Voltage Drop Calculator
Calculate voltage drop in electrical circuits for proper wire sizing
Quick Examples
What is Voltage Drop?
Voltage drop is the reduction in voltage as electrical current flows through the resistance of wires in a circuit. When electricity travels through conductors, the resistance of the wire causes some of the electrical energy to be converted to heat, resulting in a lower voltage at the load than at the source.
According to the National Electrical Code (NEC), the maximum recommended voltage drop for branch circuits is 3% and for feeders is 2%, with a combined maximum of 5% to the farthest outlet. Excessive voltage drop can cause equipment malfunction, overheating, reduced efficiency, and shortened lifespan of electrical devices.
Why is Voltage Drop Important?
Equipment Performance
Motors, lights, and electronic devices require proper voltage to operate efficiently. Low voltage can cause motors to overheat and lights to dim.
Energy Efficiency
Voltage drop represents wasted energy as heat. Minimizing voltage drop improves system efficiency and reduces energy costs.
Code Compliance
The NEC sets voltage drop limits to ensure safe and proper operation of electrical systems. Compliance is required for inspections.
Equipment Safety
Excessive voltage drop can cause overheating in wires and equipment, creating potential fire hazards and safety risks.
Voltage Drop Formulas
Single-Phase Formula
VD = 2 × I × R × D / 1000Where VD is voltage drop (volts), I is current (amperes), R is resistance (ohms per 1000 feet), and D is one-way distance (feet). The factor of 2 accounts for the round-trip current path.
Three-Phase Formula
VD = √3 × I × R × D / 1000For three-phase systems, the square root of 3 (approximately 1.732) is used instead of 2. This formula applies to balanced three-phase loads.
Percentage Formula
VD% = (VD / V) × 100Voltage drop percentage is calculated by dividing the voltage drop by the source voltage and multiplying by 100. NEC recommends keeping this below 3% for branch circuits.
How to Reduce Voltage Drop
Increase Wire Size
Use larger gauge wire (lower AWG number) to reduce resistance. This is the most common solution for excessive voltage drop.
Reduce Distance
Minimize the length of wire runs by relocating power sources or loads closer together when possible.
Increase Voltage
Use a higher supply voltage (e.g., 240V instead of 120V) to reduce current and therefore voltage drop for the same power delivery.
Use Copper Conductors
Copper has lower resistance than aluminum. While more expensive, copper requires smaller wire sizes for the same voltage drop.
Frequently Asked Questions
What is an acceptable voltage drop?
The NEC recommends a maximum voltage drop of 3% for branch circuits and 2% for feeders, with a combined maximum of 5%. For sensitive equipment, aim for even lower values (1-2%).
Why does wire size affect voltage drop?
Larger diameter wires have lower resistance per unit length. Lower resistance means less voltage is lost as current flows through the wire. This is why increasing wire size (going to a lower AWG number) reduces voltage drop.
Should I use copper or aluminum wire?
Copper has about 60% lower resistance than aluminum, making it more efficient. However, aluminum is lighter and less expensive. For the same performance, aluminum requires approximately two gauge sizes larger than copper. The choice depends on your budget, installation requirements, and local code.
How does temperature affect voltage drop?
Wire resistance increases with temperature. Standard resistance values are given at 75°C. This calculator applies a temperature correction factor to account for operation at different temperatures, making calculations more accurate for real-world conditions.
What is the difference between single-phase and three-phase calculations?
Single-phase uses a multiplier of 2 (accounting for current flow in both conductors), while three-phase uses √3 (approximately 1.732) due to the phase relationships. Three-phase systems typically have about 13% less voltage drop than single-phase for the same load.
Is this calculator accurate for real installations?
This calculator uses industry-standard resistance values and formulas. However, actual installations may have additional factors like connections, conduit fill, and harmonics. Always consult local electrical codes and consider hiring a licensed electrician for critical installations.