Voltage Drop Calculator
Size a run so the load still gets clean voltage. Keeps you under the 3% branch / 5% total recommendation.
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Voltage drop is the loss of voltage as current flows through a conductor's resistance. The NEC recommends keeping it under 3% on a branch circuit and 5% total for feeder plus branch (210.19(A) and 215.2(A), Informational Notes). For single-phase, Vd = (2 × K × I × L) ÷ CM.
The voltage drop formula
Single-phase: Vd = (2 × K × I × L) ÷ CM Three-phase: Vd = (√3 × K × I × L) ÷ CM K = 12.9 (copper) or 21.2 (aluminum) Ω·cmil/ft · I = load amps · L = one-way length (ft) · CM = conductor circular mils. Percent drop = Vd ÷ system voltage × 100.
Worked example
A 120 V, 20 A single-phase circuit on #10 copper (10,380 cmil) running 150 ft one-way: Vd = (2 × 12.9 × 20 × 150) ÷ 10,380 = 7.46 V, or 6.2% — over the 3% branch recommendation. Upsizing to #8 copper (16,510 cmil) drops it to 3.9%, and #6 (26,240 cmil) to 2.5%.
Voltage drop on a 120 V, 20 A single-phase copper circuit at common run lengths
| One-way length | #12 AWG | #10 AWG | #8 AWG |
|---|---|---|---|
| 50 ft | 3.3% | 2.1% | 1.3% |
| 100 ft | 6.6% | 4.1% | 2.6% |
| 150 ft | 9.9% | 6.2% | 3.9% |
NEC references
- NEC 210.19(A), Informational Note — 3% recommended maximum voltage drop on a branch circuit
- NEC 215.2(A)(1), Informational Note — 5% recommended maximum for feeder + branch circuit combined
- NEC Chapter 9, Table 8 — conductor properties — circular mils and DC resistance
How to calculate voltage drop
- Pick the conductor material (copper or aluminum) and the system voltage.
- Enter the load current in amps and the one-way run length in feet.
- Choose a wire size — Fieldwatt computes percent drop and end voltage instantly.
- If it exceeds 3% on a branch (or 5% total), upsize to the recommended conductor shown.
Frequently asked questions
What is the NEC voltage drop limit?
For most circuits the NEC does not mandate a limit — it recommends a maximum of 3% on a branch circuit and 5% total for feeders plus branch circuits (210.19(A) and 215.2(A) Informational Notes). Some applications (fire pumps, sensitive equipment) and local codes make a limit mandatory.
How do I calculate voltage drop?
Use Vd = (2 × K × I × L) ÷ CM for single-phase (use √3 instead of 2 for three-phase), where K is 12.9 for copper or 21.2 for aluminum, I is load amps, L is the one-way length in feet, and CM is the conductor's circular mils. Divide Vd by the system voltage for percent drop.
Is 3% voltage drop too much?
3% is the NEC's recommended maximum for a branch circuit, so it's the edge of the recommendation, not a failure. Many electricians design to 2% on long or motor circuits to leave headroom. Above 3% on a branch (or 5% total) you should consider upsizing the conductor.
Does voltage drop change the required wire size?
Yes. On long runs, voltage drop — not ampacity — often sets the conductor size. A wire large enough to carry the current can still drop too much voltage over distance, so you upsize until the percent drop falls within the recommendation.
Built for the field
- Live results as you type — every value recomputes instantly.
- Works fully offline once the app has loaded.
- Cites the governing NEC table for every result.
- Save calculations to a job and build a material list (Pro).
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For guidance only. Always verify against the current National Electrical Code and your local amendments. Fieldwatt does not replace an engineer of record.