How do you determine wire size per NEC?

Wire sizing per NEC requires checking multiple constraints simultaneously: (1) base ampacity from Table 310.16, (2) temperature correction from Table 310.15(B)(1), (3) conduit fill adjustment from Table 310.15(B)(3)(a), and (4) termination temperature limit from NEC 110.14(C). The required wire size is the smallest size that satisfies ALL constraints. For continuous loads, multiply the load current by 1.25 per NEC 210.19(A)(1).

What is the 75°C termination rule?

Per NEC 110.14(C), most equipment terminals and circuit breakers are rated at 75°C. Even when using 90°C insulation (like THHN), you can use the 90°C ampacity for derating calculations, but the final ampacity must not exceed the 75°C column value for the wire size. This is a common controlling factor — especially at standard temperatures with few conductors, where there is no significant derating.

When do I need to derate for conduit fill?

Conduit fill derating per NEC Table 310.15(B)(3)(a) applies when more than 3 current-carrying conductors share a raceway or cable. With 4-6 conductors the factor is 0.80 (80% of rated ampacity), 7-9 is 0.70, and 10-20 is 0.50. Grounding conductors and neutrals that carry only unbalanced current are not counted. This derating is cumulative with temperature correction.

Why does 90°C wire sometimes size the same as 75°C wire?

At standard conditions (30°C ambient, 3 or fewer conductors), there is no derating to apply. Since the termination limit is based on the 75°C column regardless of insulation, the 90°C insulation provides no advantage in wire sizing. The benefit of 90°C insulation appears when derating is needed — you start from a higher base ampacity, so after temperature correction or conduit fill reduction, you may still meet the termination limit in a smaller wire size.

Wire Size Calculator

Calculate the minimum wire size per NEC Table 310.16 with temperature correction, conduit fill derating, and termination rating verification. Shows which constraint controls your wire size.

Load Current

Current (amps)

Continuous load (125%)

Conductor Material

Insulation Type

Derating Conditions

Ambient Temperature

Conductors in Raceway

Termination Ratingper NEC 110.14(C)

Results

NEC 2026

Enter load current to see the recommended wire size.

NEC Table 310.16 — base ampacity

Table 310.15(B)(1) — temperature correction

Table 310.15(B)(3)(a) — conduit fill adjustment

NEC 110.14(C) — termination rating

Understanding NEC Wire Sizing

Multi-Factor Sizing

Wire sizing isn’t a single table lookup. The NEC requires you to check base ampacity (Table 310.16), apply temperature correction (Table 310.15(B)(1)), conduit fill derating (Table 310.15(B)(3)(a)), and verify the termination rating (110.14(C)). The required wire size is the largest demanded by any single constraint.

Temperature Correction

Conductor ampacity ratings in Table 310.16 assume a 30°C (86°F) ambient temperature. When conductors are installed in hotter environments — attics, rooftops, boiler rooms — the ampacity must be reduced. At 50°C ambient, a 90°C conductor loses 18% of its rated ampacity.

Conduit Fill Derating

When more than 3 current-carrying conductors share a raceway, heat dissipation is reduced and ampacity must be derated. With 4-6 conductors the factor is 0.80; at 10-20 conductors it drops to 0.50. This is the most commonly overlooked derating in panel feedthrough and commercial installations.

The 75°C Termination Rule

Per NEC 110.14(C), most circuit breakers and equipment terminals are rated at 75°C. Even when using 90°C insulated wire (THHN), you can use the higher insulation rating for derating calculations, but the final ampacity cannot exceed the 75°C column value. This is why the “controlling factor” often shows as termination limit.

This tool provides calculations for informational purposes only and does not constitute professional engineering, construction, or safety advice. Verify all results independently before use in design, construction, or compliance decisions. See our Terms of Service.

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