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Rooftop Conduit Ampacity (120°F)
Find the real-world power limit of your wire in hot or crowded conditions.
Quick Answer
At 120°F (50°C), #12 THHN is significantly derated. After temperature and grouping adjustments, its reliable ampacity drops well below 20A.
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 50°C (122°F)
x 0.82
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.
4 Current-Carrying Conductors
x 0.80
Calculated Derated Value19.7 A
Terminal Limit (75C) 25 A
Terminal Rule
The final ampacity cannot exceed the rating of the termination temperature (NEC 110.14(C)).