If voltage drop exceeds limits → .
Finally, you must ensure that in the event of a short circuit, enough current flows to trip the breaker instantly. This involves ensuring the resistance of the cable (impedance) is low enough.
VD=(mV/A/m)×Ib×L1000cap V cap D equals the fraction with numerator open paren m cap V / cap A / m close paren cross cap I sub b cross cap L and denominator 1000 end-fraction cable sizing formula
It≥InCa×Cg×Ci×Cfcap I sub t is greater than or equal to the fraction with numerator cap I sub n and denominator cap C sub a cross cap C sub g cross cap C sub i cross cap C sub f end-fraction Cacap C sub a
Are you sizing for a system or a 230V AC residential circuit? Knowing the voltage will help refine the math! Electrical Cable Size Calculator & Cable Size Chart Amps If voltage drop exceeds limits →
Short-circuit thermal check (k for Cu/PVC = 115): [ A_min = \frac10,000 \times \sqrt0.1115 \approx 27.5 \text mm^2 ] 25 mm² is slightly below → upgrade to 35 mm² for fault withstand.
The fundamental relationship is derived from and power loss : VD=(mV/A/m)×Ib×L1000cap V cap D equals the fraction with
If the cable size selected from the Thermal and Voltage Drop steps is smaller than $A_min$, the cable size must be increased to match $A_min$ to prevent insulation damage or conductor fusion during a fault.
Even if a cable can handle the heat, it might be so long that the voltage drops too much by the time it reaches the equipment. Most codes (like the NEC or BS 7671) limit voltage drop to 3% for lighting and 5% for other uses.
: The millivolt drop per ampere per meter (found in manufacturer data sheets). : The length of the run in meters. If the calculated VDcap V cap D
The designer calculates the effective current requirement ($I_eff$) using the denominator above, then looks up a cable table to find a size where the tabulated $I_z \geq I_eff$.