Formula for single-phase (approximate): [ V_drop = \frac2 \times L \times I \times R_cable1000 ] Where: ( L ) = one-way length (m), ( I ) = actual load current (A), ( R_cable ) = resistance per km (from cable data, e.g., 7.98 Ω/km for 2.5 mm² copper).
Cable Size (mm²) = (25A x 50m x 0.05) / (0.017 ohm-mm²/m x 20V) ≈ 6mm² calculate the cable size
[ I = \fracP\sqrt3 \times V_L \times \textPower Factor ] Formula for single-phase (approximate): [ V_drop = \frac2
A common mistake in cable sizing is relying solely on standard ampacity tables without accounting for installation conditions. A cable’s ability to dissipate heat is heavily influenced by its environment. Standard tables typically assume a specific ambient temperature (e.g., 30°C in air or 20°C underground). However, real-world conditions rarely align perfectly with these ideals. Engineers must apply "derating factors" to the cable’s rated capacity based on ambient temperature, the method of installation (e.g., in a conduit, buried underground, or on a cable tray), and proximity to other current-carrying cables. For instance, when multiple cables are grouped in a confined space, they radiate heat onto one another, significantly reducing each cable's capacity to carry current. The calculation must adjust for these variables to ensure safety under "worst-case" scenario conditions. For instance, when multiple cables are grouped in
I = 10,000W / 400V = 25A
Finally, verify the calculated cable size against the minimum cable size requirements for your specific application. You can consult national electrical codes, such as the NEC (National Electric Code), or consult with a qualified electrician.