Voltage Drop Calculator

What this voltage drop calculator is for

Every conductor has resistance. Push current through a long or undersized wire and the load sees less voltage than the panel delivers. This voltage drop calculator estimates drop in volts and percent for single-phase and three-phase runs—helpful during rough-in planning, troubleshooting dim lights at the far end of a branch, or comparing AWG sizes before you order wire.

Why voltage drop matters

Excessive drop wastes energy as heat, reduces motor torque, causes incandescent dimming, and can make electronic power supplies misbehave. Industry guidance commonly limits branch circuits to about 3% drop and total feeder plus branch to about 5% of system voltage—though local code and equipment listings may differ. Treat these thresholds as planning anchors, not automatic pass/fail for every jurisdiction.

How the calculation works

The tool uses DC resistance per 1,000 feet for standard copper AWG sizes (14 through 2/0). Aluminum conductors apply a 1.6× resistance multiplier relative to copper of the same gauge. Resistance per foot is:

R = (Ω per 1,000 ft ÷ 1,000) × material multiplier

Voltage drop depends on phase:

• Single-phase: Vd = 2 × length × current × R (factor of 2 accounts for outbound and return conductors over one-way length).
• Three-phase: Vd = 1.732 × length × current × R (√3 line-to-line geometry).

Percent drop = Vd ÷ system voltage × 100. Voltage at load = system voltage − Vd.

Reading pass/fail guidance

• ≤ 3%: Generally good for branch-circuit planning.
• 3–5%: Acceptable in many total-circuit narratives—confirm combined feeder drop if this is only the branch segment.
• > 5%: Excessive for most residential and commercial comfort targets—upsizing AWG, shortening the run, or reducing load current is warranted.

AWG scenario table

After you calculate, the scenario table compares 12, 10, 8, and 6 AWG using your current, length, voltage, phase, and material inputs. Scan how much each step up in conductor size reduces drop—often the fastest way to decide whether 12 AWG is sufficient or 10 AWG is worth the material cost on a long garage subfeed.

Copper vs aluminum

Aluminum conductors of the same AWG have higher resistance than copper. The calculator applies a 1.6× multiplier on resistance, a common planning approximation. Actual values vary with temperature, stranding, and installation conditions. For critical feeders, use manufacturer tables at the operating temperature your code article specifies.

Related electrical and mechanical tools

Feeder sizing often pairs with transformer and load planning—see the transformer calculation table calculator. For hydronic and pump head work on the same project, the boiler feed pump calculator and swimming pool pump size calculator cover fluid-side sizing. Fabrication teams may cross-check conduit bends with the bend allowance calculator when routing long pulls.

What this tool does not model

• AC impedance, power factor, harmonics, or parallel conductor adjustments.
• Ambient temperature derating, conduit fill, or burial depth corrections.
• Voltage drop on the utility service entrance or combined multi-segment paths.
• Motor starting (inrush) drop, which can briefly exceed steady-state estimates.

Sample branch-circuit check

A 120 V single-phase garage circuit draws 20 A over 100 ft one-way on 12 AWG copper. Resistance ≈ 1.93 Ω/1000 ft → R ≈ 0.00193 Ω/ft. Vd = 2 × 100 × 20 × 0.00193 ≈ 7.72 V (≈ 6.4% drop)—above the 3% branch comfort zone, so stepping up to 10 AWG or shortening the run is prudent. Run the calculator with your exact values to compare the scenario table across 12, 10, 8, and 6 AWG without manual table lookup.

Disclaimer

Educational planning only. Electrical work must comply with NEC (or local equivalent), equipment listings, and licensed design where required. Verify conductor ampacity, overcurrent protection, and voltage drop with qualified personnel before energizing any circuit.