AmpSizerAbout
AmpSizer answers one kind of question: how big does this have to be? The wire, the breaker, the ground, the conduit, the generator, the transformer. Each gets its own calculator, because the arithmetic is shared and everything after it — which table applies, which ceiling bites first — is not.
Why it exists
There is no shortage of electrical calculators. There is a shortage of ones that finish the job. Wire manufacturers publish good tools and stop at a single number, because the tool exists to sell cable. General calculator sites do voltage drop and nothing else. And the biggest cluster of searches in this subject — people looking for a wire gauge chart — is answered almost entirely by articles with a table pasted into them and no way to check your own numbers against it.
The gap is not arithmetic. It is that ampacity and voltage drop are two separate rules, and almost every tool treats them as two separate tools. A conductor has to satisfy both, the bigger requirement wins, and which one wins changes with the length of the run. Every sizing answer here says which rule decided it and shows the size the other rule would have accepted.
Where the numbers come from
Nothing is invented and nothing is averaged out of other websites. Ampacity is NEC Table 310.16. Resistance and conductor area are Chapter 9, Table 8. Raceway areas are Chapter 9, Table 4 and the fill percentages are Table 1. Ground wires are Table 250.122, box fill is 314.16, motor currents are Tables 430.248 and 430.250, and extension cords use Table 400.5(A)(1) because a cord is not a raceway. Each tool names its own tables under How we calculate, and the whole list plus the full conductor chart is on the methodology page.
The reference charts and the calculators read the same data files. That is deliberate: a site where the chart says one thing and the widget says another is worse than no site at all.
What this is not
It is not a design, a permit, or a substitute for an electrician, and it does not pretend the code is simpler than it is. Three things regularly make a correct table give a wrong answer on a real job: your jurisdiction may be on a different NEC edition or amend it, your equipment terminals carry their own temperature rating, and the conditions of use change the derating. Those are judgement calls, and they belong to somebody licensed to make them and inspected on it.
We also publish nothing about working on live equipment. If a question can only be answered by putting a meter on an energised conductor, it is not a question this site answers.
Nothing you type is sent anywhere. Every calculation happens in your own browser, no account is needed to use any tool, and the site keeps no record of what you worked out.
Found a figure that looks wrong, or a table we are missing? Tell us — corrections get made and requests get read.