Voltage Drop Calculator

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Use this calculator to estimate voltage drop, load voltage, and wire loss, or find the smallest listed wire size that meets your drop target.

Choose whether you already know the wire size or want the calculator to suggest the smallest size that meets your drop target.
DC and single-phase use a round-trip path factor of 2. Three-phase uses sqrt(3).
Copper and aluminum have different resistance, so the same size wire can have a different voltage drop.
Enter the source voltage before the wire run. Example: 12, 24, 120, 208, 240, 480.
Use the expected load current on the circuit.
Enter the one-way distance from source to load. Do not double it yourself.
Used in Check a wire size mode. Includes common building-wire sizes from 14 AWG through 1000 kcmil.
Used in Find the smallest wire size mode. A common planning target is 3% for a branch circuit, but local design rules may differ.
Advanced options

AC details

Resistance only uses the 20 C reference resistance adjusted to your selected conductor temperature and ignores reactance. Approximate impedance adds a generic reactance assumption, so use a cable manufacturer's table when construction-specific accuracy matters.
Only used for AC impedance mode. Power factor describes how much of the current does useful work. Leave the default if you are unsure.

Resistance temperature adjustment

The built-in table is referenced to 20 C, then corrected to this assumed conductor temperature with separate copper and aluminum coefficients. This is not an insulation rating or an ampacity check.

Number display

Changes how many digits are shown. It does not change the math.
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How to use our Voltage Drop Calculator

  1. Choose What do you want to do?, then select System type and Conductor material.
  2. Enter Supply voltage (V), Load current (A), and One-way length (ft). Use the one-way distance only; the calculator applies the correct path factor for you.
  3. In check mode, choose Wire size (AWG or kcmil). In size mode, enter Target voltage drop (%). Open Advanced options only if you want to change the AC method, power factor, or conductor temperature.
  4. Click Calculate, then read Voltage drop, Voltage drop percent, Estimated voltage at load, and Estimated resistive power loss. In size mode, also check Smallest wire size that meets target.
  5. Sanity-check the result: if Voltage drop percent looks much higher than expected, recheck One-way length (ft), confirm the right System type, and make sure you did not enter the round-trip distance by mistake.

Definitions

Voltage drop: The volts lost in the wire run between the source and the load.

Voltage drop percent: Voltage drop divided by Supply voltage (V), shown as a percent. This is usually the easiest result to compare with a design target.

Estimated voltage at load: The voltage left at the far end after the calculated drop is subtracted from the source voltage.

Estimated resistive power loss: The approximate I-squared-R heating in the current-carrying conductors. It is calculated separately from reactive voltage drop.

Path factor used: The multiplier used for voltage drop. DC and single-phase use 2, while balanced three-phase uses square root of 3, about 1.732 [3].

Resistance only: A planning estimate that temperature-corrects the built-in 20 C DC resistance and ignores reactance. For AC, it effectively assumes a power factor of 1.

Approximate impedance with power factor: An AC planning method that combines temperature-adjusted DC resistance with a generic reactance assumption and the entered Power factor (0 to 1). A cable manufacturer's table is better for construction-specific design.

Wire size (AWG or kcmil): The conductor size from the built-in list, from 14 AWG up to 1000 kcmil.

Voltage Drop Planning GuideCommon percent-drop targets used for quick circuit planning. Use this as a planning reference, then confirm project and code requirements separately.Voltage Drop Planning GuideCommon percent-drop targets used for quick circuit planningHigh0 %100 %Voltage drop percent of source voltage
Voltage Drop Planning Guide
Use this as a planning reference, then confirm project and code requirements separately.

Common mistakes and quick fixes

Mistake: Entering the full out-and-back distance in One-way length (ft) .
Fix: Enter only the source-to-load distance in One-way length (ft) . The calculator already applies the return-path factor for DC and single-phase.

Mistake: Leaving Target voltage drop (%) blank in size mode.
Fix: In Find the smallest wire size mode, enter a value greater than 0 in Target voltage drop (%) before you calculate.

Mistake: Using the wrong System type for a three-phase run.
Fix: Set System type to Three-phase AC when the load is balanced three-phase, so the calculator uses the correct factor.

Mistake: Comparing Estimated voltage at load to the wrong source value.
Fix: Make sure Supply voltage (V) matches the actual source voltage of the circuit you are checking, such as 120, 208, 240, or 480.

Mistake: Editing Power factor (0 to 1) for DC runs.
Fix: For System type set to DC, ignore AC-only settings. The DC result uses resistance only.

Mistake: Treating Meets your target? as a full code approval.
Fix: Use Meets your target? only as a voltage-drop check. Still verify ampacity, insulation, overcurrent protection, and local code rules separately.

