Cable Voltage Drop Calculator — Single Phase
Solve the single-phase voltage drop equation for drop, current, length or cable resistance — with a circuit diagram, cable-size reference tables, and an interactive drop-vs-length graph.
The source delivers Vs; resistance in the go-and-return conductor pair drops Vd along the way, so the load only sees VL = Vs − Vd. The diagram and its labels update with your inputs above.
Hover or touch the line to read the drop at any length. The amber marker is your calculated operating point; the dashed lines mark the commonly cited 3% and 5% guideline levels for V₀.
Length ↑
Vd grows directly with distance — doubling the run doubles the drop for the same current and cable size.
Current ↑
Vd is directly proportional to load current — a circuit that draws twice the current sees twice the drop on the same cable.
Cable size ↑
A larger cross-section lowers R and cuts Vd roughly in proportion — upsizing the cable is the standard fix for excess drop over long runs.
Supply voltage
V₀ doesn't change Vd itself, but a higher V₀ makes the same Vd a smaller percentage — this is one reason higher-voltage distribution reduces drop problems.
How to Use This Calculator
The voltage drop equation links four quantities: cable length L, load current I, cable resistance R, and the resulting drop Vd. Pick the one you want solved and fill in the other three.
1. Choose what to solve for
Select Vd, L, I or R under "Solve for". The greyed-out field is the one being calculated.
2. Pick a cable size, or enter R directly
The cable-size preset fills in R for common copper and aluminum conductor sizes. Switch it to "custom" to type your own value from a datasheet.
3. Enter length and current
Length is the one-way run from source to load — the formula's factor of 2 already covers the return conductor. Current is the load's full-load or design current.
4. Set the supply voltage
V₀ doesn't affect Vd itself, but it lets the tool show the drop as a percentage and flag it against the common 3%/5% guidance.
5. Read the result, diagram and graph
The result card shows Vd (or whichever variable you solved for) with a pass/caution/fail badge. The diagram updates with your values, and the graph shows how Vd changes with length.
6. Switch to Advanced for more depth
Advanced mode adds ±1% sensitivity, load-side voltage, power lost in the cable, and the maximum length for 3% and 5% drop.
What This Tool Offers
Built around the standard simplified single-phase voltage drop formula used for quick cable-sizing checks.
Solve any of four variables
Find Vd, length, current or cable resistance — the equation is rearranged for you.
Labeled circuit diagram
A source-to-load schematic showing the go and return conductors, current direction, and where the drop occurs — updates live with your inputs.
Cable resistance reference tables
Standard copper and aluminum conductor resistance by cross-section, ready to plug straight into the calculator.
Interactive drop-vs-length graph
See Vd or %Vd plotted against length, with 3%/5% guideline lines and a hover readout at any distance.
Unit conversion
Length in meters or feet, resistance in Ω/km, Ω/1000ft or Ω/m — convert independently without retyping.
Pass/caution/fail guidance
The result is checked against commonly cited 3% and 5% voltage-drop guidance so you can see at a glance whether a run needs a bigger cable.
What Is Cable Voltage Drop?
Every real conductor has resistance, so as current flows along a cable some voltage is lost before it reaches the load — this is voltage drop. For a single-phase circuit, current must travel out to the load and back through a return conductor, so the total resistance in the circuit is twice the one-way conductor resistance. Excessive voltage drop can cause dim lighting, motors that run hot or stall, and equipment operating outside its rated voltage range, which is why electrical codes and design guides set recommended limits.
Voltage Drop Formula
Rearranged for the other three variables:
I = (1000 · Vd) / (2 · L · R)
R = (1000 · Vd) / (2 · L · I)
with L in meters and R in ohms per kilometer (Ω/km) per conductor, giving Vd in volts.
