Is length one-way or round-trip?
Enter one-way length. The single-phase formula already includes the return path factor of 2.
Voltage Drop Calculator
Estimate how much voltage you lose over a cable run based on current, length, conductor material, and wire size.
Estimate voltage drop
Enter load current, one-way length, system type, material, and AWG size.
About this calculator
How to use Voltage Drop Calculator, plus the formulas and logic behind the results.
Tool description
The Voltage Drop Calculator estimates conductor voltage drop for sizing checks and long cable runs. It uses circular-mil AWG data with standard K factors.
How to use
- Choose single-phase/DC or three-phase.
- Select copper or aluminum and the AWG size.
- Enter current (A), one-way length (ft), and source voltage for percent drop.
- Select Calculate to view volts dropped and percent of source.
Formulas & reference
single/DC: VD = (2 × K × I × D) ÷ CMthree-phase: VD = (1.732 × K × I × D) ÷ CM% drop = (VD ÷ Vsource) × 100
Estimate basis
- Single-phase / DC: VD = (2 × K × I × D) ÷ CM
- Three-phase: VD = (√3 × K × I × D) ÷ CM
- K ≈ 12.9 copper, 21.2 aluminum (ohm·CM/ft)
- D is one-way length in feet
Calculation logic
Circular mils come from a built-in AWG table. K depends on material. The system type selects the 2-wire or √3 multiplier. Percent drop uses the optional source voltage.
This is a common estimating formula for copper/aluminum conductors. Temperature, conduit fill, power factor, and local code rules may change real-world drop.
Voltage Drop Calculator FAQ
Common questions about this calculator and how to use the results.
What does K mean?
K is a resistivity constant in ohm·circular-mils per foot for the conductor metal.
Is this code-compliant sizing?
It is an estimate. Always verify with your local electrical code and manufacturer data.
Why include source voltage?
It converts absolute drop into a percentage of supply voltage.
Does PF matter?
This estimate assumes essentially resistive/DC-style drop. Low power factor can increase AC drop.
Can I use meters instead of feet?
Convert meters to feet first (m × 3.28084).