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Voltage Drop Calculator

Wires have resistance, so voltage falls along their length. Check that a cable run keeps the drop within acceptable limits.

How it works Report a problem

Voltage Drop Calculator

Calculate wire voltage loss — NEC compliance, wire sizing & efficiency

Vdrop = I × R  |  R = ρ × (2L / A)  |  NEC recommends ≤ 3% drop
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Voltage Drop
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Received Voltage
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Wire Resistance (R)
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Power Lost
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Drop %0%
0%3% NEC10%+
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Step-by-Step
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About the Voltage Drop Calculator

Wires have resistance, so voltage falls along their length. Check that a cable run keeps the drop within acceptable limits.

How to use the Voltage Drop Calculator

  1. Choose the Wire Material and Wire Gauge (AWG).
  2. Enter the Source Voltage, the Current (Load) and the One-Way Wire Length.
  3. Press Calculate Voltage Drop.

Formula and method

Voltage drop = I × R, where the resistance is R = ρ × (2 × length) ÷ cross-section area. The length is doubled because current flows out and back. Copper resistivity is about 1.68 × 10⁻⁸ Ω·m.

Worked example

15 A over 30 m (one way) of 12 AWG copper (3.31 mm²) gives a resistance of 0.30 Ω and a drop of about 4.6 V, which is 3.8% of a 120 V supply.

Tips

  • A common guideline is to keep the drop below 3% on a branch circuit.
  • Use a thicker wire (lower gauge number) or shorter run to reduce the drop.
  • Follow your local electrical code, and use a qualified electrician for installations.

Frequently asked questions

Is copper or aluminium better?
Copper conducts better, so a copper wire drops less voltage than an aluminium wire of the same size.