How this site is built, and how to check it

What an answer here is made of

Every result is four things printed together: the number, the arithmetic that produced it, the constant that went into the arithmetic with the document it was published in, and a sentence naming which condition decided the result. If you disagree with an assumption, you can see which one it was and change it.

Nothing is transcribed from memory or copied from another chart. A value reaches a page in one of three ways, and each way is named where the value appears.

What the numbers rest on

A definition

Conductor size. The American Wire Gauge is a geometric series, ASTM B258, so the cross-section of every size from 18 AWG to 4/0 is computed from the gauge number, and the kcmil sizes above that are their own definition. No table is involved.

A published constant

Resistivity and its temperature coefficient, from Electrical Tech Note 212, Conductor Properties, Biosystems & Agricultural Engineering Department, Michigan State University, © 2009/24, Tables 212.2 and 212.3 and Equations 212.1 and 212.2, read 2026-09-03. That document is used for resistivity K and the temperature coefficient alpha only.

Arithmetic

Resistance is resistivity times length over area. Volts lost is current times resistance over the round trip: twice the one-way length for single phase, √3 times it for three phase. The full working, with every intermediate value, is on the formula page.

What is deliberately not here

No table from the National Electrical Code is reproduced, transcribed or paraphrased on this site. The 3% and 5% figures that every chart quotes are informational notes in that code, not requirements, and the calculator says so beside the result rather than presenting them as a rule.

Conductor ampacity is the thing people ask for most and it is absent for the same reason: the tables that answer it are published by a standards body, and this site will not print a number it cannot source. That page arrives when a licensed copy is on the desk, and not before. Conduit fill needs conduit and conductor dimensions from manufacturers' published data, which is sourceable and not yet sourced.

How a build is checked before it is published

The resistance figures are computed from the definition and the constant above, then compared against the source document's own resistance table before anything is written: 23 of 23 values within 1% (Table 212.1, direct-current resistance at 75 °C), and 10 of 10 circular-mil areas within the same tolerance. If a value drifts past 1% the build stops, because an unvalidated computation is worth less than the table it replaced.

The calculator page carries a correct answer in its HTML before any script runs, so a reader with scripts off still gets a result. Every page is then rendered in a real browser at six screen widths, from a 390px phone to a 1440px desktop, and the build fails on a layout defect, a contrast failure or a missing piece of metadata.

What runs in your browser

Nothing from anywhere else. No analytics, no tracking, no cookies, no account, no advertising, and no request to any other domain. The only links that leave this site go to the two documents the numbers rest on. The JavaScript that updates the calculator is inline and small enough to read.

When the pages change

There is no schedule. A page changes only when the arithmetic, the constant or the words do, and the footer of every page carries the date it was published and the date of the last change.

Independence

This site is not affiliated with, endorsed by, or a publication of any standards body, wire manufacturer or distributor. Nothing is sold here and nothing is sponsored. NFPA, NEC and National Electrical Code are marks of the National Fire Protection Association, named on this site only to identify the document being discussed.

It shows a calculation and its assumptions. It does not tell you what to install, and it is not a substitute for the code edition your jurisdiction has adopted or for the judgement of a licensed electrician.