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Voltage Drop Calculator

Voltage drop in volts and percent for a run, from the circular-mil formula with NEC Chapter 9 Table 8 areas.

Inputs

The circuit

The current the circuit will carry.

The voltage at the source end of the run.

Single phase uses 2 in the formula, three phase 1.732.

Distance from the source to the load. Do not double it - the formula does.

The conductor

The size installed or proposed.

Copper or aluminum. Aluminum needs roughly two sizes larger for the same ampacity and is not made at 14 AWG.

Result

Voltage drop
—
Voltage lost in the run
—
Voltage reaching the load
—
Conductor circular mils
—
Circular mils needed for 3 percent
—
Circular mils needed for 5 percent
—
Power lost in the conductors
—
—
Where this stops applying
—
Calculation details
Drop constant K
—
One-way length
—
Phase multiplier
—

What to check

  • Load amperes, phase and nominal voltage
  • One-way run length, not the round trip
  • Conductor size and material

Standards referenced

  • NFPA 70 National Electrical Code (NEC) — 2023 edition · Branch circuits, conductors, protection, raceways, and services. Local AHJ adoptions vary; the enforcing edition is set by the jurisdiction.

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