At first glance, calculating the correct cable size for an electrical installation seems mundane. The common instruction is straightforward: determine the load current, consult a table (like those from NEC, IEC, or BS standards), and pick a cable that handles that current. If the run is long, check the voltage drop. Done.
But this approach misses the deeper, more interesting reality. Choosing a cable is not a simple lookup; it is an act of engineering compromise, a balancing act between physics, economics, and safety. The question "how to calculate the cable size" is actually four intertwined questions in a trench coat. The most obvious factor is the cable's current-carrying capacity (ampacity). A wire is not a frictionless pipe for electrons; it has resistance. When current flows, power is dissipated as heat ( ( P = I^2R ) ). This heat must escape into the surroundings. If the current is too high, the insulation melts, the conductor oxidizes, or worse, a fire starts. how to calculate the cable size
Here lies the first interesting complication: . A 10 mm² copper cable can carry 60 amps in free air at 30°C, but only 45 amps when buried in hot thermal insulation. Why? Because the insulation traps heat. Derating factors for ambient temperature, grouping of cables, and soil thermal resistivity transform a simple table into a multivariate equation. To truly "calculate" the size, you must model the thermal circuit—a concept analogous to Ohm's law where temperature rise is "voltage," heat flow is "current," and thermal resistance is... resistance. 2. The Voltage Drop: The Silent Efficiency Killer If you only size for thermal limits, long runs will disappoint you. A motor at the end of a 300-meter cable might receive 200V instead of 230V. It will draw more current, overheat, and fail early. Voltage drop ( ( V_{drop} = I \times R_{cable} \times length ) ) is not just a nuisance—it is an economic and performance constraint. At first glance, calculating the correct cable size