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    Engineering / Conductor Voltage Drop

    Voltage Drop Calculator

    Conductor voltage drop and end-of-line voltage for a run, with AWG resistance tables, NEC percentage checks and three-phase support.

    ToolCalculatorExecutionIn browserCategoryEngineering
    VD
    WIRE
    OHM
    ⚙
    ZorvLabsOHMIC ENGINE
    SYSTEM STATUS
    CIRCUIT: CLOSED
    CIRCUIT PARAMETERS
    NEC DATA
    ESTIMATED
    CUSTOM Z
    WIRE MATERIAL
    WIRE SIZE (AWG/kcmil)
    CONDUIT TYPE
    SYSTEM VOLTAGE
    PHASE
    DISTANCE
    LOAD (AMPS)
    POWER FACTOR
    CIRCUIT THERMAL MAP
    THERMAL: WARM
    VOLTAGE INTEGRITY GAUGE
    CALCULATION OUTPUT
    4.17
    VOLTAGE DROP (V)
    PERCENTAGE DROP3.47%
    VOLTAGE AT LOAD115.84 V
    ESTIMATED OHMS0.12 Ω
    NEC COMPLIANCE (3%)FAIL (>3%)
    FEEDER LIMIT (5%)PASS (<5%)
    FORMULA ENGINE
    VD = (2 × K × L × I) / CM
    (Simplified DC Approx)
    engineering methodology

    A clear path from inputs to results.

    Voltage drop is what the cable takes before the load sees it. This calculator uses tabulated conductor resistance by gauge and material, applies the correct single- or three-phase factor, and checks the result against the usual 3% branch-circuit and 5% total limits.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Choose the conductor material and gauge.
    2. Select the conduit type, or enter a custom impedance.
    3. Enter the system voltage and whether it is single or three phase.
    4. Enter the one-way run distance and the load current.
    5. Set the power factor for AC loads, then read the drop and the compliance checks.

    The formula

    Vd=2IRL (single phase),Vd=3IRL (three phase),%=VdV×100V_d = 2IRL \ \text{(single phase)}, \qquad V_d = \sqrt{3}IRL \ \text{(three phase)}, \qquad \% = \frac{V_d}{V}\times 100

    Reference: NEC Chapter 9 Table 8 (conductor DC resistance); NEC 210.19(A) informational note on 3% / 5% voltage drop.

    Worked example

    3 AWG copper, 500 ft, 20 A at 120 V

    Inputs
    • 3 AWG copper, PVC conduit.
    • 500 ft one way, 20 A, 120 V, single phase.
    • Power factor 0.85.
    Calculation path
    1. 1

      Tabulated resistance for 3 AWG copper: 0.245 Ω per 1,000 ft.

    2. 2

      Loop resistance: 0.245 × 0.5 × 2 = 0.245 Ω.

    3. 3

      Drop: 20 × 0.245 = 4.90 V.

    4. 4

      Percentage: 4.90 / 120 = 4.08%.

    5. 5

      That exceeds the 3% branch-circuit guideline but stays under the 5% total.

    Result

    4.90 V drop (4.08%) · 115.1 V at the load · fails the 3% branch check.

    Interpreting the result

    A 4% drop is not a safety problem but it is a performance one: motors run hot, electronics see brownout conditions, and the load draws more current to compensate, which increases the drop further. The fix is a larger conductor, not a larger breaker.

    Assumptions & limitations

    • Resistance values are the tabulated DC figures at 75 °C; AC resistance is slightly higher because of skin effect.
    • Power factor is applied as a simple multiplier on the resistive drop; a full impedance calculation also needs reactance.
    • The run is a single cable with no parallel conductors.
    • Ambient temperature and conduit fill derating are not applied to the ampacity, only to the drop.

    Frequently asked questions

    What is the maximum acceptable voltage drop?

    Common practice is 3% on a branch circuit and 5% from the service entrance to the furthest outlet. These are performance recommendations rather than absolute code limits in most jurisdictions.

    Does power factor matter for voltage drop?

    Yes on AC circuits. A low power factor increases the current for the same real power, and the reactive component adds to the drop. The calculator applies the power factor to the resistive term as a first-order correction.

    Why does three-phase drop use √3 instead of 2?

    Because the return current flows in the other phases rather than a neutral, so the loop factor changes. For a balanced three-phase load the drop is √3·I·R·L.

    Should I size for ampacity or voltage drop?

    Both. Ampacity is a safety limit that must never be exceeded; voltage drop is a performance limit. On long runs the drop governs; on short runs ampacity does.

    Methodology last reviewed 2026-10-06. Results are informational and not financial or engineering advice — see our disclaimer.