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Electrical Tool

Cable Ampacity Calculator

Apply NEC 310.15 / IEC 60364 temperature and conduit-fill derating to a conductor and get the final ampacity plus a recommended breaker size.

Updated: June 29, 2026Planning reference

Quick Answer

Check derated cable ampacity before assigning a breaker or loading a circuit.

Select conductor size, material, insulation rating, ambient temperature and the number of current-carrying conductors. The calculator applies temperature and conduit-fill correction factors and reports the final derated ampacity, a recommended breaker and whether it covers the design load.

Conductor Setup

Input values

Use practical planning values. Final conductor sizing and overcurrent protection must be verified against the applicable NEC/IEC edition, manufacturer data and the authority having jurisdiction.

Conductor

Installation & Load

Formula basis

Derated ampacity = base ampacity × temperature correction factor × conduit-fill factor. Base values follow NEC 310.15(B)(16); only current-carrying conductors count toward fill derating.

Formula / Method

How cable ampacity derating works

Base ampacity from NEC 310.15(B)(16) assumes 30 °C ambient and ≤ 3 current-carrying conductors in a conduit. A temperature correction factor and a conduit-fill factor are multiplied against the base value to give the final derated ampacity, which must meet or exceed 125% of the continuous load current.

Derated ampacity = base × temperature CF × fill CF

Inputs

Required inputs

Use conductor size (AWG or mm²), material, insulation temperature rating, ambient temperature, number of current-carrying conductors and an optional design load.

Uses

Where this helps

Useful for branch circuits, feeders, distribution boards, rack power circuits, conduit fill checks and early electrical coordination.

FAQ

Ampacity notes

Which conductors count toward fill derating?

Only current-carrying conductors. Equipment grounding conductors and the neutral of a balanced three-phase circuit are generally not counted.

Can this replace electrical design?

No. It is a planning calculator. Final conductor sizing, protection and code compliance must be checked by a qualified person and the authority having jurisdiction.

Why does the breaker have to be below the ampacity?

The overcurrent device protects the conductor, so its rating must not exceed the derated ampacity while still covering the continuous load (typically at 125%).

Learn More

Reference links

Project

Need help validating cable sizing and electrical design?

ITCOREOPS can support cable ampacity checks, circuit load planning, rack power, structured cabling, documentation, labeling and infrastructure handover.

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Disclaimer

Planning reference only

This calculator provides a simplified planning estimate only. It does not replace qualified electrical design, NEC/IEC code-compliance review, conductor manufacturer data, short-circuit analysis, local regulations or professional inspection. All electrical work must be carried out by a licensed electrician.

Workflow

Cable ampacity should be verified before circuits are installed or loaded.

Use this calculator before installing feeders, branch circuits or distribution boards to confirm derated ampacity, recommended breaker size and code headroom.

Use exact inputs ↑

Support

Need help with electrical or infrastructure planning?

For cable sizing, circuit load, rack power, structured cabling and operational infrastructure support, visit ITCOREOPS.

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Feedback

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Worked example

Inputs: 10 mm² copper, XLPE (90 °C), reference Iz ≈ 65 A (installation method C, 3 loaded conductors, 30 °C). Derating: ambient 40 °C → Ca = 0.91; 3 bunched circuits → Cg = 0.70.

Result: Iz′ = 65 × 0.91 × 0.70 ≈ 41 A. (Base Iz comes from the table for the actual installation method.)