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.

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What 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) / CM

Where: 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) / CM

The 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 SizeCircular MilsAmpacity (75°C)
14 AWG4,11020A
12 AWG6,53025A
10 AWG10,38035A
8 AWG16,51050A
6 AWG26,24065A
4 AWG41,74085A
2 AWG66,360115A
1/0 AWG105,600150A

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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