Voltage Drop Calculator (Copper & Aluminum)

Long runs to condensers and rooftop units can drop enough voltage to affect motor performance.

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Conductor areas from NEC Chapter 9 Table 8; approximate K at ~75 °C. Verify with CEC/NEC requirements.

Formula

Single-phase: VD = 2 × K × I × L ÷ CM
Three-phase: VD = 1.732 × K × I × L ÷ CM

Variables

K: — 12.9 (copper) or 21.2 (aluminum) Ω·cmil/ft
L: — One-way length (ft)
CM: — Conductor area in circular mils

Example calculation

10 AWG copper, 24 A, 100 ft one-way, 240 V single-phase → 5.97 V (2.5%).

Practical HVAC use

Check condenser and long branch circuits before installation.

Common mistakes

Entering round-trip length (the formula already doubles it).
Ignoring voltage drop at motor starting.

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Educational tool. Results are estimates from the stated formulas; always confirm with manufacturer data and the code adopted in your jurisdiction. All calculators