Electrical calculators

Sizing wire, boxes and services

These calculators cover the everyday sizing questions on a residential or light commercial job: what wire gauge a circuit needs, how much voltage it loses over a long run, whether a box is big enough for its conductors and devices, the service size for a dwelling, and conversions between watts, amps, volts and ohms. Each result cites the 2023 NEC table or section it comes from, such as Table 310.16 for ampacity and 314.16 for box fill.

There is no waste factor in the usual sense, but there are allowances. Measure wire runs along the actual route rather than in a straight line, and leave at least 6 inches of free conductor at each box (NEC 300.14). Voltage drop is not a hard limit for most branch circuits; the NEC informational notes suggest about 3% for a branch circuit and 5% for feeder and branch combined, and the voltage drop calculator checks against both.

Ampacity figures assume the conditions behind Table 310.16: no more than three current-carrying conductors in a raceway or cable and an ambient temperature of 30°C (86°F). Bundled cables, hot attics and rooftop conduit need adjustment and correction factors. Section 240.4(D) also limits the overcurrent protection on small copper conductors to 15 amps for 14 AWG, 20 amps for 12 AWG and 30 amps for 10 AWG.

Electrical work usually needs a permit and, in many places, a licensed electrician. Use these numbers to plan a job or check a design, then follow the code edition your jurisdiction has adopted, which may be older than 2023 or locally amended.

How each calculator is specified, tested and corrected is described on our methodology page.