Disclaimer

Electrical work can kill people and burn buildings down. An undersized conductor does not fail the moment it is installed — it runs hot for years and fails later, usually inside a wall. Everything on this site is written with that in mind, and so is this page.

A calculator on this site takes the values you enter and applies a published formula to published code tables. It knows nothing else. It cannot see the equipment at either end of the circuit, the temperature rating of the terminals it lands on, the condition of existing conductors, how many other circuits share the raceway, what the ambient temperature is in the space the run passes through, or what your local amendments say.

Every one of those can change the correct answer, and several of them can change it by more than one conductor size. A result here is a starting point for a decision, not the decision.

The National Electrical Code is not law until a state or municipality adopts it, and jurisdictions adopt different editions on different schedules and amend them locally. A figure that is compliant in one state can be non-compliant a few miles away.

The authority having jurisdiction — the inspector responsible for your installation — has the final word on interpretation, and that word outranks any chart on any website, including this one. Where an inspector disagrees with a number published here, the inspector is right for the purposes of your job.

Figures on this site follow NFPA 70-2023. The 2026 edition is published and adoption is under way state by state, while some jurisdictions are still enforcing 2020 or earlier. Confirm which edition applies where you are, and check any figure you rely on against that edition's own tables before you install to it.

Conductor values are transcribed by hand from published tables, so a transcription error is possible. If you find one, taking a moment to get in touch is genuinely useful — and in the meantime, the code book on your desk beats this site every time.

Nothing here authorises anyone to carry out electrical work they are not qualified or permitted to do. Requirements for who may install, alter and inspect wiring vary by jurisdiction, as do permit requirements for the work itself. Panel work in particular exposes live conductors that remain energised with the main breaker off.

If you are not sure whether a job is within your competence, that uncertainty is the answer: hire an electrician.

The site is provided as is, without warranty of any kind. To the extent the law allows, its operator accepts no liability for loss, damage or injury arising from use of the calculators, the reference pages or anything else published here, including work carried out on the basis of a result they produced. The terms of use set out the rest.

Last updated: September 2026