NEC Cable Ampacity Calculator
Wire Type
Insulation Temperature Rating
Installation
°C
Rooftop Proximity
Continuous Load
Final Derated Ampacity40.0 A
Base Ampacity (NEC Table)40 A
Temp Correction Factor1
Conduit Fill Factor1
Rooftop Temp Adder+0°C
Continuous Load Factor1
Recommended Breaker40 A
A conductor can only carry so much current before its insulation overheats, and the National Electrical Code adjusts that limit for real installation conditions. This calculator looks up the base ampacity from NEC Table 310.16 (raceway) or 310.17 (free air) for copper conductors, then applies temperature correction, conduit-fill adjustment, rooftop heating, and the continuous-load factor to give a final derated ampacity and a recommended breaker.
Formula
Ifinal = Ibase × Ctemp × Cfill × Ccont
- Ifinal
- Final derated allowable ampacity (A)
- Ibase
- Base ampacity from NEC Table 310.16 or 310.17 for the gauge and insulation rating
- Ctemp
- Temperature correction factor (Table 310.15(B)(1)) at the effective ambient temperature
- Cfill
- Conduit-fill adjustment factor (Table 310.15(C)(1)) for the conductor count
- Ccont
- Continuous-load factor: 0.80 for continuous loads, 1.0 otherwise
How it works
- Select the wire gauge (14 AWG through 2000 kcmil), insulation temperature rating (60, 75, or 90 °C), and whether the run is in conduit or free air to pull the base ampacity from the correct NEC table.
- Enter the ambient temperature and the number of current-carrying conductors. Temperature correction comes from Table 310.15(B)(1) and the conduit-fill adjustment from Table 310.15(C)(1); add a rooftop distance to apply the sunlight temperature adder.
- Toggle continuous load to apply the 80% factor (the inverse of the 125% sizing rule), then read the final derated ampacity and the largest standard NEC 240.6 breaker at or below it.
Worked example
6 AWG copper THHN at the 75 °C rating, in conduit with 3 current-carrying conductors, 40 °C ambient, feeding a continuous load.
- Base ampacity (Table 310.16, 6 AWG, 75 °C column): 65 A.
- Temperature correction at 40 °C for 75 °C insulation: 0.88.
- Conduit-fill factor for 3 conductors: 1.00.
- Continuous-load factor: 0.80.
- Final ampacity: 65 × 0.88 × 1.00 × 0.80 = 45.76 A.
Final derated ampacity ≈ 45.76 A; recommended standard breaker is 45 A.
Frequently asked questions
- Why is the base ampacity adjusted for temperature and conduit fill?
- Conductors are rated assuming a 30 °C ambient and no more than three together. Hotter surroundings and bundling both reduce the conductor’s ability to shed heat, so the NEC multiplies the base value by correction and adjustment factors to keep the insulation within its temperature rating.
- When does the continuous-load factor apply?
- A continuous load is one expected to run at its maximum for three hours or more. For these loads the conductor and overcurrent device must be sized at 125% of the load, which is equivalent to derating the conductor ampacity to 80%, the factor this tool applies.
- Why does proximity to a rooftop matter?
- Conduit exposed to sunlight on or above a roof runs significantly hotter than the surrounding air. The NEC adds a temperature adder (up to 33 °C within half an inch of the roof) to the ambient before the temperature correction factor is selected.
- Does this calculator cover aluminum conductors?
- No. The Table 310.16 and 310.17 values built into this tool are for copper conductors. Aluminum and copper-clad aluminum have their own ampacity columns with lower values and would require a different lookup.