Cable Ampacity and Derating Factors Explained

Heat Balance

Current produces conductor losses. Heat must pass through insulation and sheath into air, soil, tray or conduit. Anything that restricts heat transfer raises conductor temperature.

Common Derating Factors

• Ambient air or soil temperature
• Multiple loaded cables grouped together
• Thermal insulation around wiring
• Burial depth and soil thermal resistivity
• Duct banks and conduit fill
• Harmonic currents and unbalanced loading

Applying Tables

Use base ampacity and correction factors from the same design standard. Multiplying unrelated tables can produce an inconsistent result. For complex circuits, use validated calculation software or specialist analysis.

Operational Margin

Consider load growth, emergency duty and cyclic loading, but avoid replacing engineering with arbitrary oversizing. Larger cable affects cost, bend radius, terminations and fault energy.