What voltage drop is acceptable?
Many DIY guides target 3% max on branch circuits and 5% total.
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.
Concise answers for common searches — definitions, steps, and comparisons.
Many DIY guides target 3% max on branch circuits and 5% total.
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 × 100Use circuit amps or watts ÷ volts from the appliance nameplate.
Choose installed or planned AWG — extension cords count too.
Measure from panel to outlet in feet (not round-trip).
Aim for 3% or less on branch circuits; upsize wire if above 5%.
Input
15 A · 12 AWG · 50 ft · 120 VOutput
~2% dropInput
20 A · 12 AWG · 100 ft · 120 VOutput
~5.3% — upsize wireCommon real-world scenarios where this tool saves time.
15 A on 12 AWG for 50 ft → ~2% at 120 V — acceptable.
100 ft run at 20 A may need 10 AWG or larger to stay under 3%.
40 A on 6 AWG for 150 ft at 240 V — check percent before install.
Step-by-step chains that connect related tools for common tasks.
| Circuit type | Target drop | Notes |
|---|---|---|
| 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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Many DIY guides target 3% max on branch circuits and 5% total — motors and workshop tools benefit from tighter drops.
Yes — undersized cords heat up and drop voltage. Use this calculator with cord length and load amps.
At 20 A and 100 ft, 12 AWG often exceeds 5% drop — 10 AWG is typically needed to stay near 3%.
Upsize wire gauge, shorten the run, or lower load amps — larger AWG has lower resistance per foot.
Yes — excessive drop can dim lights, slow motors, and cause nuisance trips on sensitive equipment.
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.
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Reviewed on 2026-03-01.
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