Voltage Drop Calculator
Calculate voltage drop for electrical circuits instantly. NEC-compliant calculations for single-phase and three-phase systems with support for all common wire gauges.
Use Calculator in AppWhat is Voltage Drop?
Voltage drop is the reduction in voltage in an electrical circuit between the source and the load. It occurs due to the resistance of the conductors carrying current. Understanding and calculating voltage drop is critical for proper electrical system design and NEC code compliance.
Excessive voltage drop can cause equipment malfunction, reduced motor efficiency, dimming lights, and overheating conductors. The NEC recommends limiting voltage drop to 3% for branch circuits and 5% total for feeders and branch circuits combined.
Voltage Drop Formula
Single-Phase Voltage Drop Formula
VD = (2 × K × I × D) / CMWhere: VD = Voltage Drop, K = Resistivity constant (12.9 for copper, 21.2 for aluminum), I = Current in amps, D = One-way distance in feet, CM = Circular mils of conductor
Three-Phase Voltage Drop Formula
VD = (1.732 × K × I × D) / CMThe three-phase formula uses 1.732 (√3) instead of 2, as three-phase circuits have a different current relationship.
NEC Voltage Drop Requirements
According to the National Electrical Code (NEC), voltage drop recommendations are found in several informational notes:
- NEC 210.19(A) Informational Note No. 4: Recommends 3% maximum voltage drop for branch circuits
- NEC 215.2(A)(1) Informational Note No. 2: Recommends 3% maximum voltage drop for feeders
- Combined Total: 5% maximum from service point to final outlet
While these are recommendations (not requirements), many electrical inspectors and engineers use them as standards for proper electrical design.
Common Wire Gauges for Voltage Drop
| Wire Size | Circular Mils | Ampacity (75°C) |
|---|---|---|
| 14 AWG | 4,110 | 20A |
| 12 AWG | 6,530 | 25A |
| 10 AWG | 10,380 | 35A |
| 8 AWG | 16,510 | 50A |
| 6 AWG | 26,240 | 65A |
| 4 AWG | 41,740 | 85A |
| 2 AWG | 66,360 | 115A |
| 1/0 AWG | 105,600 | 150A |
Voltage Drop Calculation Example
Problem:
Calculate the voltage drop for a 120V single-phase circuit with 20 amps load current, using 12 AWG copper wire over a 100-foot one-way distance.
Solution:
VD = (2 × K × I × D) / CM
VD = (2 × 12.9 × 20 × 100) / 6,530
VD = 51,600 / 6,530
VD = 7.9 volts (6.58%)
This exceeds the 3% recommendation. Consider using 10 AWG wire or reducing the circuit length.
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