Overview
The wire sizing calculator determines the minimum conductor size for a circuit by checking ampacity (Table 310.16), applying ambient temperature correction factors (Table 310.15(B)(1)), conduit fill adjustment factors (Table 310.15(B)(3)(a)), and the 125% continuous load multiplier.
The output shows each constraint’s result and highlights the controlling factor that determines the final wire size.
Inputs
- Load current: Circuit amperage in amps
- Continuous load: Yes/No (applies 125% multiplier per NEC 210.19(A)(1))
- Conductor material: Copper or aluminum
- Insulation type: THHN/THWN-2 (90°C), THWN (75°C), TW (60°C), etc.
- Ambient temperature: Actual temperature where conductors are installed (°C)
- Conductors in raceway: Number of current-carrying conductors in the conduit
- Termination rating: 60°C or 75°C (per NEC 110.14(C))
Multi-Factor Convergence
The calculator evaluates four constraints simultaneously:
- Base ampacity from Table 310.16 for the selected insulation temperature rating
- Temperature-corrected ampacity using the ambient temperature correction factor
- Conduit-fill-derated ampacity using the conductor count adjustment factor
- Voltage drop requirement (if circuit length is provided)
The final wire size is the largest required by any constraint. The output clearly labels which factor controls: “Wire size controlled by: conduit fill derating (8 AWG required) > ampacity (10 AWG sufficient).”
NEC References
- NEC Table 310.16 — allowable ampacities of insulated conductors
- NEC Table 310.15(B)(1) — ambient temperature correction factors
- NEC Table 310.15(B)(3)(a) — adjustment factors for more than 3 conductors in raceway
- NEC 210.19(A)(1) — 125% sizing for continuous loads
- NEC 110.14(C) — termination temperature limitations
- NEC 310.10(G) — parallel conductor requirements (1/0 AWG minimum)
Try it yourself
Calculate voltage drop for single-phase and three-phase circuits per NEC 2026 standards.
Calculate residential service size per NEC 2026 optional method with demand factor breakdown.
Calculate wire size per NEC Table 310.16 with temperature and conduit fill derating.
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