Voltage Drop Calculator — Volts and Percent
Estimate circuit voltage drop.
Project & Input Data
Your project criterion: enter the value from your governing code, specification or approved supplier data. No standard value is assumed.
Your project criterion: enter the value from your governing code, specification or approved supplier data. No standard value is assumed.
Your project criterion: enter the value from your governing code, specification or approved supplier data. No standard value is assumed.
Engineering Assumptions
Direct calculation — Voltage-drop equation
- Conductor resistance is taken from the manufacturer or code data used by the project.
- All percentages and inputs are user-defined; the calculation is purely arithmetic and assumes no particular code or specification.
Formula Summary
- ΔV(V) = k × I × L × R ÷ 1000, k=2 single-phase, k=√3 three-phase
- ΔV% = ΔV ÷ V_system × 100
Results
Enter your data and press Calculate to see the results.
Engineering Insight
A negative remaining allowance means the limit is exceeded and calls for a larger cable section or a shorter route.
Recommended Action
Compare the calculated drop against your project's allowed limit before finalising the chosen section.
Voltage drop decides whether a cable run is acceptable long before ampacity does on long circuits. Enter the design current, the one-way run length and the cable resistance per kilometre from your manufacturer data, and compare the result with the percentage limit you have adopted for the installation.
Method and assumptions
Voltage-drop equation
- ΔV(V) = k × I × L × R ÷ 1000, k=2 single-phase, k=√3 three-phase
- ΔV% = ΔV ÷ V_system × 100
- Conductor resistance is taken from the manufacturer or code data used by the project.
- All percentages and inputs are user-defined; the calculation is purely arithmetic and assumes no particular code or specification.
Worked example
A 63 A three-phase feeder, 55 m long, on a cable of 0.727 Ω/km, checked against a 3% limit on 400 V.
| Result | Value |
|---|---|
| Voltage drop | 4.36 V |
| Voltage drop percent | 1.09 % |
| Remaining allowance | 1.91 % |
These figures are produced by the same calculator engine used on this page.
Frequently asked questions
- Where do I get resistance per km?
- From the cable manufacturer's data for the conductor material, size and operating temperature. The tool has no cable table and assumes no value.
- Is reactance considered?
- No. The equation is resistive only (ΔV = k·I·L·R ÷ 1000). For large cables where reactance matters, use an impedance-based method.
- Which percentage limit applies?
- The allowable percentage is a project/regulatory criterion you enter. Alasos does not claim compliance with any electrical code.
Next step and related links
Related calculators
- Current CalculatorElectrical
- Cable SizingElectrical
- Breaker SizingElectrical
Free calculators are open to everyone, and CSV export is free for everyone too. Project saving and PDF/Excel reports come with Professional, and integrated workflows with Premium.