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Professional voltage drop calculator for electricians and engineers. Calculate voltage drop, wire size, and power loss for electrical circuits. NEC compliant tool with comprehensive formulas and examples based on National Electrical Code standards.
Voltage drop is the reduction in electrical potential (voltage) as current flows through a conductor's resistance. Every wire has inherent resistance that opposes current flow, causing energy loss in the form of heat and reducing the voltage available at the load end.
Basic Formula:
Vd = I Ć R
Where: Vd = Voltage Drop, I = Current (Amperes), R = Resistance (Ohms)
The National Electrical Code (NEC) provides recommendations for maximum voltage drop:
Branch Circuits: Maximum 3%
From panel to outlet/equipment
Feeders: Maximum 2%
From service to panel
Combined Total: Maximum 5%
Feeder + branch circuit combined
Note: These are recommendations, not requirements. However, following them ensures optimal system performance and efficiency.
Vd = 2 Ć K Ć I Ć L / CM
Or simplified: Vd = I Ć R (where R = 2 Ć L Ć resistance per foot)
Vd = Voltage drop (volts)
K = Resistivity constant (12.9 for copper, 21.2 for aluminum)
I = Current (amperes)
L = One-way length (feet)
CM = Circular mils (wire cross-sectional area)
2 = Factor for round-trip distance (hot + neutral)
Vd = ā3 Ć K Ć I Ć L / CM
Or simplified: Vd = 1.732 Ć I Ć R
ā3 (1.732) = Three-phase factor
Other variables same as single-phase
Note: Three-phase uses ā3 instead of 2 because of the phase relationship between conductors.
% Vd = (Vd / Vs) Ć 100
% Vd = Percentage voltage drop
Vd = Voltage drop (volts)
Vs = Source voltage (volts)
P = I Ć Vd
Or: P = IĀ² Ć R
P = Power loss (watts)
I = Current (amperes)
Vd = Voltage drop (volts)
R = Resistance (ohms)
Note: This power is wasted as heat in the conductors.
| AWG Size | Copper (Ī©/1000ft) | Aluminum (Ī©/1000ft) | Max Ampacity |
|---|---|---|---|
| 14 AWG | 3.07 | 5.06 | 15A |
| 12 AWG | 1.93 | 3.18 | 20A |
| 10 AWG | 1.21 | 1.99 | 30A |
| 8 AWG | 0.764 | 1.26 | 40A |
| 6 AWG | 0.491 | 0.808 | 55A |
| 4 AWG | 0.308 | 0.508 | 70A |
| 2 AWG | 0.194 | 0.319 | 95A |
| 1 AWG | 0.154 | 0.253 | 110A |
| 1/0 AWG | 0.122 | 0.201 | 150A |
| 2/0 AWG | 0.0967 | 0.159 | 175A |
| 3/0 AWG | 0.0766 | 0.126 | 200A |
| 4/0 AWG | 0.0608 | 0.100 | 230A |
Note: Resistance values at 75Ā°C (167Ā°F). Ampacity values for 75Ā°C insulation in conduit. Always consult NEC Table 310.16 for specific applications.
ā Advantages:
ā Disadvantages:
ā Advantages:
ā Disadvantages:
To achieve the same current-carrying capacity and voltage drop, aluminum wire must be approximately 2 AWG sizes larger than copper:
Copper
12 AWG
ā Aluminum
10 AWG
Copper
10 AWG
ā Aluminum
8 AWG
Copper
8 AWG
ā Aluminum
6 AWG
Scenario: 120V circuit, 15A load, 75 feet from panel
Wire: 12 AWG copper
Calculation:
ā Result: Exceeds 3% - Consider 10 AWG
Scenario: 240V feeder, 100A load, 150 feet
Wire: 1 AWG copper
Calculation:
ā Result: Within 2% - Acceptable for feeder
Scenario: 480V 3-phase, 50A motor, 200 feet
Wire: 6 AWG copper
Calculation:
ā Result: Well within 3% - Good design
Voltage drop is the reduction in voltage as electrical current flows through a conductor's resistance. It's critical for electrical system design because excessive voltage drop can cause equipment malfunction, reduced efficiency, overheating, and NEC code violations. The National Electrical Code recommends keeping voltage drop under 3% for branch circuits and 5% total for combined feeder and branch circuits.
Voltage drop is calculated using Ohm's Law: Vd = I Ć R, where I is current in amperes and R is the total conductor resistance in ohms. For single-phase circuits, multiply the one-way length by 2 to account for both conductors. For three-phase circuits, multiply by ā3 (1.732). This calculator automatically performs these calculations based on wire gauge, material, and circuit type.
According to NEC Article 210.19(A) and 215.2(A), voltage drop recommendations are: 3% maximum for branch circuits, 2% maximum for feeders, and 5% maximum combined total. For sensitive equipment like motors and electronics, professional electricians recommend staying under 2% to ensure optimal performance and equipment longevity.
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This voltage drop calculator is developed based on established electrical engineering principles and NEC guidelines. All formulas and resistance values are derived from industry-standard references including NEC tables, IEEE standards, and manufacturer specifications. The calculator uses wire resistance data at 75Ā°C (167Ā°F) as specified in NEC Table 8 for accurate real-world calculations. Regular updates ensure compliance with the latest electrical codes and industry best practices.