Voltage Drop Calculator - Wire Length, Current and Cable Size

Agarapu Ramesh — Editor and content reviewer
Editorially reviewed Reviewed by Agarapu Ramesh, science educator (chemistry). LinkedIn
Last reviewed: May 2026 | Ohm's law and standard electrical engineering formulas

Voltage drop is the voltage lost in the wire before power reaches the load. Long runs, high current and small conductors make it worse. The load may still run, but motors start weaker and electronics complain.

Formula at a glance

  • single-phase DC/2-wire: Vdrop = 2 x I x R x one-way length
  • three-phase: Vdrop = 1.732 x I x R x one-way length
  • drop percent = Vdrop / supply V x 100

Field note: Voltage drop is not the same as breaker protection. A cable can be safe from overheating and still deliver poor voltage at the far end.

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Voltage Drop Calculator

Calculate voltage loss in electrical wires

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Formulas

1-PhaseVd = 2 × I × R × L ÷ 1000
3-PhaseVd = √3 × I × R × L ÷ 1000

NEC Guidelines

Circuit TypeMax Drop
Branch Circuits3%
Feeders2%
Total Combined5%

Wire Resistance (Ω/1000ft)

AWGCopperAluminum
142.5254.04
121.5882.54
100.9991.60
80.6281.00

Related

Wire GaugeOhm's Law

How to use the Voltage Drop Calculator

Use this for a first sizing pass, then check the actual code table, installation method, conductor material and temperature rating. A calculator can point you in the right direction. It cannot inspect the job.

Worked example

Example: a 20 A load with 1.5 V drop on a 120 V circuit has 1.25% voltage drop.

Practical checks before you trust the number

  • Use one-way length, then the formula handles the return path where needed.
  • Copper and aluminum have different resistance.
  • Temperature, conduit fill and installation method can change ampacity.

Common mistake

Voltage drop is not the same as breaker protection. A cable can be safe from overheating and still deliver poor voltage at the far end.

Sources and references

Related calculators

Frequently Asked Questions

Voltage drop (single-phase): V_drop = 2 × L × I × R per meter. For three-phase: V_drop = √3 × L × I × R per meter. L is one-way length, R is per-meter resistance of the conductor. Example: 4 mm² Cu, 30 A, 30 m run → V_drop = 2 × 30 × 30 × 0.0046 = 8.3 V (3.6% on 230 V).

Single-phase: V_drop = 2 × L × I × R/m. Three-phase: V_drop = √3 × L × I × R/m. L is one-way circuit length. R/m comes from cable tables (copper at 20°C: about 0.018 Ω/m for 1 mm², 0.0046 Ω/m for 4 mm²).

3% on branch circuits, 5% on feeders, totaling 5% maximum from supply to load. Many codes are stricter for sensitive loads — 1% for medical equipment, 2% for some sensitive electronics. Always check the load's tolerance and the applicable code.

Drop scales linearly with length. Doubling the length doubles the voltage drop for the same current and gauge. So a 30 A circuit on 4 mm² that drops 3% in 25 m drops 6% in 50 m and 9% in 75 m. Always size up for long runs to keep drop under code limits.

Three-phase: V_drop = √3 × L × I × R/m. Use line voltage as the reference for percentage drop. The √3 factor reduces the drop compared to single-phase for the same current and length, which is one reason three-phase systems handle longer runs better.

Bigger wire (lower R), shorter run (lower L), or higher voltage (higher reference for percentage drop). For long runs, doubling wire size halves voltage drop. Going from 230 V single-phase to 415 V three-phase at the same kW reduces current by √3 and drop accordingly. For solar feeders, we frequently upsize cable.

Yes, this is one of its main uses. Enter cable length, current, voltage, and material; the calculator checks if a given gauge meets the code drop limit. If not, it suggests the next size up. Always confirm against local code and apply temperature derating.