What is here
- Voltage drop calculator: volts lost and percent drop for single-phase, three-phase and DC circuits, from 12 V RV and solar runs to 480 V feeders.
- Wire gauge chart: diameter in inches and mm, circular mils, mm² and DC resistance of copper and aluminum for every AWG size from 4/0 to 40.
- AWG to mm² converter: metric area for every size in the wire chart, and the IEC 60228 cable sizes on either side of each size from 20 AWG to 500 kcmil.
Where the numbers come from
AWG diameters follow ASTM B258, calculated and rounded the way NBS Handbook 100 describes. Copper resistance uses the International Annealed Copper Standard as published in NBS Handbook 100 (10.371 ohm-cmil/ft at 20°C), and aluminum uses the EC-H19 values in NBS Handbook 109 (61.0% of copper's conductivity, 17.002 ohm-cmil/ft). The voltage drop constants, 12.9 for copper and 21.2 for aluminum at 75°C, are the ones IAEI publishes from the DC resistance values in Chapter 9, Table 8 of the National Electrical Code, and they also follow from the NBS resistivities raised to 75°C. The wire table was checked against NBS Handbook 100 and against MWS Wire Industries and Southwire data before it went live; the results are printed under the wire gauge chart.
What is not here
These pages cover the physics of wire: size, area and resistance. They do not list ampacity, conduit fill or box fill. For those, use the electrical code your jurisdiction enforces, the equipment installation manual and your inspector. NFPA publishes the National Electrical Code as NFPA 70 and offers free read-only access to its codes online, though not every edition is available that way.