Voltage Drop Calculator — Wire & Cable (Free)
Find volts and percent drop from AWG, current, and one-way length
Required Parameters
Waiting for input data...
Quick Answer
Single-phase / DC: VD = 2 × I × R_one-way. Three-phase: VD = √3 × I × R. Keep branch-circuit drop near 3%. Enter one-way length only — the formula already includes the return path.
Voltage Drop Calculator
Find cable voltage drop from AWG, load current, one-way length, and source voltage. Results include volts lost, percent drop, voltage at the load, and conductor resistance at 75 °C.
Quick start: 12 AWG, 20 A, 50 ft, 120 V copper → about 3.9 V / 3.2%. Enter one-way length only.
Voltage drop formulas
Single-phase and DC (2-wire):
VD = 2 × I × R
Three-phase (line-to-line):
VD = √3 × I × R
| Symbol | Meaning |
|---|---|
| VD | Voltage drop (V) |
| I | Load current (A) |
| R | One-way conductor resistance (Ω) at 75 °C |
The factor of 2 (or √3) already includes the return path. If you enter the out-and-back distance, the result will be roughly doubled.
Worked example — 12 AWG, 20 A, 50 ft, 120 V
- 12 AWG copper ≈ 3.31 mm². At 75 °C, 50 ft (15.24 m) is about 0.097 Ω one-way.
- VD = 2 × 20 × 0.097 ≈ 3.9 V
- Percent = 3.9 / 120 × 100 ≈ 3.2%
- Voltage at the load ≈ 116 V
That is just over a 3% branch-circuit target. Shorten the run or step up to 10 AWG. Use the wire size calculator if you do not know the gauge yet.
How much drop is acceptable?
NEC language on voltage drop is an informational note, not a mandatory table: about 3% on a feeder or branch circuit and 5% total to the farthest outlet. Motors, LED drivers, and 12 V DC runs suffer well before 5%. On a 12 V rail, 0.5 V lost is already 4%.
DC voltage drop
Use the DC (2-wire) option. The formula is the same as single-phase: VD = 2 × I × R. Low-voltage DC is less forgiving — keep the cable short and thick.
Why 75 °C resistance?
Loaded building wire runs hot. DC resistance at 75 °C is about 21% higher than 20 °C handbook values and matches common THWN / NEC Chapter 9 practice. Using 20 °C numbers understates the drop.
Related tools
- Wire Size Calculator — pick AWG from amps and length
- AWG Wire Size Chart — diameter and mm² for a known gauge
- Ohm's Law Calculator — current from watts and volts
- Three-Phase Power Calculator — line current for 3φ
- Amps to Watts Calculator — P = I × V
Design Notes
R is the one-way conductor resistance at 75 °C. The factor of 2 accounts for the return path. Three-phase line-to-line drop uses √3. Voltage drop is not a mandatory NEC table, but 215.2(A)(1) Informational Note No. 2 recommends 3% on feeders or branch circuits and 5% total.
Common Mistakes
- 1
Using one-way resistance without the return path (forgetting the ×2).
- 2
Entering round-trip length and also applying ×2 — that doubles the drop.
- 3
Ignoring temperature. Resistance at 75 °C is about 21% higher than at 20 °C.
Engineering Handbox
1. 12 AWG copper ≈ 1.93 Ω/kft at 75 °C 2. One-way R = 1.93 × 0.050 = 0.0965 Ω 3. VD = 2 × 20 × 0.0965 = 3.86 V → 3.2%
Knowledge Base
How do I calculate voltage drop in a wire?
VD = 2 × I × R for a 2-wire DC or single-phase run, where R is the one-way conductor resistance. Example: 20 A through 50 ft of 12 AWG copper (≈ 0.096 Ω one-way at 75 °C) → VD ≈ 3.9 V, about 3.2% on 120 V.
Do I enter one-way or round-trip length?
One-way. The calculator multiplies by 2 (or √3 for three-phase). If you enter the out-and-back distance, the drop will be roughly doubled.
What voltage drop is acceptable?
NEC Informational Note language is 3% on a feeder or branch circuit and 5% total to the farthest outlet. It is not a mandatory table, but motors, LED drivers, and long 12 V DC runs suffer well before 5%.
How do I calculate DC voltage drop?
Same as single-phase: VD = 2 × I × R. Low-voltage DC is less forgiving — 0.5 V lost on a 12 V rail is already 4%. Use the DC circuit option and a short, thick cable.
What if I do not know the AWG yet?
Use the wire size calculator. It picks the smallest AWG that meets both ampacity and your drop limit, then you can confirm the drop here.
Why is resistance taken at 75 °C?
Loaded building wire runs hot. 75 °C DC resistance is about 21% higher than 20 °C handbook values and matches common THWN / NEC Chapter 9 practice.