Voltage Drop Calculator
Check whether your wire is big enough. Pick an AWG size (14–4), enter one-way run length, current and system voltage, and get voltage drop in volts and percent, the delivered voltage at the load, power lost in the cable, and a good/borderline/excessive verdict with a full AWG comparison table.
TheCalculators — Voltage Drop Calculator
Printed thecalculators.com
Voltage Drop
4.76 V
3.97% of 120 V · ≈ 115.24 V at the load
- Wire
- 12 AWG copper
- Resistance
- 1.588 Ω per 1000 ft
- One-Way Length
- 100 ft
- Phase Factor
- 2 (single phase, out & back)
- Load Current
- 15 A
- Drop (V)
- 4.76
- Drop (%)
- 3.97%
- Delivered Voltage
- 115.24 V
- Power Lost in Cable
- 71.5 W
- Verdict
- Borderline
Voltage drop verdict
4.0%
Borderline (3–5%)
How the drop is computed
- 1FormulaVD = 2 × I × L × R ÷ 1000 (single phase: out and back)
- 2Substitute2 × 15 A × 100 ft × 1.588 ÷ 1000 = 4.764 V
- 3Percent of source4.764 ÷ 120 × 100 = 3.97%
- 4Cable heating lossVD × I = 4.764 × 15 = 71.5 W wasted as heat
Every gauge at this run length & current
Same length, current and phase — only the conductor size changes.
Uses uncoated copper resistance at 75 °C per NEC Chapter 9 Table 8. The NEC recommends ≤ 3% for a branch circuit and ≤ 5% combined feeder + branch. Aluminum conductors need larger sizes.
Most useful next
Formula
VD = k × I × L × R ÷ 1000 · k = 2 (1φ) or √3 (3φ) · R = Ω per 1000 ft · % = VD ÷ source V × 100
Pro tips
- • Long runs are the silent killer: doubling length doubles drop — check before adding a shed or EV charger circuit.
- • If you are above 3%, jump two wire sizes rather than one; resistance falls roughly 40% per size step.
FAQ
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