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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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Every conductor resists a little, so a load at the end of a long cable receives less voltage than the source provides: that loss is voltage drop. This calculator estimates it from the current, the run length and the conductor's resistance, so cables can be sized before they are installed.

How to use this calculator

  • Enter the current the load draws (amps).
  • Enter the one-way cable length and the conductor details (material/size, or its resistance per unit length).
  • Press calculate to see the voltage drop and the voltage remaining at the load.

Why long runs punish small cables

Drop grows with length and current and shrinks with thicker conductors, which is why a garden pump at the end of 50 metres of thin cable sulks while the same pump near the house runs fine. Standards typically cap design drop at about 3 to 5% of supply voltage; beyond that, motors overheat, lights dim and electronics misbehave.

Frequently asked questions

What is the voltage drop formula?
For a DC or single-phase run: V drop = 2 × current × resistance per unit length × length (the 2 covers the outward and return conductors).
How do I reduce voltage drop?
Use a thicker conductor, shorten the run, reduce the current, or raise the system voltage (drop is a smaller percentage of a bigger supply).
Why does voltage drop matter for solar and batteries?
Low-voltage systems suffer proportionally: losing 1 volt from 12 is over 8%, while the same volt from 230 is negligible. Short, thick cables are critical at 12 and 24 volts.