Cable De Rating Factor
A cable de-rating factor is a numerical multiplier applied to a cable's tabulated ampacity to adjust for actual installation and environmental conditions, ensuring the cable does not exceed its permitted operating temperature. Typically ≤ 1.0 under less favorable conditions (e.g., high ambient temperature, grouping, insulation), it may exceed 1.0 under more favorable conditions (e.g., cooler ambient).
On a shop floor, cable de-rating factors are applied during design, installation changes, and compliance reviews to ensure correct cable sizing for feeders and branch circuits. Engineers obtain standard ampacity from CEC/NEC tables, identify applicable factors (ambient temperature, grouping, insulation, burial), look up each factor from standard tables, multiply them together, and apply the total factor to the standard rating. This ensures safe routing, cable management, and compliance with WorkSafeBC/OSHA requirements, preventing overheating, insulation degradation, and electrical fires.
How is a cable de-rating factor mathematically applied during design?
The derated ampacity is computed as I_a = I_s × (k_ambient × k_grouping × k_insulation × k_burial × ...), where I_s is the standard ampacity and each k_i is a dimensionless factor from standard tables. All relevant factors are multiplied together. Alternatively, to ensure the cable is sufficient for a protective device rating I_n, use I_required = I_n / (k_1 × k_2 × ...) and select a cable with I_s ≥ I_required.
What environmental and installation parameters typically drive de-rating?
Primary drivers include air ambient temperature, ground temperature and burial depth, soil thermal resistivity, grouping/bunching of circuits or cables, thermal insulation around the cable, and installation method (e.g., conduit, tray, free air). These parameters are codified in NEC/CEC/IEC tables and must be systematically checked for each installation.
How does cable de-rating intersect with protective device selection?
The protective device rating must be coordinated with the derated cable ampacity so the cable never carries current above its derated rating. The design ensures I_a ≥ design load and I_s ≥ I_n / k_total. Mismatched ratings (e.g., breaker sized to base ampacity but cable heavily derated) can be a hazardous condition requiring correction.