Percentage Voltage Drop Calculator
Solve for percentage voltage drop, the drop in volts or the rated voltage. In Advanced mode, work out the drop of a real cable from current, length, size and material, find the longest run or smallest conductor for your limit, and see diagrams, charts and reference tables.
Drop is wasted voltage
Voltage lost in the cable never reaches the load. It also turns into heat, I²R, in the conductor.
Low voltage suffers most
The same volts lost is a bigger percentage of 12 V than of 230 V, so DC and extra-low-voltage runs need thick cable.
Length and size
Drop grows with current and length and falls with conductor area. Doubling the area halves the resistance and the drop.
Resistive approximation
This tool uses conductor resistance only. For large cables on AC, reactance and power factor matter and raise the true drop.
How to Use This Calculator
Percentage voltage drop compares the volts lost to the rated supply voltage. Choose which quantity to solve, enter the other two, and use the cable calculator to find the drop of a real run.
1. Choose what to solve for
%VD (from drop and rated voltage), Vd (the volts lost for a given percentage) or Vrated (the supply voltage that gives a given percentage).
2. Enter your values
Type a number, tap a preset (12 V to 400 V, 1 to 10 %), or drag the slider under each field. Sliders are logarithmic for the voltage fields.
3. Pick the units
Voltage drop in mV or V, rated voltage in V or kV, cable length in metres or feet. Values convert when you change a unit.
4. Calculate and read the result
Press Calculate & show result. It scrolls to the result card with the answer, the steps, the voltage at the load and a rating against the 3 % and 5 % guidelines.
5. Check a real cable
Advanced is on by default: enter current, one-way length, conductor size, material and system. See its drop, the longest run, the smallest size that meets your limit and the power lost.
6. Send it back, or load a real case
Press "Use the cable's voltage drop" to feed it into the main calculator. Use Basic for a simpler view, or load an example from the tables below.
What This Tool Offers
Built on the standard definition %VD = (Vd ÷ Vrated) × 100, extended with a cable voltage-drop calculator.
Solve in any direction
Find the percentage, the volts lost or the rated voltage, with each step shown.
Real cable calculator
Copper or aluminium, mm² or AWG, DC, single-phase or three-phase, with a temperature correction.
Design answers
The maximum cable length and the minimum standard size that keep the drop within your limit, plus I²R power loss.
Type, preset or drag
Every main input has quick presets and a draggable slider for fast "what if" exploration.
Charts and diagram
A drop scale with 3 % and 5 % guidelines, %VD curves for different cable sizes, and a circuit showing the voltage at the load.
Real examples and references
House circuits, LED strips, motors, solar and EV charging, plus conductor resistance and regulation guideline tables.
Real-world cable examples
Typical cases computed with this page's resistive model (conductor resistance only, at the temperature shown). Click Load to try one.
| Case | Setup | Vd | %VD | Result |
|---|
Voltage drop guidelines
| Source | Commonly cited limit |
|---|---|
| NEC (informational note) | 3 % on a branch circuit, 5 % feeder plus branch |
| BS 7671 (UK) | 3 % lighting, 5 % other uses |
| AS/NZS 3000 | 5 % from origin to any point |
| Motor starting (typical guidance) | about 10 – 15 % transient dip |
These are commonly cited values. Check the current edition of the code that applies to you.
Conductor resistance reference
Copper at 20 °C. The model column uses ρ = 0.0175 Ω·mm²/m. The IEC 60228 column shows the standard maximum values, which are a little higher because of stranding.
| Size | Area (mm²) | Cu, this page (mΩ/m) | Cu, IEC 60228 max (mΩ/m) | Al, this page (mΩ/m) |
|---|
Constants and unit reference
| Quantity | Value |
|---|---|
| Copper resistivity (20 °C) | 0.0175 Ω·mm²/m (annealed copper 0.017241) |
| Aluminium resistivity (20 °C) | 0.0283 Ω·mm²/m |
| Temperature coefficient | Cu 0.00393 /°C · Al 0.00403 /°C |
| Three-phase factor | √3 ≈ 1.732 |
| 1 foot | 0.3048 m |
| 1 kV | 1,000 V |
Validation and sources
Cable drop: single-phase or DC V_d = 2 · I · R · L, three-phase V_d = √3 · I · R · L (resistive)
Resistivity of copper and aluminium at 20 °C, with the standard linear temperature coefficient
Each result is cross-checked by recomputing V_d = %VD × V_rated / 100
Conductor data and guideline limits are indicative. For installation work, follow the applicable wiring code and the cable manufacturer's tables, which also account for reactance and installation method.
Formulas
Voltage at the load = V_rated − V_d
R(T) = ρ₂₀ [1 + α (T − 20)] / A (Ω per m, A in mm²)
V_d = k · I · R · L with k = 2 (DC, single-phase) or √3 (three-phase)
L_max = (limit % · V_rated / 100) / (k · I · R) · A_min = k · ρ · L · I / (limit % · V_rated / 100)
Power lost = 2 I² R L (DC, single-phase) or 3 I² R L (three-phase)
Results are first-order estimates for design checks, not a replacement for a full cable-sizing calculation.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| %VD | Percentage voltage drop | % |
| V_d | Voltage drop | V |
| V_rated | Rated system voltage | V |
What it means
Percentage voltage drop is the voltage drop V_d expressed as a percentage of the rated voltage V_rated: %VD = (V_d / V_rated) × 100. This is a standard way to specify allowable voltage drop in electrical installations. For example, many codes limit the drop to 3% for branch circuits and 5% for feeders. Example: With V_d = 5V and V_rated = 230V, %VD = (5/230)×100 ≈ 2.17%, which is acceptable. This metric is useful for quickly assessing whether a cable is adequately sized. Over‑sizing cables reduces voltage drop but increases cost, so a balance is needed.
Worked example
Percentage Voltage Drop – Practical Example
Real‑World| Parameter | Value |
|---|---|
| Vd | 2 V |
| Vrated | 230 V |
| Formula | %VD = (Vd / Vrated) × 100 |
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
Percentage voltage drop %VD = (V_d / V_rated) × 100 gives the drop as a percentage of nominal voltage. This is used to check if the drop is within acceptable limits (typically 3‑5%). Engineers use it to verify compliance with standards and to adjust cable sizes. This ensures reliable operation of sensitive equipment.
- Compliance with voltage drop standards (e.g., NEC)
- Quality of supply assessment
- Design optimisation for cable size and cost
- Diagnosis of undervoltage issues
- Educational understanding of voltage drop limits
Related calculators · Electrical Installations
Recently updated formulas
Suggest an improvement
Found a mistake? Have a better example? Let us know.
You may send up to 3 suggestions per 24 hours.