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Voltage Drop Calculator

Estimate voltage drop over a wire run using the NEC formula, by gauge and length.

Introduction

Voltage Drop Calculator — Estimate voltage drop over a wire run using the NEC formula, by gauge and length. Enter Source voltage (V), Current (A), One-way length (ft), Wire gauge, Conductor material, Phase to get an instant, accurate result.

Formula

NEC Chapter 9, Table 8 approximate method: VD = (2 × K × I × L) / CM for single-phase, or VD = (√3 × K × I × L) / CM for three-phase, where K = 12.9 ohm-cmil/ft for copper (21.2 for aluminum), I is load current, L is one-way conductor length in feet, and CM is the conductor's circular-mil area.

Step-by-step

  1. Enter the Source voltage (V).
  2. Enter the Current (A).
  3. Enter the One-way length (ft).
  4. Enter the Wire gauge.
  5. Enter the Conductor material.
  6. Enter the Phase.
  7. Click Calculate to see your result instantly.

Real-world example

Example: With Source voltage (V) = 120, Current (A) = 20, One-way length (ft) = 100, Wire gauge = 12, Conductor material = copper, Phase = no, the Voltage Drop Calculator gives Voltage Drop V: 7.902, Voltage Drop Pct: 6.585, Circular Mils Used: 6530.

Frequently Asked Questions

What voltage drop percentage is acceptable?
The NEC does not mandate a limit for most branch circuits, but its informational notes recommend keeping voltage drop to 3% or less on a branch circuit and 5% or less total (feeder + branch circuit combined) for reasonable efficiency and equipment performance.
Why does one-way length matter, not round-trip?
The formula already accounts for both the outgoing and return conductor (via the factor of 2 for single-phase, or √3 for three-phase), so you enter only the one-way distance from source to load.
Does wire material matter?
Yes — aluminum has higher resistance than copper for the same gauge, so its K constant (21.2) is larger, producing more voltage drop than copper (K=12.9) at the same size and length.
What inputs does the Voltage Drop Calculator need?
You'll need to provide: Source voltage (V), Current (A), One-way length (ft), Wire gauge, Conductor material, Phase. All fields use sensible defaults, so you can see a working example immediately and then adjust the values to match your own numbers.
How accurate is the Voltage Drop Calculator?
The Voltage Drop Calculator applies the formula exactly as calculated — NEC Chapter 9, Table 8 approximate method: VD = (2 × K × I × L) / CM for single-phase, or VD = (√3 × K × I × L) / CM for three-phase, where K = 12.9 ohm-cmil/ft for copper (21.2 for aluminum), I is load current, L is one-way conductor length in feet, and CM is the conductor's circular-mil area. — so results are precise for the inputs you provide. Accuracy depends on entering correct, realistic input values.
Is the Voltage Drop Calculator free to use?
Yes, the Voltage Drop Calculator is completely free, requires no signup, and runs instantly in your browser.

About the Voltage Drop Calculator

The Voltage Drop Calculator uses a real, verifiable formula — NEC Chapter 9, Table 8 approximate method: VD = (2 × K × I × L) / CM for single-phase, or VD = (√3 × K × I × L) / CM for three-phase, where K = 12.9 ohm-cmil/ft for copper (21.2 for aluminum), I is load current, L is one-way conductor length in feet, and CM is the conductor's circular-mil area. — so results are accurate every time, not an approximation.