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Voltage Drop Calculator

Percent voltage drop and receiving-end voltage for a feeder or branch circuit.

About this calculator

Voltage drop along a feeder is driven by the conductor's resistance and reactance, the load current, the length of the run, and the load's power factor: VD = k · I · L · (R·cosθ + X·sinθ), where k is 2 for single-phase (accounting for the out-and-back conductor path) or √3 for three-phase, and R, X are per-unit-length conductor properties. This calculator looks up representative resistance and reactance for the selected conductor size and material, then reports both the absolute drop and the percentage relative to the source voltage, along with the resulting receiving-end voltage.

Use it to check whether a feeder or branch circuit conductor is large enough before finalizing a design, or to explain why a piece of equipment at the end of a long run is seeing low voltage. NEC 210.19(A) and 215.2(A) informational notes recommend keeping combined branch-circuit-plus-feeder drop to 5%, with no more than 3% on either segment alone — this calculator flags results against a 3% default you can adjust.

Common pitfall: this is the standard approximate voltage-drop formula, accurate for typical power factors and conductor sizes, but it is not exact for very large conductors or unusual X/R ratios, and it does not account for conductor temperature above 75°C, harmonics, or unbalanced loading. It also assumes a lagging (inductive) power factor; for a leading power factor the reactive term subtracts rather than adds, and the actual drop would be slightly lower than shown here.

Assumptions

  • Lagging (inductive) power factor assumed; a leading power factor would produce slightly less drop than shown.
  • Conductor resistance and reactance are representative THWN/THHN values in non-metallic conduit, not a substitute for the governing code table.
  • Balanced loading assumed for the three-phase case; steady-state conditions (not motor starting).

When to use this calculator

Appropriate for

  • Checking whether a feeder or branch-circuit conductor keeps voltage drop within a target percentage before finalizing a size
  • Explaining why equipment at the end of a long run sees reduced voltage
  • Comparing conductor sizes or materials for a balanced, steady-state load at a typical (lagging) power factor

Not suitable for

  • Very large conductors or unusual X/R ratios, where the approximate R·cosθ + X·sinθ formula loses accuracy
  • Code-compliance sizing or final conductor selection — the drop limits used here are informational recommendations, not the governing ampacity or termination rules
  • Transient, motor-starting, or harmonic conditions, and unbalanced systems, none of which this steady-state model captures

What this calculator does not cover

  • Uses the standard approximate formula (R·cosθ + X·sinθ) — adequate for typical feeders, less exact for very large conductors or unusual X/R ratios.
  • Conductor R and X are representative 75 °C values in non-metallic conduit; steel raceway raises reactance, and the governing code table always takes precedence.
  • Steady-state load current only — motor-starting dip is a separate calculation (see the motor starting calculator), and a leading power factor slightly reduces the drop below this result.
  • One segment at a time — the recommended limits apply to the combined feeder-plus-branch drop, which you must total across segments yourself.
  • As with every calculator on this site: results are preliminary and educational, are not verified for any specific installation, and must be reviewed against the applicable code edition and stamped by a licensed Professional Engineer before real-world use.

Frequently asked questions

What voltage drop percentage is acceptable?

NEC informational notes recommend no more than 3% on a branch circuit or feeder alone, and no more than 5% combined from source to the farthest outlet. These are recommendations, not hard code requirements, but are widely used as the design target.

Why does power factor affect voltage drop?

Voltage drop depends on both the conductor's resistance and reactance. The load's power factor determines how much of the current is in phase with the voltage (interacting with resistance) versus 90° out of phase (interacting with reactance), so the formula weights R and X by cosθ and sinθ respectively.

Does this calculator size the conductor for me?

It evaluates the voltage drop for a conductor size you select. Increase the size and re-run the calculation if the drop exceeds your limit — pair it with the cable ampacity calculator to confirm the size is also thermally adequate.

References

  • NEC 210.19(A) Informational Note No. 4 and 215.2(A) Informational Note No. 2 — recommended voltage drop limits
  • IEEE Std 141 (Red Book), Ch. 3 — Voltage Considerations

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