CONDUCTORS · CALCULATOR

Ampacity calculator

Free ampacity calculator: get temperature correction factor (equation 310.15(b)). The formula and a worked example are shown; verify against the NEC.

Written by Chameera Sampath · Not yet reviewed by a licensed electrician. See the methodology.

US (NEC) only · NFPA 70 / NEC — locally adopted editionDefaults are US values. Verify the locally adopted NEC edition and project data.Official reference ↗

Project inputs

Measurements

Tabulated ampacity at table ambient (user-verified)

Conductor temperature ratingTable base ambient

°C

Actual ambient temperature

°C

Verified adjustment factor (more than three conductors)

Terminal temperature ampacity limit

Updates as inputs change. Unit changes preserve the same physical values.

THE MATH BEHIND THE ANSWER

Method & assumptions

I′ = I × √((Tc − Ta′) / (Tc − Ta)) (NEC Equation 310.15(B)); Iusable = min(I′ × Cadj, Iterminal)

US (NEC) only. Temperature correction uses NEC Equation 310.15(B), permitted as an alternative to the banded correction factors of Table 310.15(B)(1); results differ slightly from the banded table. Enter tabulated ampacity and the adjustment factor for the actual conductor and installation; no NEC ampacity table is included. Engineering arithmetic, not installation approval.

Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.

Sources and validation →

Worked example

Tabulated ampacity at table ambient (user-verified): 55 A · Conductor temperature rating: 90 °C · Table base ambient: 30 °C · Actual ambient temperature: 40 °C · Verified adjustment factor (more than three conductors): 1 · Terminal temperature ampacity limit: 50 A

Temperature correction factor (Equation 310.15(B)): 0.913 · Corrected ampacity (Equation 310.15(B) × adjustment): 50.208 A · Usable ampacity (terminal limit applied): 50 A

This example uses the stated inputs; results for the entered values appear above.

Explore the conductors branch →See the related electrical concepts →Size the wire by ampacity and voltage drop →See the standard breaker sizes →Read the AWG wire gauge chart →All wire sizing pages, 20 A to 200 A →

Estimating and invoicing tool

Ampxa is developing an estimating and invoicing application for electrical contractors. The list below receives notice when it opens.