Wire size and breaker calculator
This applies the two things most wire-size tools skip: the 125% factor for continuous loads under NEC 210.19(A)(1), and the fact that aluminium carries meaningfully less current than copper of the same gauge.
It reads the 75°C column of NEC 310.16, which is the usual limit because most terminations are rated 75°C even where the conductor insulation is rated higher.
Continuous means the load runs for three hours or more at a stretch.
The 125% rule
For a continuous load — anything running three hours or more without a break — the branch circuit and overcurrent device must be sized for 125% of the load. A 40 A continuous load therefore needs a circuit rated for at least 50 A.
This catches people out on EV chargers, electric water heaters, and commercial lighting, all of which run for hours. A 48 A charger is a 60 A circuit, not a 50 A one.
What this calculator cannot know
Four things change ampacity and none of them can be inferred from a load figure.
- Termination temperature rating — most breakers and lugs are 75°C, which caps the conductor regardless of its insulation rating
- Conduit fill and bundling — more than three current-carrying conductors in a raceway requires derating under 310.15(C)(1)
- Ambient temperature — hot attics and rooftops derate significantly under 310.15(B)
- Voltage drop on long runs, which frequently forces a larger conductor than ampacity alone would
Aluminium
Aluminium carries roughly two AWG sizes less current than copper — 2/0 aluminium is about equivalent to 1/0 copper. It's cheaper for large feeders and service entrances, which is where it's normally used.
It also requires terminations rated for it and an approved antioxidant compound on connections. Aluminium branch-circuit wiring in older homes is a known hazard and a different problem entirely.
Questions
Can I just use this to wire something?
No. It gives you a conservative starting point for planning and for checking someone else's number. Real installations depend on termination ratings, fill, ambient temperature, and local amendments — and in most jurisdictions the work requires a permit and inspection.
Why does 12 AWG show 25 A when everyone says 20 A?
Because 25 A is the 75°C ampacity from 310.16, while the 20 A figure comes from 240.4(D), which restricts overcurrent protection on small conductors to 20 A for 12 AWG regardless of ampacity. The breaker column reflects the standard sizes in 240.6(A); for small conductors the 240.4(D) limit governs.
Does this cover motor circuits?
Not properly. Motors are sized from the tables in NEC Article 430 rather than nameplate current, because starting current and overload protection are handled separately. Treat a motor circuit as out of scope here.