Voltage Drop Calculator
Calculate voltage drop in volts and percent for any conductor run. Single-phase formula VD = (2 × L × I × R) ÷ 1000; three-phase formula VD = (1.732 × L × I × R) ÷ 1000.
Quick Answer: Common Voltage Drop Values
12 AWG Copper at 120 V, 20 A:
- 25 ft = 1.61 % drop
- 50 ft = 3.22 % drop
- 75 ft = 4.83 % drop
- 100 ft = 6.45 % drop
10 AWG Copper at 240 V, 30 A:
- 50 ft = 1.52 % drop
- 100 ft = 3.04 % drop
- 150 ft = 4.56 % drop
- 200 ft = 6.08 % drop
6 AWG Copper at 240 V, 50 A:
- 50 ft = 1.00 % drop
- 100 ft = 2.00 % drop
- 150 ft = 3.00 % drop
- 200 ft = 4.00 % drop
Solid copper at 75 °C, calculated from NIST Handbook 100 constants, as in the calculator. Stranded conductors measure slightly higher.
Voltage Drop Calculator
Calculate voltage drop for both copper and aluminum conductors
Input Parameters
Copper:
Meets your voltage-drop target (3 %)
Aluminum:
Above your voltage-drop target (3 %)
Calculation Results
Voltage Drop Visualization
Copper Wire
2.02 % DropMeets your voltage-drop target
4.86 V
235.14 V
97 W
10 AWG
Aluminum Wire
3.33 % DropAbove your voltage-drop target
8.00 V
232.00 V
160 W
10 AWG
Material Comparison: Aluminum has about 64 % higher resistance than copper.
DC resistance of solid conductors at 75 °C, calculated from NIST Handbook 100 and 109 constants. Stranded conductors measure slightly higher, and AC reactance is not included.
Complete Guide to Understanding Voltage Drop
What is Voltage Drop?
Voltage drop is the reduction in voltage that occurs as electricity travels through a conductor. This loss happens because all conductors have resistance, which opposes the flow of current. The longer the wire and the higher the current, the more voltage is lost along the way.
The Voltage Drop Formula
- VD = Voltage drop (volts)
- L = One-way length (feet)
- I = Current (amps)
- R = Resistance (ohms/1000 ft)
Three-Phase Formula:
Important: The factor of 2 counts both conductors of a single-phase or DC circuit: current goes out on one and returns on the other.
Why Voltage Drop Matters
Effects of Excessive Voltage Drop
- • Motors: Overheat, reduced torque, premature failure
- • Lights: Dimming, flickering, reduced lifespan
- • Electronics: Malfunction, random resets, data corruption
- • Equipment damage: Compressors, pumps fail to start
Benefits of Proper Voltage Management
- • Efficiency: Equipment operates at rated performance
- • Longevity: Extended equipment lifespan
- • Reliability: Stable voltage for sensitive electronics
The wire resistance calculator lists the resistance of each size and the basis of each column.
Troubleshooting Voltage Drop Problems
Diagnosing Excessive Voltage Drop
Symptoms to Watch For
- • Lights dim when motors start
- • Lights brighten when loads turn off
- • Motors struggle to start or overheat
- • Electronic equipment randomly resets
- • Flickering lights, especially at end of circuit
Testing Procedure
- 1. Measure voltage at panel with no load
- 2. Apply full rated load to circuit
- 3. Measure voltage at load while running
- 4. Calculate: Drop = (V₁ - V₂) ÷ V₁ × 100
- 5. Check all connections with IR camera
- 6. Verify wire size matches circuit rating
Common Causes & Solutions
Poor Connections
Cause: Loose terminals, corrosion, improper torque
Solution: Retorque all connections to manufacturer specs, replace corroded terminals
Undersized Conductors
Cause: Wire sized for ampacity only, not distance
Solution: Replace with larger wire, add parallel conductor, or relocate panel closer to load
Damaged Conductors
Cause: Nicked wire, water damage, overheating
Solution: Megger test insulation, check for hot spots, replace damaged sections
Advanced Voltage Drop Calculations
Parallel Conductor Calculations
Where N = number of parallel conductors per phase
DC Circuit Calculations
Same as single-phase AC, but no power factor
Solar/Battery Systems
- • MPPT controllers need minimum voltage
- • Battery charging efficiency critical
- • Consider 48 V systems for long runs
Voltage Drop with Reactive Loads
AC Impedance Formula
Where X = inductive reactance (ohms)
Installation Best Practices to Minimize Voltage Drop
Design Phase
- Locate panels centrally to minimize runs
- Use 240 V for high-current or long-distance loads
- Consider multiple circuits vs one large feeder
- Use voltage drop software for complex systems
Installation Phase
- Torque all terminals to manufacturer specs
- Use compression lugs for large conductors
- Minimize splices and junction points
- Keep conductors away from heat sources
Verification Phase
- Measure actual voltage at source first
- Test voltage drop under full load conditions
- Document readings for future reference
Comprehensive Voltage Drop FAQ
Q: How do you calculate voltage drop for a 100 foot run?
A: For single-phase: Voltage Drop = (2 × Length × Current × Resistance) ÷ 1000.
Q: Why is my voltage drop so high with aluminum wire?
A: Aluminum wire has approximately 61 % of the conductivity of copper, meaning it has 64 % more resistance. For the same ampacity, aluminum wire must be 1-2 sizes larger than copper to achieve similar voltage drop.
Q: Does voltage drop affect 240 V circuits?
A: Yes, but the percentage impact is half that of 120 V circuits. A 7.2 V drop is 6 % on a 120 V circuit but only 3 % on a 240 V circuit. This is why 240 V is preferred for long-distance, high-current applications.
Q: Should I calculate voltage drop for LED lighting?
A: Yes, LED drivers are sensitive to voltage variations. While LEDs use less current than incandescent bulbs, voltage drop still affects driver efficiency and can cause flickering or premature driver failure.
Q: What's the voltage drop formula for 3-phase circuits?
A: For 3-phase: VD = (√3 × Length × Current × Resistance) ÷ 1000. The factor is 1.732 instead of 2.
Q: Can I use voltage drop to size my wire?
A: Voltage drop is one factor in wire sizing. You must also consider ampacity (NEC Table 310.16), overcurrent protection, and grounding requirements. Always use the larger wire size when multiple factors apply.
Q: Why does distance matter more at 12 V than 120 V?
A: A 1.2 V drop is 1 % at 120 V but 10 % at 12 V. Low voltage systems require significantly larger conductors to maintain acceptable voltage drop percentages.
Q: How does temperature affect voltage drop?
A: Wire resistance increases about 0.4 % per degree Celsius above 20 °C. In a hot attic (50 °C), resistance can be 12 % higher than at room temperature, increasing voltage drop proportionally.
Q: How do I calculate voltage drop for DC circuits?
A: DC voltage drop uses the same formula as single-phase AC: VD = (2 × L × I × R) ÷ 1000. However, DC circuits don't have power factor considerations, making calculations simpler.
Q: What is voltage drop for parallel conductors?
A: For parallel conductors, divide the resistance by the number of conductors. Two parallel 1/0 AWG conductors have half the resistance of a single 1/0.
Q: How do I measure actual voltage drop?
A: Measure voltage at the panel and at the load while under full current. The difference is your voltage drop. For accuracy, use a true RMS meter and ensure connections are tight.
Q: How does voltage drop affect solar panels?
A: Solar DC circuits are particularly sensitive to voltage drop because MPPT controllers need minimum voltage to operate.