Which NEC tables does this calculator use?

Base ampacity values come from NEC 2023 Table 310.16. Ambient temperature correction is from Table 310.15(B)(1). Bundling adjustment is from 310.15(C)(1).

Can I use the 90°C column to apply derating and then land on 75°C equipment?

Common practice is to apply correction/adjustment to the 90°C column for conductors with 90°C insulation, but the final ampacity cannot exceed the ampacity permitted by the termination rating (often 75°C). This tool enforces the lower final limit.

Does the calculator apply the small conductor rule (240.4(D))?

We display an advisory note for 14/12/10 AWG copper conductors. Always size overcurrent protection per NEC 240.4(D) where applicable.

Are rooftop, ambient correction below 30°C, or special conditions included?

This version applies ambient corrections for temperatures above 30°C and standard bundling adjustments. Rooftop adjustments, more specialized conditions, or increases for lower ambient are not included and should be evaluated separately.

Which NEC edition is referenced?

NEC 2023 (NFPA 70). Local jurisdictions may adopt different editions; always verify with your AHJ.

NEC Ampacity Calculator

Q: Does the tool consider the 60°C vs 75°C termination rule?

Yes. The allowable base ampacity uses the lower of the conductor insulation rating and the equipment termination temperature rating.

This professional-grade NEC ampacity calculator determines the allowable current-carrying capacity of conductors using NEC Table 310.16, with ambient temperature and conductor bundling adjustments per NEC 310.15. Built for engineers, electricians, and plan reviewers who need fast, reliable, and code-aligned results.

Calculator

Conductor Material

Copper

Aluminum

Conductor Size (AWG/kcmil)

Conductor Insulation Rating (°C)

Termination Temperature Rating (°C)

Ambient Temperature

°C °F

Current-Carrying Conductors in the raceway/cable

Results

Selected Temperature Column —

Base Ampacity at 30°C (NEC 310.16) —

Ambient Correction Factor, Famb —

Bundling Adjustment, Fbundling —

Adjusted Allowable Ampacity —

Data Source and Methodology

NFPA 70 — National Electrical Code (NEC) 2023, Article 310: Table 310.16 (Allowable Ampacities), Table 310.15(B)(1) (Ambient Temperature Correction), and 310.15(C)(1) (Adjustment for More Than Three Current-Carrying Conductors). Official reference: NFPA 70, 2023 Edition.
CerroWire Ampacity Charts (industry summary): cerrowire.com

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The Formula Explained

We compute the allowable ampacity using the lower applicable temperature column and apply ambient and bundling factors:

$$ I_{allow} \;=\; I_{base}(T_{sel}) \times F_{amb}(T_a, T_{sel}) \times F_{bundling}(N_{ccc}) $$

Where the selected temperature column is the lower of the conductor insulation rating and the equipment termination rating:

$$ T_{sel} \;=\; \min\left(T_{insulation},\, T_{termination}\right) $$

Ambient correction factor is taken from NEC Table 310.15(B)(1) based on ambient temperature ranges; bundling adjustment is from 310.15(C)(1).

Glossary of Variables

How It Works: A Step-by-Step Example

Case: Copper 4 AWG, 90°C insulation, 75°C terminations, ambient 40°C, six current-carrying conductors.

Select the temperature column: Tsel = min(90°C insulation, 75°C termination) = 75°C.
From NEC Table 310.16, Ibase for 4 AWG copper at 75°C is 85 A.
Ambient correction Famb for 36–40°C at 75°C column is 0.88 (NEC Table 310.15(B)(1)).
Bundling adjustment Fbundling for 4–6 CCC is 0.80 (NEC 310.15(C)(1)).
Compute:
$$ I_{allow} = 85 \times 0.88 \times 0.80 = 59.84 \text{ A} \approx 60 \text{ A} $$

Note: Overcurrent protection must also comply with NEC Article 240. The “small conductor rule” of 240.4(D) may limit 14/12/10 AWG copper branch-circuit conductors to 15/20/30 A regardless of higher thermal capacity.

Frequently Asked Questions (FAQ)

Does this tool consider the lower of insulation and termination ratings?

Yes. It selects the lower temperature column between the conductor insulation rating and the termination rating, per NEC requirements.

How are ambient temperature corrections applied?

The calculator maps your ambient temperature to the correct NEC Table 310.15(B)(1) range for the selected temperature column and applies that factor to the base ampacity.

How are bundling adjustments determined?

We apply the 310.15(C)(1) “more than three CCC” table based on the number of current-carrying conductors in the raceway or cable.

Are rooftop or special conditions included?

No. Rooftop sunlight adjustments, conductors in parallel, and other special cases require additional calculations and code sections. Consult the NEC and your AHJ.

Which NEC edition does the calculator use?

NEC 2023. Local adoption may vary; verify with your Authority Having Jurisdiction (AHJ).

Is the small conductor rule enforced?

We display an advisory for 14/12/10 AWG copper. Overcurrent protection must comply with NEC 240.4(D) where applicable.

How should I report or verify a result?

Use the “Copy Results” button to capture your inputs and outputs. Always cross-check with the NEC tables and project specifications.

Tool developed by Ugo Candido. Content verified by CalcDomain Editorial Team.
Last reviewed for accuracy on: September 14, 2025.