Calculating Cable Size [repack] Jun 2026

What is the (e.g., in conduit, direct buried, open air)?

This guide follows and British Standard BS 7671 (which aligns with many other national codes). For US/Canada (NEC), the process is similar but uses different tables (AWG, 60/75/90°C columns, and NEC ampacity tables). calculating cable size

Since 13.7V < 20.75V, the .

For long runs (>50m), voltage drop often drives cable size larger than ampacity requires. What is the (e

Calculating the correct cable size is one of the most critical aspects of electrical design. It is a balancing act between safety, efficiency, and cost. A cable that is too small will overheat, posing a fire hazard and causing voltage drops that damage equipment. A cable that is too large is an unnecessary waste of money and creates difficulties in termination and installation. Since 13

Multiply length limits by ~0.55 (same mm²).

It≥InCa×Cg×Cd×Cicap 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 d cross cap C sub i end-fraction Cacap C sub a = Ambient temperature correction factor Cgcap C sub g = Grouping or clustering correction factor Cdcap C sub d = Installation depth correction factor (for buried cables) Cicap C sub i = Thermal insulation correction factor 4. Select Candidate Cable Size from Ampacity Tables Cross-reference your calculated Itcap I sub t