Formula Variables
| Symbol | Meaning | Typical unit |
|---|---|---|
| Vd | Voltage dropped along the feeder | volts (V) |
| L | One-way cable length, source to load | m, ft |
| I | Load (design) current | amps (A) |
| R | Resistance per conductor, per unit length | Ω/km, Ω/1000ft, Ω/m |
| V₀ | Nominal supply voltage (for %Vd only) | volts (V) |
Reference Table — Standard Copper Conductor Resistance
Approximate stranded copper conductor DC resistance at 20°C, by cross-sectional area:
| Size | R (Ω/km) | R (Ω/1000ft) |
|---|---|---|
| 1.5 mm² | 12.1 | 3.69 |
| 2.5 mm² | 7.41 | 2.26 |
| 4 mm² | 4.61 | 1.41 |
| 6 mm² | 3.08 | 0.939 |
| 10 mm² | 1.83 | 0.558 |
| 16 mm² | 1.15 | 0.351 |
| 25 mm² | 0.727 | 0.222 |
| 35 mm² | 0.524 | 0.160 |
| 50 mm² | 0.387 | 0.118 |
| 70 mm² | 0.268 | 0.0817 |
| 95 mm² | 0.193 | 0.0588 |
| 120 mm² | 0.153 | 0.0466 |
| 150 mm² | 0.124 | 0.0378 |
Reference Table — Standard Aluminum Conductor Resistance
| Size | R (Ω/km) | R (Ω/1000ft) |
|---|---|---|
| 16 mm² | 1.91 | 0.582 |
| 25 mm² | 1.20 | 0.366 |
| 35 mm² | 0.868 | 0.265 |
| 50 mm² | 0.641 | 0.195 |
| 70 mm² | 0.443 | 0.135 |
| 95 mm² | 0.320 | 0.0975 |
| 120 mm² | 0.253 | 0.0771 |
| 150 mm² | 0.206 | 0.0628 |
Conversion Table — Units
| Quantity | Conversions |
|---|---|
| Length | 1 m = 3.28084 ft · 1 ft = 0.3048 m |
| Resistance | 1 Ω/km = 0.3048 Ω/1000ft · 1 Ω/1000ft = 3.28084 Ω/km |
| Resistance | 1 Ω/km = 0.001 Ω/m · 1 Ω/m = 1000 Ω/km |
Reference Table — Common Voltage Drop Guidance
Widely cited rule-of-thumb limits used in many design guides. Always confirm the specific figure required by the electrical code or standard governing your installation.
| Circuit type | Commonly cited limit |
|---|---|
| Branch / final circuit | ≈ 3% of nominal voltage |
| Feeder + branch combined | ≈ 5% of nominal voltage |
| Motor starting circuits | Often allowed higher, per equipment manufacturer |
Worked Example
Vd = (2 × 20 × 16 × 7.41) / 1000
Vd = 4,742.4 / 1000
Vd = 4.74 V (≈ 2.06% of V₀ — within the typical 3% guideline)
Reference & Method Note
Frequently Asked Questions
In a single-phase circuit, current flows out to the load through one conductor and back through another. Both conductors add resistance, so the total resistance in the loop is twice the one-way (single-conductor) resistance — that's the 2 in the formula.
L is the one-way distance from source to load. The formula already multiplies by 2 to account for the return conductor, so entering the round-trip distance would double-count it and overstate the drop.
This varies by electrical code and installation type. Many guides commonly cite around 3% for a branch circuit alone and 5% for a feeder plus branch circuit combined, but you should confirm the exact figure required for your jurisdiction and application.
Yes. Conductor resistance rises with temperature, so a cable running hot under load has a higher R — and therefore a higher Vd — than the 20°C reference value used in the tables here.
A three-phase circuit doesn't use a separate full-current return conductor in the same way, so its simplified formula uses a factor of √3 (≈1.732) times L·I·R instead of the single-phase factor of 2. This calculator covers single-phase circuits only.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| V_d | Voltage drop | V |
| L | Cable length (one-way) | m |
| I | Load current | A |
| R | Cable resistance (per km) | Ω/km |
What it means
The voltage drop in a single‑phase cable is given by V_d = (2 × L × I × R) / 1000, where L is the cable length in metres, I is the current in amperes, and R is the resistance of the cable per kilometre (Ω/km). The factor 2 accounts for both the outgoing and return conductors. Voltage drop must be limited to ensure proper operation of equipment (typically <3‑5%). Example: A 100m cable carrying 50A, with a resistance of 0.5Ω/km, has V_d = (2 × 100 × 50 × 0.5) / 1000 = (2 × 100 × 50 × 0.5) / 1000 = (5000) / 1000 = 5V. This is the drop in the cable; the load will see a voltage lower than the supply by 5V.
Worked example
Cable Voltage Drop – Practical Example
Real‑World| Parameter | Value |
|---|---|
| L | 100 m = 0.1 km |
| I | 20 A |
| R | 0.5 Ω/km |
| Formula | Vd = (2 × L × I × R) / 1000 |
Common mistakes
Watch for unit mismatches (W vs kW, single- vs three-phase) and remember to include power factor or efficiency where the formula requires it.Applications
Cable voltage drop (single phase): V_d = (2 × L × I × R)/1000 accounts for the resistance of the two conductors. This ensures that the voltage at the load is within acceptable limits. Engineers use it to size cables for voltage drop, to comply with standards, and to ensure proper operation of equipment.
- Cable sizing for voltage drop compliance
- Power distribution design in buildings
- Transformer and cable loss estimation
- Motor starting voltage dip analysis
- Educational understanding of voltage drop
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