Core answer: AWG runs backward — smaller numbers are thicker wires, and every −6 steps doubles the diameter (12AWG≈2.05mm carries ~20A); for hardness, use HRC for hardened steel, HB for annealed/soft metals, and HV as the near-universal scale; energy units chain as 1kWh = 3.6MJ = 860kcal = 3412BTU.
Wire gauge: AWG to mm²
| AWG | Diameter | Cross-section | Typical copper ampacity |
|---|---|---|---|
| 18 | 1.02 mm | 0.82 mm² | ~7A |
| 16 | 1.29 mm | 1.31 mm² | ~10A |
| 14 | 1.63 mm | 2.08 mm² | ~15A |
| 12 | 2.05 mm | 3.31 mm² | ~20A |
| 10 | 2.59 mm | 5.26 mm² | ~30A |
| 8 | 3.26 mm | 8.37 mm² | ~40A |
Ampacity varies with insulation rating and bundling — derate 20–30% in conduit or hot environments.
AWG current-capacity table
Rule of thumb: every −6 AWG doubles the diameter (quadruples the cross-section); every −3 AWG doubles the cross-section. Metric equivalents: 12AWG≈3.3mm², 14AWG≈2.1mm², 16AWG≈1.3mm².
Aluminum wire needs about two AWG steps thicker (or ×1.6 cross-section) for the same current as copper.
Hardness scales compared
| Scale | Indenter | Best for | Typical reading |
|---|---|---|---|
| HRC (Rockwell C) | diamond cone | hardened steel, blades | files 62–65, knives 56–60 |
| HB (Brinell) | steel/tungsten ball | annealed steel, aluminum, cast iron | structural steel 120–180 |
| HV (Vickers) | diamond pyramid | thin layers, coatings, universal | pure copper ~50, nitrided layer 1000+ |
Rough conversions: HRC 60 ≈ HB 601 ≈ HV 697; HRC 30 ≈ HB 283. Conversions are empirical — measure in the native scale when precision matters.
Energy unit conversions
| Unit | Equals | Everyday anchor |
|---|---|---|
| 1 kWh | 3.6 MJ | one hour of a 1kW appliance |
| 1 kWh | 860 kcal | ~1.5 hamburgers of energy |
| 1 kcal | 4.184 kJ | food labels use kcal |
| 1 BTU | 1.055 kJ | air conditioners and furnaces |
| 1 Wh | 3.6 kJ | battery specs (mAh × V = mWh) |
Example: wire for a 30A EV charger
A Level-2 charger draws 30A continuous. NEC-style continuous-load derating (×1.25) means sizing for 37.5A:
- 10AWG copper ≈ 30A — not enough for a continuous load
- 8AWG copper ≈ 40A — correct choice
Voltage drop on a 15m run at 30A through 8AWG (3.3Ω/km): 2×15m×0.0033×30 ≈ 3V — acceptable. Skipping the derating logic and matching “30A wire to a 30A load” is the classic overheating scenario.
Example: which hardness scale to specify
Ordering a machined shaft, quenched and tempered:
- Hardened to ~HRC 40? Specify HRC — the diamond cone handles hard surfaces with a small indentation
- Annealed aluminum bracket? HB — HRC’s sharp cone sinks too deep in soft metal and reads unreliable values
- Nitrided surface layer 0.3mm deep? HV with low load — HB’s ball would punch through the layer
Writing “hardness 40” with no scale is meaningless — always attach HRC/HB/HV.
Common mistakes
- “Bigger AWG = thicker wire”: it is the opposite — 8AWG dwarfs 18AWG. The number counts historical drawing dies.
- Matching ampacity exactly to load: continuous loads (chargers, heaters) need ×1.25 headroom; exact matches run hot for hours.
- Mixing hardness scales without conversion: “HRC 60” and “HB 600” describe similar hardness, but “HRC 60” vs “HV 60” are worlds apart (HV 60 is soft copper).
- kW vs kWh: kW is power (rate), kWh is energy (amount). A 2kW heater running 3h uses 6kWh — bills charge kWh.