What is Voltage Drop?
Voltage drop is the reduction in voltage that occurs as electrical current flows through the resistance of conductors. While ampacity ensures wires don't overheat, voltage drop affects performance and efficiency.
Why Voltage Drop Matters
- Equipment Performance: Motors may not start or run properly
- Light Quality: Lamps operate dimmer than rated
- Energy Efficiency: Wasted power dissipated as heat
- Appliance Lifespan: Low voltage can damage electronics
- Code Compliance: Excessive drop violates NEC recommendations
NEC Voltage Drop Standards
NEC Article 210.19(A) Informational Note No. 4
The NEC provides recommendations (not mandatory requirements) for voltage drop:
NEC Recommendations
- Branch Circuits: Maximum 3% voltage drop
- Feeders: Maximum 2% voltage drop
- Combined (Feeder + Branch): Maximum 5% total
Important Distinction: Recommendation vs. Requirement
While not mandatory under the NEC, these limits are:
- Considered best practice and industry standard
- Often enforced by local codes and inspectors
- Required for optimal equipment performance
- Essential for warranty compliance on many devices
- May become mandatory in future NEC editions
Calculating Voltage Drop
Basic Voltage Drop Formula
Single-Phase: VD = 2 × K × I × L / CM
VD = Voltage drop (volts), K = Resistivity constant (12.9 for copper, 21.2 for aluminum), I = Current (amps), L = One-way length (feet), CM = Wire circular mils
Three-Phase: VD = 1.732 × K × I × L / CM
Simplified Formula Using Wire Resistance
VD = 2 × I × R × L
VD = Voltage drop (volts), I = Current (amps), R = Wire resistance (ohms per 1000 feet), L = One-way length (thousands of feet), 2 = Accounts for round trip
Wire Resistance Table
| AWG Size | Copper (Ω/1000ft) | Aluminum (Ω/1000ft) |
|---|---|---|
| 14 AWG | 3.07 | 5.06 |
| 12 AWG | 1.93 | 3.18 |
| 10 AWG | 1.21 | 2.00 |
| 8 AWG | 0.764 | 1.26 |
| 6 AWG | 0.491 | 0.808 |
| 4 AWG | 0.308 | 0.508 |
Step-by-Step Voltage Drop Calculation
Example: Residential Branch Circuit
Given
- Circuit: 120V, 20A continuous load
- Wire: 12 AWG copper
- Distance: 75 feet one-way
Calculation: VD = 2 × 20A × 1.93 × (75/1000) = 5.79 volts
Percentage: (5.79 / 120) × 100 = 4.83%
Result
4.83% EXCEEDS 3% LIMIT - Solution: Upsize to 10 AWG
Voltage Drop Limits by Application
Critical Applications (1-2% Maximum)
- Medical equipment and life safety systems
- Computer and server rooms
- Precision electronic equipment
- Emergency lighting circuits
Standard Applications (3% Maximum)
- General lighting circuits
- Receptacle outlets
- Most residential circuits
- Commercial branch circuits
Practical Wire Sizing Guidelines
When to Upsize Wire for Voltage Drop
- Under 50 feet: Standard ampacity sizing usually sufficient
- 50-100 feet: Check voltage drop, may need to upsize 1 gauge
- 100-150 feet: Likely need to upsize 1-2 gauges
- Over 150 feet: Calculate voltage drop, may need 2-3 gauge upsize
Voltage Drop Mitigation Strategies
1. Upsize Conductors
Most common solution. Going up one or two wire gauges typically resolves voltage drop issues.
2. Use Higher Voltage
240V circuits have half the percentage drop of 120V circuits for the same load and wire size.
3. Reduce Circuit Length
- Relocate sub-panel closer to loads
- Use multiple shorter circuits instead of one long circuit
- Strategic panel placement in building design
4. Use Copper Instead of Aluminum
Copper has 37% lower resistance than aluminum for the same gauge.
Common Voltage Drop Mistakes
1. Only Checking Ampacity
The most common error. Wire may be safe from overheating but still have excessive voltage drop.
2. Forgetting the Round Trip
Voltage drop occurs in both hot and neutral conductors. Always multiply by 2 for single-phase.
Tools and Calculators
Use our Voltage Drop Calculator to quickly determine if your wire size is adequate for your circuit length and load. The calculator automatically applies the correct formulas and provides recommendations.
Conclusion
While the NEC voltage drop limits are recommendations rather than strict requirements, following them is essential for proper equipment operation, energy efficiency, and professional installations. Always calculate voltage drop for circuits over 50 feet, and don't hesitate to upsize wire when needed—the small additional cost of larger wire is far less than the problems caused by excessive voltage drop.
Remember: meeting ampacity requirements ensures safety, but controlling voltage drop ensures performance. Both are critical for successful electrical installations.