1. Definition
Cable ampacity is the maximum safe continuous current a cable can carry under specified ambient temperatures and installation conditions, while maintaining conductor temperature within the insulation's allowable limit. It applies only to steady-state long-term operation, excluding startup inrush current and short-circuit transient current.
2. Influencing Factors
Ampacity is not a fixed value. It varies with heat dissipation conditions and cable specifications:
- Conductor Material & Size: Copper has higher ampacity than aluminum; larger cross-section improves current capacity.
- Insulation Rating: High-temperature resistant insulation (XLPE) allows higher continuous current than ordinary PVC.
- Ambient Temperature: Higher ambient temperature reduces heat dissipation and lowers effective ampacity.
- Installation Method: Free-air laying provides optimal cooling. Conduit, multi-cable stacking and tray installation cause heat accumulation and derate ampacity.
- Soil Thermal Resistance: Dry and dense soil worsens heat dissipation and reduces ampacity for buried cables.
- AC/DC Difference: AC skin effect and proximity effect decrease the effective conductive area, resulting in lower ampacity than DC operation.
3. Standard Calculation Method
Engineering ampacity calculation follows IEC 60287 and GB/T 16895, using base ampacity multiplied by environmental and layout correction factors.
Formula
I_allow=I_base * K1 * K2*K3*K4
Parameter Explanation
- I_base: Standard base ampacity under ideal single-cable free-air conditions.
- K₁: Ambient temperature correction factor.
- K₂: Multi-cable parallel layout correction factor.
- K₃: Enclosed/stacked installation correction factor.
- K₄: Soil thermal resistance and burial depth correction factor.
The corrected allowable ampacity must exceed the maximum continuous load current. Short-circuit thermal stability must be verified separately.
4. Technical Notes
- Ampacity defines long-term continuous safety limits, not for transient startup or overload current.
- All ampacity values are condition-based. Any layout or environmental change requires recalculation.
- Correct ampacity selection avoids cable overheating, insulation degradation, and fire risks.
