How we calculate

Every calculator on this site shows its formula, and every number it uses has at least two published sources. This page explains the method once, so each calculator can stay short.

Physics first

The math starts from physical laws, not from lookup tables:

• Volts, amps and watts. Power is volts times amps. For AC loads the power factor comes in, and three-phase circuits add the square root of 3.
• Resistance of a wire. A conductor’s resistance grows with its length and shrinks with its cross-section area. Copper and aluminum each have a published resistivity, and resistance rises with temperature at a known rate, so the calculators work at the conductor temperature they state.
• Voltage drop. The drop is the current times the resistance of the whole path, out and back for a single-phase run.
• Fill. How full a conduit is comes from the area of each wire, including its insulation, compared with the inside area of the conduit.

Maker data for sizes

Physical sizes come from the people who make the products: wire diameters from wire makers’ spec sheets and the ASTM standards they build to, conduit inside diameters from conduit makers, and box volumes from box makers. When two makers disagree, the calculator uses the figure that gives the more cautious answer and says so.

Two sources for every number

Before a figure goes live it has to match at least two published sources, such as a standards body, a maker’s spec sheet or a university or government reference. Another calculator site never counts as a source. Each calculator lists its sources at the bottom, with the date the data was last checked.

What your inspector decides

Electrical codes set limits that physics alone doesn’t, and they change with each edition and with local amendments. This site doesn’t reproduce code tables. Where a result depends on a code rule, the page names it as a typical figure and tells you to check your code edition and your local inspector (AHJ).

Before you wire anything

These calculators give estimates worked out from physics and published maker data. They are not engineering, design or code advice, and they do not replace an inspection.

Electrical codes differ by edition and by town. Your local inspector, the authority having jurisdiction (AHJ), has the final say on wire size, conduit, boxes and breakers.

Mains wiring can kill and can start fires. Unless you are trained for it, hire a licensed electrician, and get a permit where your town requires one.