About This Site

This site is a set of electrical calculators and reference charts for sizing conductors and checking voltage drop. It exists because the arithmetic is simple, the code rules around it are not, and most of the answers online are either a bare number with no source or a table photographed out of a book with no edition on it.

It is built for the people who need a number in the middle of a job: electricians checking a long run before they pull it, solar and marine installers working at 12 or 24 volts where voltage drop decides the conductor, and homeowners trying to find out whether the wire they already bought is the wire the circuit needs.

Every conductor figure here is transcribed from the National Electrical Code, not calculated from a rule of thumb. Ampacity comes from NEC Table 310.16. Resistance and reactance come from NEC Chapter 9, Table 8 for direct current and Table 9 for alternating current, including the separate values Table 9 publishes for PVC, aluminum and steel raceway. Conductor areas in circular mils come from Table 8. The site targets NFPA 70-2023.

What follows is the part most sites leave out, and it is the reason to trust this one. The figures were not checked against NFPA 70 itself. The code book is not freely available, so every value was instead compared against two independent reproductions of the same table — different publishers, different code editions, neither copied from the other — by parsing each reproduction and diffing it cell by cell against the data the calculators read. Nothing was checked by eye. That method catches transcription error, which is the mistake that actually happens. It cannot catch an error common to both reproductions.

The consequence is different for each table, and the difference matters:

• Table 310.16, ampacity. All 88 values agreed across two reproductions. The copper half is confirmed against a 2023-edition reproduction directly.
• Chapter 9 Tables 8 and 9, resistance and reactance. Every value agreed across two reproductions, and the tables are confirmed identical from the 2005 edition through the 2017 edition. The 2020 and 2023 editions were not directly confirmed — no free reproduction of Chapter 9 at those editions could be found. A table unchanged across four editions is unlikely to have moved since, but that is an inference, not a source, and it is stated here as one.
• Below 14 AWG there is no code rating at all. Table 310.16 stops there and NEC 240.4(D) stops there. For the fine conductor sizes used in 12 and 24 volt work, the calculator applies our own conservative limit, not a code figure: the same current per circular mil that 240.4(D) permits on 14 AWG copper, which is 15 amperes over 4,110 circular mils. Where that limit decides an answer the calculator says so in the working, and says the figure is ours. It stands in for a code rating that does not exist, and it will be replaced when a published fine-gauge current-rating table can be sourced to the same standard.

Figures are never rounded, reformatted or “corrected” on their way onto a page. A published value of 2.00 stays 2.00. Where a number is derived rather than published — a temperature correction, a size the code table does not list — the page says so at the point of use rather than only here.

Every formula this site uses is printed on the page that uses it, with its variables defined and a worked example alongside. Calculators show the resistance and reactance they used for the result, so you can check the answer against the code book yourself rather than taking it on trust. Nothing is hidden behind a script you cannot read.

That is deliberate. A calculator that returns a number and no working is impossible to verify and impossible to argue with, which is exactly the wrong property for a tool people use to size conductors.

It is not a substitute for a licensed electrician. It is not an authority having jurisdiction. Results are for planning: they do not account for the condition of an existing installation, the equipment you are connecting to, or anything an inspector will see that a web page cannot.

Local amendments govern. Adoption of the NEC runs state by state and often city by city, and a jurisdiction may enforce an older edition, a newer one, or one with its own amendments layered on top. The edition your inspector enforces outranks anything published here, and so does the inspector. For anything affecting code compliance or safety, confirm the result with a licensed electrician. The full disclaimer sets out the limits, and if you find a figure here that is wrong, you can report a correction.

Last updated: September 2026