The first place to look is in the instructions for your microwave. If it says it "requires" a certain kind of circuit, that is what you are obligated to install. If it "recommends" such a circuit, I'd go ahead and do it anyhow.
If it says 15A, you're free to put in a 20A one.

If this is a dedicated outlet that is in the space of a builtin unit, you can get by with 15A if that's what the manufacturer says is OK. If it's not in the microwaves dedicated space, and we're talking about a kitchen, then you can't use a 15A whether you intend to dedicate it to the microwave or not. The small appliance circuits must be 20A.

check for threads by 'bigbird'. He is Canadian and well up on their requirements which are slightly different than the US requirements.

Yes, there the countertop receptacle circuits are one of the known differences between the CEC and the US NEC. I don't know the Canadian code at all.

It depends on the wattage. If it is the typical 1200 watt oven, it will require about 10.5 amps, therefore, a 15 amp circuit is ok so long as there are not other loads on the same circuit.

The amperage number here is only a ballpark figure. It could be more depending intrepretation of the nameplate data. These numbers are based on 1200 watts input, not output. If the oven puts out 1200 watts and is 95% efficient, it uses 1269 watts input or about 11.5 amp.

I always dedicate a circuit for the microwave, usually 15 amp But if it is a microwave/range hood combo unit, you will need 20amp circuit. Is the microwave in a cabinet, or counter top, as counter tops circuits are now 20 amp anyway

Here is a link that might be useful: Pulling Kitchen Wire

as counter tops circuits are now 20 amp anyway

Op is in Canada. I thought I remembered bigbird as saying that 15 amps are their requirement. I could be wrong.

"If the oven puts out 1200 watts and is 95% efficient, it uses 1269 watts input or about 11.5 amp."

Good luck finding a high voltage power supply and magnetron with that high an efficiency.

You are doing rather well to get even 75% efficiency.
Generating RF is not an efficient process.

I belive you can look at the label and decide quickly what you bottom line need.

If it's an 1100 watt unit and 80% efficient then it's
drawing 20% more than 1100 watts. or about 1300 watts.

That's 10amps ie 10A x 120 volts = 1200 watts.

Most breakers don't flip until you hit about 120% of the
rating or in a 15A that would be 18A. I should think
with an 1100 watt unit with a 1300 watt tag (on the back) a 15amp circuit would be fine.

Those are the facts as I see it.

In my locality, all kitchen circuits are to be 20 map by code.

Correct me if I'm wrong but, AFAIK, there are few if any small appliances that require a 20A circuit, else they would come specifically equipped with a 5-20P NEMA plug (one configured thusly):

True. A powerful argument supporting the first and second post in this thread.

Since I've not seen a single plug like this above: (small appliance equipped with a 5-20P NEMA plug) I'll conclude now that 15A is indeed sufficient.
In my case, the dedicated circuit IS in a cabinet.
I haven't owned a microwave oven for more than ten years so I was wondering where to go for guidance.
The first answer was sufficient.
This answer above is a killer.
Because now I also understand better how 20A circuits relate to 15A appliances.
Interesting! Very interesting! But..

"Correct me if I'm wrong but, AFAIK, there are few if any small appliances that require a 20A circuit, else they would come specifically equipped with a 5-20P NEMA plug (one configured thusly)"

The NEC does not cover the manufacture of appliances, that is covered by NEMA (and not always very well).

Since the equipment is not 'permanently installed' the NEC has limited reach.

UL should be a check also, but there are any number of appliances available that use more than 80% of a 15 A circuit and should require a 20 amp circuit and plug.

They often cone with warnings about making sure nothing else is on the circuit the equipment is plugged into.

The starting surge to charge up the high voltage power supply does also not have to be included in the nameplate rating.

But as I stated before, if we're talking about a kitchen, and you're plugging anything OTHER than a switched or a refrigerator it needs to be 20A.

Found this thread when looking for an answer. Might be different per unit, but I have a 1000W Maytag wall mount (fan/microwave/light) and found the user manual which says it's a requirement for it to be on a 15 or 20 AMP, but only recommended that it have it's own circuit. An inspector flagged it when I was selling my condo and I wasn't about to add a circuit. In my case there's one other outlet that shares the circuit and I've never had an issue during 10 years of using the kitchen.

Observe all governing codes and ordinances.

Required:
■ A 120 Volt, 60 Hz, AC only, 15- or 20-amp electrical supply
with a fuse or circuit breaker.

Recommended:
■ A time-delay fuse or time-delay circuit breaker.
■ A separate circuit serving only this microwave oven

Maytag User Manual Home Depot

I've always been a little confused on this issue. I know that the circuits must be 20 amp, but I thought the receptacle can be a 15 A receptacle, i.e. one which accepts parallel blades only. Is that right, Ron?

The code specifically allows a 15 amp rated receptacle on a 20 amp circuit, but I'm not sure that answers the OP's question about the microwave.

Apparently the instructions for the appliance do not require (only recommend) a dedicated circuit. Based on that information, I think this part of the code applies.

110.3(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.

Bruce

If it must be 20A circuit and it’s dedicated...how can you install a 15A outlet? I thought you could only use 15A outlet on 20A circuit if you were using more than one outlet?

A duplex receptacle counts as two.