Maximum cable length calculator
Estimate the longest one-way cable run before reaching a user-selected voltage drop. This is a simplified resistance-only screening calculation, not a complete cable-sizing method.
How the calculation works
We first estimate conductor resistivity at operating temperature: ρ(T) = ρ20 × [1 + α × (T − 20)]. Then:
Lmax = (U × allowed_drop/100 × A) / (k × I × ρ(T))
Here k = 2 for a single-phase circuit with a return conductor, and k = √3 for a balanced three-phase circuit. Lmax is the one-way length, not the sum of conductor lengths. Copper uses ρ20 = 0.01724 Ω·mm²/m and α = 0.00393/°C; aluminium uses 0.02826 and 0.00403/°C.
Worked example
For a 230 V single-phase circuit, 20 A, 6 mm² copper, 70 °C conductor temperature and a chosen 3% drop budget, the calculated limit is approximately 50.2 m one-way. This is an estimate, not a regulatory maximum.
Important limitations
Do not use this result alone to choose a cable. The simplified model omits conductor reactance, power factor effects, connection resistance, fault-loop impedance, protective device operation, current-carrying capacity, grouping, installation method, and applicable national requirements. For long runs and larger conductors, AC effects may be material. Verify with manufacturer data and applicable standards.
Frequently asked questions
Is 3% always the permitted voltage drop?
No. The percentage is a user-defined design target. Requirements depend on the installation and applicable standards.
Can I use the same length for aluminium?
No. Aluminium has higher resistivity than copper at equal cross-section, so the estimated maximum run is shorter under the same conditions.