Cable Derating Factor Table [updated] Online

As the surrounding temperature rises, the cable’s internal heat adds to the ambient heat, accelerating insulation aging. Derating tables provide multipliers for different temperature ranges. For example, a standard PVC-insulated cable rated for 30°C might have a derating factor of 0.88 at 40°C, and 0.75 at 50°C. Conversely, at very low temperatures, the factor can exceed 1.0, allowing higher currents.

In the field of electrical engineering, selecting the correct cable size is not merely a matter of matching a conductor to a load current. A cable that performs perfectly in open air at 20°C can overheat, fail, or become a fire hazard when installed in a hot, enclosed conduit alongside several other circuits. To address this, engineers rely on the cable derating factor table —a set of standardized multipliers that adjust a cable’s current-carrying capacity (ampacity) based on real-world installation conditions. What is a Derating Factor? A derating factor is a numerical value (always less than or equal to 1.0) by which a cable’s base ampacity is multiplied. The base ampacity is the current a cable can carry under ideal reference conditions (e.g., 30°C ambient temperature for thermoplastic cables, free air, single circuit). When conditions deviate from this ideal, the cable’s ability to dissipate heat diminishes. Since heat is the primary enemy of insulation, the current must be reduced—or "derated." Key Factors in Derating Tables Derating factor tables are organized around three main environmental and installation parameters: cable derating factor table

To use it:

| Ambient Temp (°C) | Derating Factor (PVC, 30°C base) | |------------------|----------------------------------| | 30 | 1.00 | | 35 | 0.94 | | 40 | 0.87 | | 45 | 0.79 | | 50 | 0.71 | As the surrounding temperature rises, the cable’s internal