There's often some confusion as to what the ampacity of a wire is, or what the overload protection can be.
As an example, let's consider humble #12 copper wire. A look at the ampacity tables might tell us it's rated at 25 amp - but a little star directs us to a place where the code says 'don't go over 20.'
That same citation also details ten different situations where the note doesn't apply, and other rules kick in. Tap rules, motors, welders, and air conditioning are among the exceptions, where we are directed, instead, to specific sections of the code.
For example, I've seen welders with duty cycles so low that you could, following the code rules, put a 100 amp breaker on that #12.
HVAC is another such example where 'following the table' isn't as simple as it seems. Sure, the wire hasn't changed ... but the nature of the equipment has changed what the wire is exposed to.
Not only do motors require a starting current far higher than their operating current - the situation is made worse in an air conditioner because that compressor is ALWAYS starting up against a full load (the internal pressure doesn't change). So, for that first few seconds, the motor is fighing an uphill battle. In effect, that 1hp motor has to produce six or seven hp just to get running. Once running, the load drops to well under 1hp (in this example).
The effect of this is that, by measuring temperatures, current draw, and voltage drops, the manufacture has demonstrated that his equipment might need a 50 amp breaker -to prevent the start-up load from tripping it needlessly - while a #10 wire stays cool enough that the insulatio is not damaged .... even under the most demanding of conditions.
This leaves the manufacturer with a choice. Does he supply the electrician with abook of engineering data and a code book, letting him figure it all out? Or, does he simply mark the nameplate : Largest breaker 50 amp, smallest wire #10?
This doesn't relieve the sparky of all his responsibility, but it sure helps. Short cycling, long wire runs, low PoCo voltage, conduit fill, etc., can all mitigate against using the smallest possible wire. Likewise, the sparky may find that he can successfully use a smaller breaker on that particular installation, without nuisance tripping at start-up.
Not mentioned in this discussion (so far) is the role of a motor starter in protecting equipment.
We're required to instal a disconnect. The code is silent about using a starter. The unit already contains a contactor - why would one add a starter?
Because a starter is simply a contactor - with overload devices added. These "heaters" are for protecting the motor against long-term operation under excessive load. One can size these overloads very close to the actual operationg current of the unit without fear of nuisance tripping. You might very well have a circuit that is protected by a 50 amp breaker, while the unit itself is protected by 25 amp overloads.