Electrical Calculators

This is the hub where the primary source is hardest to reach, and the one where saying so matters most.

The calculators

• Voltage Drop CalculatorConductor table from NBS Circular 31, a public-domain NIST publication
• Watt and Electricity Cost CalculatorConversion factors from NIST SP 811, quoted as printed

The National Electrical Code is NFPA 70. Unlike the IRC, which several cities and states publish as free PDFs that reproduce the model text in full, the NEC is readable only through a JavaScript application behind a login. Every reproduction of its tables that turned up in searching sat on a commercial calculator site, a continuing-education provider or a document-sharing service.

So there is a hard line running through this section. Anything derivable from physics or from a public-domain government publication is here and fully sourced. Anything that exists only inside NFPA 70 is not here at all, and is named as absent rather than approximated.

What that means in practice

Voltage drop is on the near side of the line. The formula is Ohm's law, and the wire table behind it, the diameters, the circular mils and the resistance per thousand feet, was published by the National Bureau of Standards as Circular 31 and is in the public domain. Even the constant K that every calculator quotes without explanation falls out of that document in two steps.

Ampacity is on the far side. How much current a conductor may carry before it becomes a fire is NEC Table 310.16, adjusted for ambient temperature, for bundling and for termination ratings. It is not reproduced anywhere on this site. A span table holds people off the ground; an ampacity table keeps a wall from catching fire, and neither is a figure to copy from a stranger.

The order the two questions come in

Size the conductor for ampacity from the code, then check voltage drop and go up if it fails. Voltage drop can only ever make a conductor larger, never smaller, so it is safe to check second and unsafe to use first.

Recommendations that read like rules

The 3% branch-circuit and 5% total figures everyone works to sit in Informational Notes, which the code itself defines as unenforceable explanatory material. That distinction could not be verified here against the primary document, so it is reported and labelled rather than asserted. What is not in doubt is that some jurisdictions adopt those figures as mandatory local amendments, which is the same pattern the concrete section keeps finding in the IRC: the model says one thing, and the place you are building says another.