This site publishes three free calculators for use in the United States:
- Voltage drop
- Wire size based on voltage drop only
- Room (window) air-conditioner sizing
The calculators are meant for field estimates and learning. They do not replace a licensed electrician, a licensed HVAC contractor, the local codebook, or manufacturer instructions. If you use the results, you do so at your own risk.
Where the numbers come from
Copper wire. Conductor area, 20 °C resistivity (10.371 ohm-circular mil/ft), and the 20 °C temperature coefficient of resistance (0.00393 per °C) are taken from the U.S. National Bureau of Standards, Copper Wire Tables, NBS Handbook 100. Resistance is then adjusted to 75 °C with that temperature coefficient. Handbook 100 is a U.S. government publication.
Aluminum wire. Resistivity and temperature coefficient are taken from the U.S. National Bureau of Standards, Aluminum Wire Tables, NBS Handbook 109. The calculators use the Handbook 109 Table 1 values for EC-H19 aluminum: 17.002 ohm-circular mil/ft at 20 °C and a temperature coefficient of 0.00403 per °C at 20 °C. Nominal AWG areas are the same American Wire Gage areas published in Handbook 100.
Room air conditioners. Sizing follows the EPA ENERGY STAR room air conditioner method, as published on Room Air Conditioners and How To Choose the Right Sized Window AC (page dated February 2, 2024). Both pages were checked on November 15, 2026.
What this site does not use
We do not copy or reproduce National Electrical Code (NEC) tables. We do not calculate ampacity, derating, bundling, terminal temperature, or conduit fill. We do not use ACCA Manual J or any other copyrighted whole-house load-calculation method. The air-conditioner tool is only for room window units, not central systems.
Review
The formulas and this wording are written to match the sources above. A review by a licensed U.S. electrician and HVAC technician is planned before launch when we can arrange it. Until then, treat every result as an estimate and verify it against the original sources and local requirements.
These tools are written for U.S. practice (feet, AWG, common U.S. voltages, and ENERGY STAR room-AC sizing). They are not directly applicable outside the United States.