AmpSizerElectrical Sizing Guides
Every question on this site starts in the same place: somebody standing at a panel or in an aisle with a phone, trying to turn a load and a distance into a size. The arithmetic is not difficult. It is that the two rules which decide the answer live in different chapters of the code book and nobody writes them down in the order you need them.
These ten guides do that. They cover the complete conductor chart with ampacity and resistance for every size, the voltage drop formula and where the 3% target comes from, what size wire a 50 amp circuit and a 100 amp service each want and why those two answers differ, the conduit fill chart, copper against aluminium, how derating actually works when a raceway is crowded or hot, Ohm’s law and the power wheel, the breaker and wire pairing chart, and the AWG to square millimetre conversion for anyone working from metric drawings.
Every table on these pages is generated at build time from the same data files the calculators read, so a chart here and an answer from a widget cannot disagree. Every figure names the NEC table it comes from.
If you want the number rather than the reasoning, start with the voltage drop calculator or the wire size calculator and come back here afterwards. The wire gauge chart is the reference the rest of the site is built on.
Ampacity derating explained
How the two derating factors work: ambient temperature and more than three conductors, why they multiply, and what the 90 °C column is really for.
AWG to mm² conversion
AWG to square millimetre conversion for every wire size, the nearest stocked metric equivalent, and the units every US electrical figure has a twin for.
Breaker and wire size chart
Every common breaker size paired with the copper and aluminium conductor it protects, the ground that goes with it, and what each circuit normally feeds.
Copper against aluminium wire
Copper and aluminium conductors compared: the two-size rule, ampacity and resistance side by side, where aluminium belongs and why it earned a bad reputation.
The voltage drop formula
The voltage drop formula worked through step by step, where the 3 and 5 percent figures come from, and why they are recommendations in the NEC rather than requirements.
NEC conduit fill chart
Conduit fill charts for EMT, PVC and rigid: how many THHN conductors of each size fit in each trade size at the 40 percent limit, computed from NEC Chapter 9.
Ohm's law explained
Ohm's law from first principles: what V = I × R means, the power wheel and its twelve formulas, and the one consequence that explains most electrical failures.
What size wire for a 100 amp service
A 100 amp service can use 4 AWG copper under the 83 percent rule, while a 100 amp sub panel needs 3 AWG — why the same rating gets two different answers.
What size wire for 50 amps
The 50 amp circuit answered properly: 8 AWG copper up to about 90 feet, 6 AWG beyond it, with the ground, the conduit and the aluminium equivalent.
Wire gauge chart
Complete wire gauge chart: AWG and kcmil size, area in mm², ampacity for copper and aluminium at 60, 75 and 90 °C, and resistance per 1,000 ft.