Voltage Drop Calculator
Calculate voltage drop over a cable run and check it against limits.
Please note: For planning and educational use only. Electrical installation is governed by wiring regulations and is dangerous when done incorrectly. Always have work designed, installed and inspected by a qualified electrician.
What the Voltage Drop Calculator does
Voltage drop is the loss along a cable caused by conductor resistance. It matters because equipment at the far end sees less than the supply voltage, and excessive drop causes dim lighting, motor overheating and nuisance tripping.
Formula
Conductor resistance R = ρ × Length ÷ AreaSingle phase drop = 2 × I × R (out and back)Three phase drop = √3 × I × RDrop % = Drop ÷ Supply voltage × 100
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Supply type | one of 2 options | Yes | — |
| Conductor material | one of 2 options | Yes | — |
| Supply voltage | V | Yes | Accepts more than 0. |
| Load current | A | Yes | Accepts more than 0. |
| One-way cable length | number | Yes | Accepts more than 0. |
| One-way cable length unit | one of 6 options | Yes | — |
| Conductor cross-section | mm² | Yes | Accepts 0.1 or more. |
| Maximum allowable drop | % | Yes | Commonly 3% for lighting, 5% for power circuits. |
How to use it
- Choose Supply type and Conductor material.
- Enter Supply voltage, Load current, One-way cable length and Conductor cross-section and 1 more.
- Select Calculate.
Worked example
20 A over 30 m of 4 mm² copper on a 230 V single-phase supply.
- Phase
- Single
- Material
- Copper
- Current
- 20 A
- Length
- 30 m
- Size
- 4 mm²
R = 0.1293 Ω, drop = 2 × 20 × 0.1293 = 5.17 V, which is 2.25% — within the 3% limit.
Frequently asked questions
What is an acceptable voltage drop?
Commonly 3% for lighting circuits and 5% for power circuits, measured from the origin of the installation. Check your local wiring regulations.
Why double the length for single phase?
Current flows out along the live conductor and back along the neutral, so it travels the cable length twice.
Method and sources
Method. Conductor resistance from resistivity, length and cross-sectional area, then drop as current times resistance, doubled for the return path in a single-phase circuit.
Applies to. None applied — permitted voltage drop is defined by national wiring regulations
Assumptions
- Resistivity is taken at a nominal conductor temperature; it rises as the conductor warms, increasing the drop under load.
- The run length entered is one-way, with the return accounted for by the multiplier.
Limitations
- Uses representative resistivity for copper and aluminium, not a certified figure for a specific cable construction.
- Reactance is ignored, which is acceptable for small conductors but understates drop on large cables and long AC runs.
- Permitted drop is set by the wiring regulations in force, and this calculator applies none — it computes a drop, it does not tell you whether that drop complies.
Sources
- The national wiring regulations applying where the installation is located — Varies by country. The permitted voltage-drop limit and the conductor data that governs a real design.