Wire Ampacity & Derating Calculator (NEC 310.16)

Find the real-world power limit of your wire in hot or crowded conditions.

1Conductor Specs

Material

Wire Material

Copper is standard. Aluminum is lighter/cheaper but requires larger sizes.

Wire Size

Conductor Size

AWG or kcmil size of the conductor.

Insulation Rating

Insulation Temp

Temperature rating of the wire insulation itself (60°C, 75°C, or 90°C).

2Conditions

Ambient Temp

Ambient Temperature

Surrounding air temperature. Higher temps reduce ampacity.

(30°C / 86°F)

20°C80°C

Current-Carrying Conductors

Conductor Count

Number of current-carrying wires in the same raceway. >3 requires derating.

No Derating

Terminal Rating

Termination Temp

Temperature rating of the equipment terminals (breakers/lugs). Usually limits the final ampacity.

Usually 75°C for modern breakers.

Allowable Ampacity
Final Ampacity
The maximum safe current the conductor can carry after all adjustments (NEC 310.15).

NEC Table 310.16 →Warning

-Amps

Base (90C)

Table Ampacity

Starting ampacity from NEC Table 310.16 before adjustments.

Derated Value

Adjusted Ampacity

Ampacity after temperature and fill correction factors are applied.

Notice: The ampacity is limited to the 75C column because the terminal temperature rating (75C) is lower than the wire insulation rating (90C).

Calculation Breakdown

Base Ampacity
Base Table Value
Starting value from NEC Table 310.16.

12 AWG Copper @ 90C column (Table 310.16)

30 A

Temp Correction
Temperature Factor
Correction factor for ambient temperatures other than 30°C (86°F).

Ambient 30°C (86°F)

x 1.00

Fill Adjustment
Fill Factor
Adjustment for more than 3 current-carrying conductors in a raceway.

3 Current-Carrying Conductors

x 1.00

Calculated Derated Value30.0 A

Terminal Limit (75C)

Terminal Rule

The final ampacity cannot exceed the rating of the termination temperature (NEC 110.14(C)).

25 A

View Table 310.15(B)(1) (Temp)View Table 310.15(C)(1) (Fill)

Popular ampacity Configurations

Rooftop Conduit Ampacity (120°F)Attic Wiring Derating (140°F)UF-B Underground Cable Ampacity Ampacity Derating: 9 Wires in Pipe

Ampacity Adjustment Formula

I_adj = I_table × F_temp × F_fill

NEC 310.15(B) & (C) Adjustments

NEC 310.15(B) outlines ambient temperature correction factors. NEC 310.15(C)(1) requires ampacity reduction when more than 3 current-carrying conductors are bundled together in a raceway or cable.

Frequently Asked Questions

When do I need to derate wire?

You must derate when (1) the ambient temperature exceeds 86°F (30°C) or (2) there are more than 3 current-carrying conductors in the same pipe/raceway.

Does the neutral wire count as a current-carrying conductor?

Typically, no. In a balanced system, it carries only unbalanced current. However, if the load is mostly non-linear (electronics, LED lights), the neutral DOES count.

Can I start derating from the 90°C column?

Yes! Even if your breakers are rated 75°C, you can use the 90°C ampacity of THHN/THWN-2 conductors as the starting point for derating calculations.

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