Updated | Derating Factor For Cables

For cables in conduit/trunking, use similar or stricter factors. NEC Table 310.15(C)(1) provides detailed adjustments.

A is a multiplier (less than 1) applied to a cable’s rated current-carrying capacity to reduce its maximum allowable load. This ensures the cable operates within safe temperature limits under real-world conditions.

): When multiple cables are bunched in a conduit or tray, they heat each other up. Grouping six circuits might reduce each cable's capacity to just of its original rating. Installation Method ( derating factor for cables

Soil’s ability to dissipate heat varies. Standard assumes ( R_th = 2.5 , K·m/W ) (damp soil). Dry or sandy soil reduces ampacity.

If a cable has a base rating of , but is installed in an ambient temperature of 40°C (factor 0.91) alongside 3 other cables (factor 0.8): For cables in conduit/trunking, use similar or stricter

: Depth of burial correction factor (for underground installations).

): 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 This ensures the cable operates within safe temperature

): Dry or sandy soil acts as an insulator, whereas wet clay helps pull heat away. Electricians' Guide: Cable Rating Factors