Wire Gauge Calculator
Enter a required ampacity and installation conditions to get the minimum AWG. Uses NEC 2020 Table 310.16 with full derating — ambient temperature, conductor count, and 110.14(C) termination cap applied in the correct NEC order. Copper and aluminum, 60–90°C insulation.
How to use this calculator
- Enter the required load current in amperes. For continuous loads (running 3+ hours), check the "Continuous load" box — the calculator applies the NEC 125% multiplier.
- Select conductor material, insulation rating, and termination rating. The termination rating caps the usable ampacity per NEC 110.14(C) regardless of insulation.
- Enter the ambient temperature and number of current-carrying conductors in the raceway or cable.
- Read the minimum AWG and the derating breakdown showing how each factor was applied.
NEC reference
This calculator implements NEC 2020 Table 310.16 in the correct order: base ampacity from the conductor insulation column, multiplied by the ambient correction factor from Table 310.15(B)(1), multiplied by the conductor-count adjustment from Table 310.15(C)(1), then capped at the termination temperature column per NEC 110.14(C). The final ampacity is the minimum of the derated value and the termination limit.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
How wire gauge is determined by the NEC
NEC Table 310.16 lists allowable ampacities for conductors in raceway, cable, or direct burial based on 30°C (86°F) ambient and not more than three current-carrying conductors. When installation conditions differ, three derating steps apply:
- Ambient correction (310.15(B)(1)): When ambient exceeds 30°C, a correction factor less than 1.0 reduces ampacity. When ambient is below 30°C, the factor exceeds 1.0 — a boost the NEC permits but most termination ratings prevent from being used.
- Conductor-count adjustment (310.15(C)(1)): More than three current-carrying conductors in a raceway or cable increases heat and reduces ampacity. Four to six conductors: 80%; seven to nine: 70%; and so on.
- Termination cap (110.14(C)): The derated ampacity may not exceed the Table 310.16 value at the lowest temperature rating of the termination or conductor, whichever is lower. Most panels and lugs are rated at 75°C; some small equipment at 60°C.
The final usable ampacity is the lesser of the derated value and the termination cap. This calculator applies all three steps and returns the smallest conductor whose final ampacity meets or exceeds your requirement.
Wire gauge chart — copper, 75°C terminations (NEC Table 310.16)
Standard conditions: 30°C (86°F) ambient, 3 or fewer current-carrying conductors, 90°C THHN/THWN-2 insulation, 75°C terminations. These are the common-case ampacities most contractors use for commercial work.
| AWG / kcmil | Ampacity (A) | Typical application |
|---|---|---|
| 14 AWG | 20 | 15A circuits (240.4(D) limits OCPD to 15A) |
| 12 AWG | 25 | 20A circuits (240.4(D) limits OCPD to 20A) |
| 10 AWG | 35 | 30A circuits, A/C equipment, dryers |
| 8 AWG | 50 | 50A circuits, ranges, large A/C |
| 6 AWG | 65 | EV chargers, sub-panels |
| 4 AWG | 85 | Small service entrance feeders |
| 2 AWG | 115 | 100A service conductors |
| 1/0 AWG | 150 | 125A feeders |
| 2/0 AWG | 175 | 150A feeders |
| 3/0 AWG | 200 | 175A feeders |
| 4/0 AWG | 230 | 200A service conductors |
| 250 kcmil | 255 | 200A+ feeders |
Source: NEC 2020 Table 310.16, 90°C copper column capped at 75°C termination. Use the calculator above for derating and non-standard conditions.
Frequently asked questions
What wire gauge do I need for 20 amps?
14 AWG copper is the minimum by Table 310.16 ampacity for a 20A load at standard conditions. However, NEC 240.4(D) limits overcurrent protection on 14 AWG to 15A — if the circuit needs a 20A breaker, 12 AWG is required. For a 20A continuous load, 10 AWG is the minimum conductor (125% of 20A = 25A; 12 AWG only reaches 25A at the edge). Always check both the ampacity requirement and the breaker size.
Does wire gauge change for continuous loads?
Yes. NEC 210.19(A)(1) requires conductors serving continuous loads (3+ hours) to be sized at 125% of the continuous current. A 20A continuous load requires the conductor to carry 25A, which pushes the minimum from 14 AWG up to 12 AWG at 75°C termination conditions. Check the "Continuous load" box and the calculator applies this multiplier before looking up the AWG.
What is the 110.14(C) termination rule?
NEC 110.14(C) limits the usable conductor ampacity to the Table 310.16 value at the lowest temperature rating of either the conductor insulation or the termination, whichever is lower. A 90°C THHN conductor in a 75°C-rated panelboard cannot use the 90°C ampacity — it is capped at the 75°C column. Most 100A+ equipment is rated 75°C. Equipment rated for only 60°C (common in smaller disconnect sizes) forces a further reduction. This is the most commonly misapplied rule in conductor sizing on the jobsite.
What wire gauge for aluminum vs copper?
Aluminum is typically two wire gauges larger than copper for the same ampacity at common sizes. A 4 AWG copper conductor at 75°C carries 85A; 2 AWG aluminum at 75°C carries 90A — close, but not identical across the full range. NEC Table 310.16 does not list aluminum below 12 AWG; the minimum aluminum conductor for any application is 12 AWG. Aluminum requires compatible terminals — AL/CU-listed or aluminum-rated lugs per NEC 110.14.
Related calculators
- Wire Size Calculator — sizes for both ampacity and voltage drop simultaneously, with OCPD selection.
- Wire Ampacity Calculator — check the derated ampacity of a specific conductor size you already have in mind.
- Voltage Drop Calculator — verify the conductor also meets the 3% and 5% NEC voltage drop recommendations.
- Conduit Size Calculator — size the raceway once the conductor gauge is determined.