The most critical constraint is thermal. When current ($I$) flows through a conductor of resistance ($R$), heat is generated at a rate of $I^2R$ (Joule's law). The cable must dissipate this heat without exceeding its insulation's maximum permissible temperature. The governing formula is deceptively simple:
For a , the approximate voltage drop ($V_d$) is: formula for cable size calculation
The formula for the required current capacity of the cable ($I_z$) is: The most critical constraint is thermal
It is important to note that resistivity ($\rho$) varies slightly based on the specific alloy of copper or aluminum used and the operating temperature. The governing formula is deceptively simple: For a
To solve for the cable size based on a permissible voltage drop (usually limited to 3% or 5% of the supply voltage), the formula becomes:
The formula for calculating the cable size (or cross-sectional area) depends on several factors including the load current, the voltage drop allowed, the type of conductor material (usually copper or aluminum), the insulation type, and the ambient temperature. A common approach to sizing cables is to ensure that the cable can carry the expected load current without overheating and that the voltage drop along the cable does not exceed a specified limit.
: This is often the same as the load current but consider factors like future expansions.