Limitations & Key Assumptions / Boundary Conditions

  • This tool estimates voltage drop only. It does not verify ampacity, overcurrent protection, conduit fill, insulation rating, termination temperature limit, or local code compliance.
  • One-way length (ft) must be entered as source to load distance only. The calculator handles the path factor internally.
  • Three-phase results assume a balanced three-phase load. Unbalanced systems can behave differently in practice [3].
  • The resistance table contains rounded 20 C reference values based on standard copper and aluminum material data, not a catalog value for every cable construction. Stranding, alloy, installation, and product construction can change actual resistance.
  • The approximate AC impedance option uses a generic 0.08 ohm per 1,000 ft reactance. Actual reactance depends on conductor spacing, raceway, cable construction, and installation, so use manufacturer data when those details matter.
  • Conductor temperature (deg C) changes resistance through a material-specific correction from 20 C. It is an assumed operating temperature, not an insulation rating or ampacity approval. Real temperature varies with load, bundling, ambient conditions, and installation method.
  • If no listed size meets the target in size mode, the tool can only report that none of the built-in sizes worked. A larger conductor family or a different design approach may be needed.
  • If Voltage drop is greater than Supply voltage (V), the calculator keeps the signed Estimated voltage at load result and flags the setup as impractical.
  • The common planning guidance of about 3% for a branch circuit and 5% total feeder plus branch is guidance, not a stand-alone pass/fail code rule by itself [2].

Methodology

Resistance basis and temperature

The built-in resistance values are rounded DC reference values at 20 C. Copper uses the International Annealed Copper Standard basis and its 20 C coefficient of 0.00393 per degree C [4]. Aluminum uses the NIST EC-H19 61 percent IACS basis and its material-specific 20 C coefficient of 0.00403 per degree C [5].

R_at_T = R_at_20C x [1 + alpha_at_20C x (T - 20)]

This fixes the reference-condition mismatch: selecting 60 C, 75 C, or 90 C now raises resistance from the stated 20 C baseline. The table is a planning reference, not a substitute for the resistance and reactance data for a specific cable. Southwire product data, for example, reports separate DC resistance, AC resistance, and reactance values tied to the product construction [6].

Voltage-drop formulas

Vdrop = 2 x I x R_per_1000ft x L_ft / 1000

This is the resistance-only formula for DC and single-phase AC.

Vdrop = 1.7320508076 x I x R_per_1000ft x L_ft / 1000

This is the resistance-only formula for balanced three-phase AC. The 1.7320508076 value is square root of 3 [3].

Vdrop = K x I x (R_per_1000ft x PF + X_per_1000ft x sqrt(1 - PF^2)) x L_ft / 1000

This is the approximate AC impedance option. K is 2 for single-phase or 1.7320508076 for balanced three-phase, PF is power factor, R is the temperature-adjusted DC reference resistance, and X is the generic planning value of 0.08 ohm per 1,000 ft. Because actual AC resistance and reactance depend on construction and installation, do not treat this option as a product-specific impedance table.

drop_percent = 100 x Vdrop / Vsupply

Vload = Vsupply - Vdrop

Ploss = 2 x I^2 x R_per_1000ft x L_ft / 1000

The two-conductor loss formula is used for DC and single-phase. Balanced three-phase resistive loss uses 3 x I^2 x R_per_1000ft x L_ft / 1000. Resistive heating is calculated from R rather than by multiplying an impedance-based voltage drop by current.

Mode logic

In Check a wire size mode, the selected Wire size (AWG or kcmil) is used directly, and the tool reports whether that size meets the entered target when a target is shown. In Find the smallest wire size mode, the calculator checks sizes from smallest to largest and returns the first one at or below the entered Target voltage drop (%).

Worked mini-example

Suppose you check a single-phase copper run at 120 V, 20 A, and 100 ft one-way with 12 AWG, 75 C conductor temperature, and resistance only. The 20 C reference is 1.588 ohms per 1,000 ft. Temperature correction gives:

R_at_75C = 1.588 x [1 + 0.00393 x (75 - 20)] = about 1.931 ohms per 1,000 ft

Vdrop = 2 x 20 x 1.931 x 100 / 1000 = about 7.72 V

Percent drop is about 6.44%, estimated load voltage is 112.28 V, and estimated resistive power loss is about 154.5 W.

How to interpret the result

A lower Voltage drop percent usually means better voltage delivered to the load and less wasted heating in the wire. If the percent drop is above your design target, common fixes are a shorter run, lower current, higher system voltage when appropriate, or a larger conductor.

Assumptions behind the estimate

The calculation assumes the selected conductor size follows the rounded reference table, the selected temperature reasonably represents conductor operating temperature, and three-phase loads are balanced. Real installations can differ because of alloy, stranding, cable construction, connections, routing, actual temperature, harmonic content, or other system details. Confirm final designs with the applicable product and code data.

Sources

  1. Nexans - Understanding Voltage Drop (plain-language explainer) - Nexans
  2. Acceptable Voltage Drop: Understanding the NEC Code - Expertce
  3. Voltage Drop - Nebulous LLC - Nebulous-llc
  4. Copper Wire Tables and International Annealed Copper Standard - National Bureau of Standards (NIST)
  5. Aluminum Wire Tables, NBS Handbook 109 - National Bureau of Standards (NIST)
  6. Cable electrical and engineering data with DC resistance, AC resistance, and reactance - Southwire