A dedicated circuit serves one piece of equipment and nothing else. The reasons are a mixture of Code requirement, manufacturer instruction and plain arithmetic.
Required by Code
Bathroom receptacles — a 20 amp circuit serving bathroom receptacle outlets and nothing else outside the bathroom.
Laundry — a 20 amp circuit for the laundry receptacles.
Small appliance circuits in the kitchen, which serve the specified areas and nothing else.
Any individual branch circuit required by a specific appliance rule.
Required by the appliance
Most large appliances specify a dedicated circuit in their installation instructions, and Code requires equipment to be installed according to those instructions.
Electric range, cooktop and wall oven — 240 V, typically 40 or 50 amps depending on nameplate.
Electric dryer — 240 V, commonly 30 amps.
Electric water heater — 240 V, commonly 30 amps.
Air conditioner or heat pump outdoor unit — sized from the MCA and MOP on the nameplate.
Furnace or air handler — a dedicated circuit, so that servicing it does not de-energise anything else.
Dishwasher and disposal — dedicated, sometimes shared between them where the ratings permit.
Built-in or over-range microwave — dedicated 20 amp.
Well pump, sump pump, septic pump — dedicated, because these are equipment nobody wants tripped by an unrelated load.
EV charging equipment — dedicated, and sized at 125% of the continuous load.
Hot tub or spa — dedicated, with GFCI protection and a disconnect within sight.
Sensible even where not required
Refrigerator and freezer. Not universally required, and a tripped kitchen circuit that also feeds the fridge is discovered when the food thaws.
Home office equipment, where a tripped breaker means lost work.
Sump pump, where a failure has consequences beyond inconvenience.
A workshop, where power tools have high starting currents.
Why the list keeps growing
Every appliance moving from gas to electricity adds a dedicated circuit and a substantial load: induction ranges, heat pump water heaters, heat pumps, EV chargers.
A house built with gas heating, gas cooking and gas water heating has three fewer large circuits than the same house electrified, and its panel was sized accordingly.
That is why a renovation programme frequently discovers the panel is the constraint rather than any individual appliance.
What a dedicated circuit is not
Not a receptacle with only one thing plugged into it. The circuit is dedicated if nothing else is wired to it, regardless of what is plugged in.
Not achieved by unplugging things. A shared circuit remains shared.
Not the same as an isolated ground, which is a different arrangement used for sensitive equipment and is rarely relevant in a house.
Identifying what you have
Turn off one breaker at a time and find what goes off. It is tedious and it is the only reliable method.
Label the panel directory as you go. An accurate directory is worth having for its own sake, and it is the document every future electrician will wish existed.
The disconnect requirement
Several of these appliances need a disconnecting means within sight of the equipment, so that a technician working on it can be certain it is dead.
Outdoor air conditioning and heat pump units, hot tubs, and in many cases furnaces and water heaters fall under this. It is a small weatherproof switch or a pull-out beside the equipment, and it is one of the items most often missing on an appliance installed by a non-electrician.
Where the panel is in the same room and in sight, a breaker with a lock-out provision can serve instead.
Sharing where it is allowed
A dishwasher and a disposal on one 20 amp circuit is permitted where the sum of their nameplate ratings fits, and it is a common arrangement in a panel short of positions.
The arithmetic has to be done rather than assumed, and a large disposal with a modern dishwasher frequently does not fit. It is worth checking the two nameplates before deciding to combine them, because the alternative is a breaker that trips whenever the household runs both.