NEC Voltage Drop Calculator

Circuit Parameters

Phase Type

Single-Phase (1ϕ) Three-Phase (3ϕ)

Conductor Material

Copper (Cu) Aluminum (Al)

Conductor Size

Source Voltage (V)

Load Current (A)

One-Way Length (ft)

Power Factor

0.01 – 1.00 (resistive = 1.0)

Circuit Type (for NEC check)

Results

Voltage Drop

—

volts

Vd %

—

percent

Voltage at Load

—

volts

Worked Steps

Reverse Calculator — Minimum Conductor Size

Max Allowed Vd%

Enter parameters above and a max Vd% to see the minimum conductor size.

Conductor Circular Mils Reference

Show / hide table

Size	Circular Mils (CM)	Approx. Area (mm²)

NEC 2023 Code References

210.19(A) Informational Note No. 4 — Conductors for branch circuits sized to prevent a voltage drop exceeding 3% at the farthest outlet and a maximum total voltage drop on both feeders and branch circuits of 5% provide reasonable efficiency of operation.

215.2(A)(1) Informational Note No. 2 — Same 3%/5% voltage drop guidance applies to feeder conductors.

310.14 — Conductor ampacity tables; conductor temperature ratings and material properties.

Formula basis — The "K factor" method uses the resistivity of the conductor material at 75°C:
K_Cu = 12.9 Ω⋅CM/ft
K_Al = 21.2 Ω⋅CM/ft
These constants assume resistivity at operating temperature; they are accepted engineering approximations used widely in NEC-based calculations.

Study aid only — verify against the adopted code edition / a licensed professional before use in real project/estimate documentation.