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Voltage Drop Calculator

By Muhammad Abdullah Rauf · Founder, EverydayTools.proUpdated 2026-03-01

How much voltage drop on my wire run?

Voltage drop wastes energy and can dim lights or stress motors. For **single-phase copper**:

**VD (volts) = 2 × R × I × L ÷ 1000**

**Percent drop = VD ÷ supply voltage × 100**

**Example — 15 A, 12 AWG, 50 ft, 120 V:**

R (12 AWG) ≈ 1.588 Ω/1000 ft

VD ≈ 2 × 1.588 × 15 × 50 ÷ 1000 = **2.4 V** → **2%**

**Rule of thumb:** aim for **3% or less** on branch circuits; **5% max** combined feeder + branch in many DIY guides.

  • **R** = wire resistance (Ω per 1,000 ft)
  • **I** = load amps
  • **L** = one-way length in feet
  • **2** = out-and-back conductor path

Single-phase voltage drop

Uses copper resistance per 1,000 ft at 75°C and round-trip conductor length for percent voltage drop.

Formula

VD = 2 × R(Ω/1000ft) × I × L(ft) ÷ 1000; % = VD ÷ V × 100

Assumptions

  • Copper conductors.
  • Single-phase AC.
  • Unity power factor approximation.

Limitations

  • Does not model power factor or harmonic loads.
  • Aluminum wire uses different resistance values.

How to use Voltage Drop Calculator

  1. Enter load amps

    Use circuit amps or watts ÷ volts from the appliance nameplate.

  2. Select wire gauge

    Choose installed or planned AWG — extension cords count too.

  3. Enter one-way length

    Measure from panel to outlet in feet (not round-trip).

  4. Read percent drop

    Aim for 3% or less on branch circuits; upsize wire if above 5%.

Voltage Drop Calculator examples

Extension cord

Input

15 A · 12 AWG · 50 ft · 120 V

Output

~2% drop

Long garage run

Input

20 A · 12 AWG · 100 ft · 120 V

Output

~5.3% — upsize wire

Who uses Voltage Drop Calculator?

Common real-world scenarios where this tool saves time.

50 ft extension cord

15 A on 12 AWG for 50 ft → ~2% at 120 V — acceptable.

Detached garage feed

100 ft run at 20 A may need 10 AWG or larger to stay under 3%.

Subpanel run

40 A on 6 AWG for 150 ft at 240 V — check percent before install.

Workflow guides

Step-by-step chains that connect related tools for common tasks.

Long outdoor run

  1. Enter planned wire and length
  2. If drop >3%, upsize AWG in Wire Gauge Calculator
  3. Confirm breaker still matches wire ampacity

Reference tables

Voltage drop targets (DIY guides)

Circuit typeTarget dropNotes
Branch circuit≤ 3%Lights, outlets, general loads
Combined feeder + branch≤ 5%Many residential guides
Motor / workshop≤ 3%Tighter drop helps starting torque

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Frequently Asked Questions

What voltage drop is acceptable?

Many DIY guides target 3% max on branch circuits and 5% total — motors and workshop tools benefit from tighter drops.

Does extension cord gauge matter?

Yes — undersized cords heat up and drop voltage. Use this calculator with cord length and load amps.

12 or 10 gauge for 100 ft?

At 20 A and 100 ft, 12 AWG often exceeds 5% drop — 10 AWG is typically needed to stay near 3%.

How do I reduce voltage drop?

Upsize wire gauge, shorten the run, or lower load amps — larger AWG has lower resistance per foot.

Does voltage drop affect appliance performance?

Yes — excessive drop can dim lights, slow motors, and cause nuisance trips on sensitive equipment.

Privacy, accuracy, and trust

Privacy

Wire length, gauge, and load amps are processed only in your browser — EverydayTools does not store circuit inputs.

Estimate only — not NEC certification or engineered voltage-drop study. Verify with licensed design for long feeds.

Part of Calculator Tools

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Wire Gauge Calculator

Free wire gauge calculator: load watts or amps → minimum copper AWG with continuous-load sizing. 60°C NM-B ampacity reference. Pair with breaker and voltage drop tools. Runs locally in your browser.

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Free electricity cost calculator: kWh = (watts ÷ 1000) × hours; cost = kWh × your $/kWh rate—stack appliances, peak/off-peak. Runs in your browser, no signup.

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Reviewed on 2026-03-01.