Methodology and sources

The charts on this site and the calculators in your browser run the same tested code, so a table and a calculator cannot disagree.

Wire ampacity

Allowable ampacity follows the 2023 National Electrical Code, table 310.16: up to three current-carrying conductors in a raceway or cable at 30 °C ambient. Small conductors are limited to 15 A (14 AWG), 20 A (12 AWG) and 30 A (10 AWG) for copper, and 15 A (12 AWG) and 25 A (10 AWG) for aluminum. 18 and 16 AWG are never offered for a branch circuit. Corrections for ambient temperature and bundling are not applied yet.

Voltage drop

Single phase: drop = 2 × K × I × L ÷ cmil. Three phase: drop = √3 × K × I × L ÷ cmil. K is 12.9 for copper and 21.2 for aluminum, I is the current in amps, L is the one-way length in feet and cmil is the conductor area in circular mils. The target is a drop of no more than 3% on a circuit, and 5% for extension cords.

Grounding conductors

Equipment grounding conductors are sized by the breaker that protects the circuit. When the hot wires are made larger for voltage drop, the ground grows in proportion to their area, but never past the hot wires. Grounding electrode conductors for services are sized by the service-entrance wire, with the conductor to ground rods limited to 6 AWG copper.

Services and sub panels

The main service of a one-family house from 100 to 400 A may use wire rated for 83% of the service. Sub panel feeders and other services are sized for the full rating at 75 °C.

Conduit and box fill

Conduit fill divides the conductor areas by the inside area of the conduit: at most 40% for three or more conductors, 31% for two and 53% for one. Box fill adds a volume per wire by size, two wires' worth per device, one for clamps and one for all grounds, using the largest conductor in the box.

Appliance power and running cost

Wattage ranges are typical values for current models. Amps are calculated as watts ÷ volts. Cost uses the US average residential price of $0.18 per kWh (EIA, August 2026) unless you enter your own. The nameplate of your own appliance always takes priority.

Generator sizes are conservative: the smallest common generator whose running rating covers the start-up surge. Power station runtimes assume 85% of the battery reaches the outlet.

Circuits and house load

Outlet counts use 180 VA per receptacle, the load-calculation value for non-dwelling buildings. Lighting counts use 80% of the breaker rating. The house load calculator follows the optional method for a single dwelling: 3 VA per square foot, 1,500 VA per kitchen and laundry circuit, fixed appliances at nameplate, the first 10 kVA at 100% and the rest at 40%, plus the larger of heating and cooling.

How the data is checked

Reference values are compared with published documents: wire, conduit and equipment manufacturers' technical tables and catalogs, and openly published government and utility documents. Each page links the documents its values were compared with. A page with any value not yet compared is marked as draft data at the bottom.

Many of those documents follow an earlier edition of the electrical code, 2017 or 2020, while the pages cite the 2023 tables. Each document's edition is shown next to its link; where the values in the 2017, 2020 and 2023 editions agree in the documents we found, the page uses them, and where we found no document, the page stays a draft.

Breaker suggestions start at 15 A. The 2023 code added 10 A to the list of standard breaker ratings; the calculators do not suggest it.