Cable Ampacity Calculator
Determine cable ampacity with NEC derating factors for temperature, conduit fill, and continuous loads.
Inputs
NEC 310.16: #14 AWG=15A; #12=20A; #10=30A; #8=50A; #6=65A (60°C THHN, 3 conductors)
NEC 310.4: THHN ampacity based on 75°C column for most terminations (NEC 110.14(C))
NEC base: 30°C; 40°C: derate to 0.87×; 50°C: 0.75×; 60°C: 0.58× of base ampacity
NEC 310.15(C)(1): ≤3 = 100%; 4–6 = 80%; 7–9 = 70%; 10–20 = 50%; 21–30 = 45%
NEC 210.19: continuous load conductor = 125% of load; breaker = 125% of continuous load
Results
Derated Ampacity
40 A
Required Breaker Size
40 A
How to Use This Calculator
- Select the Conductor Size in AWG — smaller AWG numbers indicate larger wires (e.g., AWG 10 is larger than AWG 14).
- Select the Insulation Type: THHN is the most common general-purpose wire; XHHW is used in wet locations.
- Enter the Ambient Temperature in °C around the conductors — NEC base is 30°C; higher temperatures require derating.
- Enter the number of Current-Carrying Conductors in the raceway — three or fewer requires no fill derating.
- Indicate whether this is a Continuous Load (3+ hours) — NEC requires conductors to be derated to 80% for continuous loads.
- Read the Derated Ampacity in amperes and the Required Breaker Size to confirm the circuit is properly protected.
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