Wire Size Calculator
Estimate cable cross-section from voltage drop and load — a planning figure, not a compliant selection.
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 Wire Size Calculator does
Cable sizing balances two constraints: the conductor must carry the current without overheating, and it must keep voltage drop within limits over the run. Long runs are usually governed by voltage drop, short runs by current capacity.
Formula
Area for voltage drop = Multiplier × I × ρ × Length ÷ Allowable dropMultiplier is 2 for single phase, √3 for three phaseArea for capacity ≈ Current ÷ (Capacity per mm² × Derating)Selected size = next standard size above the larger requirement
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 0 or more. |
| 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 | — |
| Maximum voltage drop | % | Yes | — |
| Derating factor | % | Yes | Reduce for grouping, insulation or high ambient temperature. |
How to use it
- Choose Supply type and Conductor material.
- Enter Supply voltage, Load current, One-way cable length and Maximum voltage drop and 1 more.
- Select Calculate.
Worked example
32 A over 25 m of copper on 230 V single phase with a 3% drop limit.
- Current
- 32 A
- Length
- 25 m
- Voltage
- 230
- Max drop
- 3
Allowable drop 6.9 V, required area 3.997 mm², capacity needs 5.33 mm² → select 6 mm², giving a 2.0% drop.
Frequently asked questions
Why did it pick a larger cable than the voltage drop needs?
Because current capacity governed instead. Short runs with heavy loads are limited by heating, not by drop.
Can I use this for a real installation?
No. Use it to understand the trade-offs, then have a qualified electrician design to the actual regulations, which account for far more variables.
Method and sources
Method. Two constraints are computed and the larger governs. Voltage drop uses conductor resistivity, length and current to find the area that keeps the drop within the limit entered. Current-carrying capacity uses a simplified capacity-per-square-millimetre figure with a derating factor. The result is rounded up to the next standard size.
Applies to. None applied — cable sizing is governed by the national wiring regulations in force where the installation is located, and those tables are not shipped here
Assumptions
- Copper or aluminium at a nominal temperature, with resistivity taken as representative rather than certified for a specific cable.
- The derating factor entered accounts for installation conditions — the calculator does not derive it.
- One circuit is considered in isolation, with no grouping effects from adjacent cables.
Limitations
- HUMAN REVIEW REQUIRED — the current-capacity side is an approximation, not a regulatory figure. Real ampacity is read from tables in the wiring regulations and varies substantially with installation method, grouping, ambient temperature and insulation type. A single capacity-per-square-millimetre figure can indicate a smaller conductor than the tables require, and an under-sized conductor is a fire risk.
- Nothing here selects a protective device, checks earth-fault loop impedance, verifies disconnection times or considers short-circuit withstand — all of which are required parts of a compliant design.
- The voltage-drop side ignores reactance, which understates drop on large conductors and long AC runs.
- This calculator is for planning and estimating only. It is not a cable schedule, and it must not be used to size a conductor for an installation.
Sources
- The national wiring regulations applying where the installation is located, and their current-carrying capacity tables — Varies by country. The ampacity figures, derating factors, permitted voltage drop and every other input to a compliant design. This calculator approximates the first and applies none of the rest.
- Conductor resistivity for copper and aluminium — Standard material data; a cable manufacturer's datasheet governs a specific construction. The voltage-drop side of the calculation, which is straightforward physics.