): Standard tables assume ~30°C for air or ~25°C for soil. If the site is hotter (e.g., 45°C), the cable can dissipate less heat and its rating must be reduced (often by ~10–25%). Cable Grouping (
In the context of electrical engineering, (also known as correction factors) are multipliers applied to the current-carrying capacity (ampacity) of a cable to ensure safe operation under non-standard conditions.
), the target ampacity must be divided by the total derating factor to find the minimum required baseline cable rating:
): Deeper cables are further from the surface and find it harder to release heat into the ground. Soil Thermal Resistivity ( Cscap C sub s derating factor for cables
Possesses higher thermal resilience. At 45°C ambient air, the derating factor typically sits near 0.88 . 2. Grouping and Bundling Factor ( Cgcap C sub g
): Dry or sandy soil acts as an insulator, whereas wet clay helps pull heat away. Electricians' Guide: Cable Rating Factors
losses). A cable relies on the temperature differential between its core and the surrounding environment to shed this heat. When ambient air or ground temperatures exceed standard reference benchmarks (e.g., 30°C in air per IEC 60364-5-52 ), heat dissipation slows down. ): Standard tables assume ~30°C for air or ~25°C for soil
For XLPE at 55°C → factor ≈ 0.85 (from detailed table). Step 2 – Grouping factor: 4 cables touching → factor = 0.70. Step 3 – Soil factor: Dry sand → factor = 0.75.
Therefore, the cable should not carry more than safely.
For cables in conduit/trunking, use similar or stricter factors. NEC Table 310.15(C)(1) provides detailed adjustments. ), the target ampacity must be divided by
): Cables in free air cool best. Those buried, in conduits, or surrounded by (like fiberglass in a wall) require significant derating because heat is trapped. Burial Depth ( Cdcap C sub d
Actual Cable Ampacity = Rated Ampacity (in ideal conditions) × Derating Factor