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Attic Wiring Derating (140°F)
Find the real-world power limit of your wire in hot or crowded conditions.
Quick Answer
In a 140°F (60°C) attic, the ampacity of standard wire is reduced by approx 30% (factor of 0.71). It is critical to ensure that your lighting and power circuits have enough overhead for these extreme temperatures.
1Conductor Specifications
-
2Conditions & Adjustment Factors
Ambient Temperature Adjustment
NEC tables assume a standard room temperature of 30°C (86°F). If your conduit is in a hot attic or on a rooftop, the wire cannot dissipate heat as well, and its ampacity must be reduced.
Current Carrying Conductors
The number of wires in a single pipe that are actually "working." Do not count ground wires. Neutrals only count if the load is non-linear (like computers or LED drivers).
Calculation SummaryCalculation based on NEC Table 310.16 (formerly 310.15(B)(16)). Final value is the lower of the adjusted ampacity or the terminal rating.
Calculation Summary
Calculation based on NEC Table 310.16 (formerly 310.15(B)(16)). Final value is the lower of the adjusted ampacity or the terminal rating.
-Amps
Base Ampacity--
Derated Value --
Base (90C) vs. Derated Value
Per NEC 110.14(C), you can use the high-heat 90°C column as your starting point for math, but your final result can never exceed the rating of your breaker's terminals (usually 75°C).
Calculation Breakdown
1
Base Ampacity Base Table ValueStarting value from NEC Table 310.16.
Base Table Value
Starting value from NEC Table 310.16.
12 AWG Copper @ 90C column
30 A
2
Temp Correction Temperature FactorCorrection factor for ambient temperatures other than 30°C (86°F).
Temperature Factor
Correction factor for ambient temperatures other than 30°C (86°F).
Ambient 60°C (140°F)
x 0.71
3
Fill Adjustment Fill FactorAdjustment for more than 3 current-carrying conductors in a raceway.
Fill Factor
Adjustment for more than 3 current-carrying conductors in a raceway.
2 Current-Carrying Conductors
x 1.00
Calculated Derated Value21.3 A
Terminal Limit (75C) 25 A
Terminal Rule
The final ampacity cannot exceed the rating of the termination temperature (NEC 110.14(C)).