Ampacity and Derating Calculation

PASS

Report date: August 1, 2026
Reference basis: 2023 NEC Table 310.16
Ambient: 30°C (86°F)

Verification note

Planning result only. Verify the adopted code, equipment markings, conductor listing, exceptions, and AHJ requirements.

2023 NEC reference — verify local adoption

This calculator models a common Table 310.16 pathway only. Confirm the adopted edition, equipment markings, conductor listing, exceptions, and authority-having-jurisdiction requirements before installation.

Calculation inputs

Conductor and conditions of use

Conductor materialConductor sizeConstructionConstruction identifies the product; Table 310.16 ampacity is unchanged.Insulation temperature ratingEquipment terminal ratingUse the lowest applicable marked termination rating.Ambient temperature unitNEC table ranges are evaluated in Celsius.Ambient temperature — °CEquivalent: 86°F.Current-carrying conductorsEnter the code-counted conductors in the raceway or cable—not grounds automatically.

Load check

Continuous load is modeled at 125%; noncontinuous load is modeled at 100%.

Continuous load — ANoncontinuous load — A

Conductor sizing comparison

Minimum size under these entered conditions

Smallest supported size that passes12 AWG Copper

20 A planning limit versus 20 A required.

Currently selectedUse recommendation in Voltage Drop

Conductor	Planning limit	Required	Result	Selection
14 AWG	15 A	20 A	Revise
12 AWGMinimum passing	20 A	20 A	Pass	Selected
Selected size details 12 AWG CopperPass Base ampacity 30 A Ambient factor × 1.00 Conductor factor × 1.00 Corrected + adjusted 30 A Terminal limit 25 A Planning limit 20 A Required 20 A Governing Small-conductor overcurrent-protection limit
10 AWG	30 A	20 A	Pass
8 AWG	50 A	20 A	Pass
6 AWG	65 A	20 A	Pass

Comparison uses the same material, insulation, terminal rating, ambient temperature, conductor count, and load entered above. It does not model parallel conductors or exceptions.

Transparent calculation

How the governing value was selected

Table ampacity

30 A

12 AWG copper at the 90°C insulation column.

Ambient correction

× 1.00

Table factor for 30°C (86°F) ambient and 90°C (194°F) insulation.

Conductor adjustment

× 1.00

100% for 3 current-carrying conductors.

Conditions-of-use result

30 A

30 × 1.00 × 1.00.

Terminal limitation

25 A

The unadjusted 75°C table ampacity caps the conditions-of-use result.

Load requirement

20 A

20 A + 125% × 0 A.

Small-conductor rule applied for this general planning check

NEC 240.4(D) generally limits this conductor to 20 A overcurrent protection. The calculator uses that lower value while excluding special permissions and exceptions.

Continue this calculation

Send these inputs to another calculator

The destination opens with compatible conduit and cable inputs already selected. This calculator and its results remain unchanged.

Use this conductor in Voltage DropTransfers conductor material and size; enter the actual circuit voltage, load, and length there.Open calculator →

Method and scope

What this calculator does—and does not establish

This workflow begins with the 2023 NEC Table 310.16 ampacity for insulated conductors rated 0–2000 V, not more than three current-carrying conductors, and a 30°C ambient basis. It applies the selected ambient correction and conductor-count adjustment, then enforces the selected terminal-temperature column.

It does not determine whether a neutral counts, whether an adjustment exception applies, equipment listing compatibility, overcurrent-device selection beyond the displayed small-conductor planning cap, rooftop conditions, dwelling-service allowances, motor/HVAC rules, cable-tray ampacity, medium-voltage conductors, parallel installations, fault current, or complete code compliance.

Read the full methodology

Primary references