Size overcurrent protection to the wire

Why this rule exists

A breaker or fuse has one core job: open the circuit before the wire overheats. Every conductor has a safe current-carrying capacity, its ampacity, set by size, insulation, and conditions. The overcurrent device must be sized to trip at or below that ampacity, so the wire never becomes the fuse. Oversize the breaker so it stops tripping and you strip the wire's protection; under a sustained overload the conductor heats, insulation degrades, and you get a fire inside the wall. The breaker protects the wire, not the appliance. Size it to the conductor.

In practice

Match breaker to conductor: 15-amp breaker on 14 AWG copper, 20-amp on 12 AWG, 30-amp on 10 AWG, as a common baseline before derating. Never install a larger breaker to cure nuisance tripping; find the overload or fault instead. When you extend or splice a circuit, do not reduce wire size below what the breaker protects. Check that terminal and device ratings match the breaker too. If the load genuinely needs more current, run the correct larger conductor and size the breaker to it, in that order.

Example

14 AWG copper: 15 A max breaker
12 AWG copper: 20 A max breaker
10 AWG copper: 30 A max breaker
Never: 20 A breaker on 14 AWG wire
That lets 14 AWG carry 20 A and overheat
The breaker protects the conductor, not the load

When it doesn't apply

Motor and transformer circuits use different rules, allowing higher upstream protection sized for inrush while separate overload devices protect the equipment; that is deliberate, not a violation. These exceptions come from the code for specific reasons; do not generalize them into a bigger breaker is fine. When the sizing is not a simple branch circuit, check the rules.