About CircuitGauge

CircuitGauge exists because sizing a circuit should not require three browser tabs: one chart for ampacity, another for voltage drop and a forum thread to find out which temperature column applies. Each calculator here does all of those checks together and tells you which one decided the answer.

What we cover

Branch circuits and feeders in US dwellings and small commercial buildings: 120, 208, 240, 277 and 480 V, copper and aluminum, NM-B, UF-B, THHN/THWN-2 and SER. Low-voltage DC systems, electronics and solar are outside the scope of this site.

How the numbers are made

Ampacity comes from NEC 2023 Table 310.16, resistance from Chapter 9 Table 8, and the small-conductor breaker limits from 240.4(D). Tables on the pages are generated by the same code that runs the calculators, so a chart and a calculator can never disagree. The full method is on the methodology page.

What we are not

We are not a licensed electrical contractor and we do not inspect installations. The pages are a design check. The code adopted in your city or county, and the inspector who enforces it, have the final word.

Corrections

If a number looks wrong, write to [email protected] with the page and the inputs. Confirmed errors are fixed, a regression test is added, and the page date is updated.