Voltage Drop

Calculate voltage drop and wire size for a given circuit.

System Phase

Single-Phase (1φ)Three-Phase (3φ)

Multiplier: 2 — current travels out and back on two conductors

System VoltageConductor MaterialLoad Current (Amps)One-Way Run Length (ft)Wire Size (AWG / kcmil) — cmil from NEC Ch. 9, Table 8Start Free Trial

How the Circular Mils (cmil) Method Works

To calculate voltage drop using the circular mils method, you rely on the physical properties of the conductor found in NEC Chapter 9, Table 8. The formula rearranges Ohm's Law (V = I × R) to solve for voltage drop based on the conductor's cross-sectional area.

General FormulaVD = (K × I × L) / cmil

K — Resistivity constant: 12.9 for Copper, 21.2 for Aluminum (at 75°C)
I — Load current in amperes
L — One-way circuit length in feet
cmil — Conductor area in circular mils (from NEC Table 8)

Single-Phase (1φ)

VD = (2 × K × I × L) / cmil

Current travels out and back — the distance is doubled.

Three-Phase (3φ)

VD = (1.732 × K × I × L) / cmil

Phase conductors are offset by 120° — multiplier is √3 ≈ 1.732.

Step-by-Step Application

1. Determine Current (I): Identify the full-load current of the load.
2. Determine Length (L): Measure the one-way distance from source to load.
3. Find cmil: Look up your wire size in NEC Chapter 9, Table 8 to get its area in circular mils.
4. Choose K: Use 12.9 for Copper or 21.2 for Aluminum.
5. Calculate: Plug values into the formula above. Compare result to 3% of system voltage.

Practical Example

100 ft copper circuit (single-phase), 50A load, #6 AWG wire (cmil = 26,240):

VD = (2 × 12.9 × 50 × 100) / 26,240 = 129,000 / 26,240 ≈ 4.92V

On a 120V circuit: 4.92 / 120 = 4.1% — exceeds the 3% recommendation. Upgrade to #4 AWG (cmil = 41,740) to bring drop to 3.09V (2.6%).

The answers we give you are for educational purposes only. Please verify with your code book, your journeyman or master electrician, and your local Authority Having Jurisdiction (AHJ).