Voltage Drop Calculator
How to Calculate Voltage Drop
Voltage Drop Formulas
Single Phase: Vd = 2 × L × I × R / 1000
Three Phase: Vd = 1.732 × L × I × R / 1000
Where:
- Vd = Voltage drop (volts)
- L = Wire length (feet)
- I = Current (amperes)
- R = Resistance (ohms per 1000 ft)
National Electrical Code (NEC) Recommendations
- Branch Circuits: Maximum 3% voltage drop
- Feeders: Maximum 3% voltage drop
- Total (Branch + Feeder): Maximum 5% voltage drop
Example Calculation
A 120V single-phase circuit with 15A load, 100ft of 12 AWG copper wire:
- Resistance of 12 AWG copper: 1.93 ohms/1000ft
- Vd = 2 × 100 × 15 × 1.93 / 1000 = 5.79V
- Percentage = 5.79 / 120 × 100 = 4.83%
About Voltage Drop Calculator
Voltage Drop Calculator - Calculate Voltage Drop Instantly
Calculate voltage drop in electrical circuits instantly with our free voltage drop calculator. Determine voltage loss for single-phase and three-phase circuits based on wire size, length, current, and conductor material.
What is Voltage Drop?
Voltage drop is the reduction in voltage as electrical current flows through a conductor due to the conductor's resistance. All wires have some resistance, which causes voltage to decrease over distance. Excessive voltage drop can cause equipment to malfunction, reduce efficiency, and create safety hazards.
National Electrical Code (NEC) Recommendations
- Branch Circuits: Maximum 3% voltage drop
- Feeders: Maximum 3% voltage drop
- Total (Branch + Feeder): Maximum 5% voltage drop
Voltage Drop Formulas
Single-Phase Formula:
Vd = (2 × L × I × R) / 1000
Three-Phase Formula:
Vd = (1.732 × L × I × R) / 1000
Where:
- Vd = Voltage drop (volts)
- L = One-way wire length (feet)
- I = Load current (amperes)
- R = Conductor resistance (ohms per 1000 feet)
Example Calculation
Problem: Calculate voltage drop for a 120V single-phase circuit with 15A load, 100ft of 12 AWG copper wire.
- Resistance of 12 AWG copper: 1.93 ohms/1000ft
- Vd = (2 × 100 × 15 × 1.93) / 1000 = 5.79V
- Percentage = (5.79 / 120) × 100 = 4.83%
- Final Voltage = 120 - 5.79 = 114.21V
Result: The voltage drop is 4.83%, which exceeds the 3% recommendation for branch circuits. Use 10 AWG wire instead.
How to Reduce Voltage Drop
- Increase Wire Size: Use larger gauge wire (lower AWG number)
- Reduce Length: Shorten the distance between power source and load
- Use Copper: Copper has lower resistance than aluminum
- Reduce Current: Lower the load current on the circuit
- Higher Voltage: Higher voltage systems have less percentage drop for the same power
Wire Resistance Chart
Resistance values in ohms per 1000 feet at 75°C:
| AWG/kcmil | Copper | Aluminum |
|---|---|---|
| 14 AWG | 3.07 | 4.93 |
| 12 AWG | 1.93 | 3.10 |
| 10 AWG | 1.21 | 1.94 |
| 8 AWG | 0.764 | 1.22 |
| 6 AWG | 0.491 | 0.791 |
| 4 AWG | 0.308 | 0.496 |
| 2 AWG | 0.194 | 0.313 |
| 1/0 AWG | 0.122 | 0.197 |
| 4/0 AWG | 0.0608 | 0.0983 |
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