Conductor sizing in a dwelling comes from two places in the Code, and using only the first is the most common mistake in amateur discussion of it.
The common residential sizes
Copper, for typical branch circuits:
| Circuit | Copper conductor |
|---|---|
| 15 A | 14 AWG |
| 20 A | 12 AWG |
| 30 A | 10 AWG |
| 40 A | 8 AWG |
| 50 A | 6 AWG |
| 60 A | 4 AWG |
Aluminium conductors are sized larger for the same rating, and the specific size depends on the application — dwelling service and feeder conductors have their own allowance in the Code.
Why the table alone is wrong
Ampacity tables list what a conductor can carry based on its insulation temperature rating. At 60°C, 12 AWG copper is listed at 20 amps; at 75°C it is listed higher.
But there is a separate rule — the small conductor rule — that limits overcurrent protection for 14, 12 and 10 AWG copper to 15, 20 and 30 amps regardless of what the ampacity table shows.
That is why the table above is the practical answer for branch circuits, and why "the chart says 25 amps" is not a basis for putting a 25 amp breaker on 12 AWG.
Termination temperature
There is a second constraint that catches people on larger circuits.
Conductors are rated at 60, 75 or 90°C, and the equipment terminals have their own rating. The circuit is limited by the lowest rating in the path, which for most residential equipment means the 60°C or 75°C column, not the 90°C one — even when the wire itself is 90°C rated.
Using the 90°C column because it is printed on the cable is a frequent error, and it produces conductors that are undersized in service.
Derating
Ampacity falls when conductors are bundled together or run in a hot space.
More than three current-carrying conductors in a raceway or cable requires a reduction. High ambient temperature — an attic in summer is the usual case — requires another.
A circuit that is correct in a basement can be undersized in an attic, and both adjustments apply together where both conditions exist.
Voltage drop
Not a safety requirement, and a performance one. Long runs lose voltage, and the Code recommends limits — commonly cited as 3% on a branch circuit and 5% overall.
On a long run to a detached garage or a well pump, voltage drop rather than ampacity is what sets the conductor size, and it is why those circuits are frequently two sizes larger than the amperage alone suggests.
Aluminium branch wiring
Houses wired with aluminium branch circuits, largely in the late 1960s and early 1970s, have a documented history of connection failures at devices.
The recognised remedies are specific: COPALUM crimps or listed AlumiConn connectors at every device. Simply replacing devices with those marked CO/ALR is a partial measure, and wire nuts intended for copper are not a repair.
Larger aluminium conductors for services and feeders are a different matter and are entirely normal in current construction.
What to check on existing work
The breaker against the wire size, which is visible at the panel. A 20 amp breaker on 14 AWG is a hazard and it is found regularly.
Terminations, for discoloration or looseness.
Extension of an existing circuit in a smaller size than the original, which is a common DIY finding.
Any of these is worth correcting on its own, and all of them are cheap next to the consequence.