The Feeder Voltage Drop Calculator is designed to help engineers, electricians, and technicians determine the voltage loss along electrical feeder lines. When electricity travels through a conductor over distance, part of the voltage is lost due to resistance. This tool ensures that the voltage at the load end remains within acceptable limits to maintain equipment performance and safety.
This calculator is especially important for long cable runs or circuits with high current loads. By knowing the voltage drop, users can make informed decisions on wire size, material (copper or aluminum), and installation practices to comply with national standards such as NEC or IEC.
formula of Feeder Voltage Drop Calculator
For Single-Phase AC Systems:
Voltage Drop (V) = 2 × I × L × R
Where:
- V = voltage drop (volts)
- I = current in amperes
- L = one-way length of feeder (meters or feet)
- R = resistance of conductor per unit length (ohm/m or ohm/ft)
For Three-Phase AC Systems:
Voltage Drop (V) = √3 × I × L × R
(√3 ≈ 1.732)
With Reactance (when applicable):
Single-phase with reactance:
V = 2 × I × L × √(R² + X²)
Three-phase with reactance:
V = √3 × I × L × √(R² + X²)
Where:
- X = reactance per unit length (ohm/m or ohm/ft), used in inductive circuits
Percentage Voltage Drop:
Voltage Drop (%) = (Voltage Drop / Source Voltage) × 100
Use this to ensure the drop stays under 3% for feeders, as recommended by industry standards.
Reference Table: Typical Resistance Values (Copper at 75°C)
| Wire Size (AWG) | Resistance (ohm/1000 ft) | Max Current (A) |
|---|---|---|
| 14 AWG | 2.525 | 15 |
| 12 AWG | 1.588 | 20 |
| 10 AWG | 0.999 | 30 |
| 8 AWG | 0.628 | 40-50 |
| 6 AWG | 0.395 | 55-65 |
| 4 AWG | 0.248 | 70-85 |
| 2 AWG | 0.156 | 95-115 |
| 1/0 AWG | 0.0983 | 150 |
Use manufacturer-specific values for aluminum.
Example of Feeder Voltage Drop Calculator
Suppose:
- System: Single-phase
- Current (I): 50 A
- One-way Length (L): 100 ft
- Wire: 6 AWG copper (R = 0.395 ohm/1000 ft)
- Source Voltage = 240 V
Step 1: Convert resistance
R = 0.395 / 1000 = 0.000395 ohm/ft
Step 2: Voltage Drop
V = 2 × 50 × 100 × 0.000395 = 3.95 V
Step 3: Voltage Drop (%)
Voltage Drop % = (3.95 / 240) × 100 ≈ 1.65%
This is within the acceptable range, so 6 AWG copper is suitable for this feeder.
Most Common FAQs
Voltage drop affects the performance of connected equipment. Too much drop can lead to poor efficiency, overheating, or failure of devices.
Yes. The formulas use different factors (2 for single-phase and √3 for three-phase) because the systems distribute current differently.
You can reduce it by increasing the wire size, using a material with lower resistance (like copper), shortening the cable run, or decreasing the load current.