used in single-phase calculations. This accounts for the phase relationship in three-phase power distribution.
Practical Implications
Low Voltage Systems (12V, 24V DC)
Voltage drop is particularly critical in low-voltage systems used for control circuits, LED lighting, and automation. A 1-volt drop in a 120V system represents less than 1% drop, but the same 1-volt drop in a 24V system represents over 4% - potentially causing equipment malfunction.
Control system designers must pay careful attention to voltage drop when powering PLC inputs and outputs, sensor circuits, and actuator systems. For automation components and control equipment specifications, PLC Department provides industrial automation solutions with detailed technical documentation to help ensure proper system design.
Motor Circuits
Voltage drop significantly affects motor performance. Starting current for motors can be 6-8 times running current, causing substantial voltage drop during startup. Excessive voltage drop can:
- Prevent motor from starting
- Cause lights to dim during motor starting (brownout)
- Reduce available starting torque
- Lead to overheating and premature motor failure
Long Distance Power Distribution
Industrial facilities, farms, and commercial campuses often require power distribution over substantial distances. In these applications, voltage drop becomes a primary design consideration, potentially requiring:
- Oversized conductors beyond ampacity requirements
- Higher distribution voltages (480V vs 240V)
- Intermediate step-up/step-down transformers
- Dedicated feeders for high-current loads
Measuring Voltage Drop
Voltage drop can be measured in operating circuits using a digital multimeter:
- Measure voltage at the power source
- Measure voltage at the load with the load energized
- The difference is the total voltage drop
- Calculate percentage: (Voltage Drop / Source Voltage) × 100
For troubleshooting, voltage measurements should be taken under load conditions, as unloaded circuits show minimal voltage drop regardless of wire sizing.
Solutions to Excessive Voltage Drop
When voltage drop exceeds acceptable limits, several solutions are available:
1. Increase Wire Size
The most common solution is using larger conductors. This reduces resistance and voltage drop proportionally. Use our Voltage Drop Calculator to determine appropriate wire sizes.
2. Reduce Circuit Length
Relocating panels closer to loads, using multiple distribution points, or rerouting circuits can significantly reduce voltage drop.
3. Reduce Current
Splitting loads across multiple circuits or reducing total connected load decreases current and voltage drop.
4. Increase System Voltage
Operating at higher voltage (480V vs 240V) reduces current for the same power level, which reduces voltage drop. This is why high-power industrial equipment typically operates at 480V or higher.
5. Use Voltage Regulation
Automatic voltage regulators or buck-boost transformers can compensate for voltage drop in critical applications, though this adds cost and complexity.
Design Best Practices
- Calculate Before Installing: Always calculate expected voltage drop during the design phase
- Consider Future Loads: Account for potential circuit additions or load increases
- Check Worst-Case Scenarios: Calculate voltage drop at maximum expected load and distance
- Verify with Measurements: Test installed circuits under load to confirm calculations
- Document Calculations: Maintain records for future modifications and troubleshooting
- Consider Ampacity: Wire size must meet both voltage drop AND ampacity requirements - use the larger size
Resources and Components
Proper electrical system design requires quality components and reliable suppliers. When implementing automation systems that require careful voltage drop consideration, sourcing the right control components is essential. For PLCs, motor drives, operator interfaces, and industrial control equipment, visit PLC Department for industrial automation components and technical support.
Use our Voltage Drop Calculator to quickly determine appropriate wire sizes for your applications, and consult the Wire Sizing Guide for additional selection criteria.