A friend has a microwave on the same circuit as electric stove. Independently they work fine but if an element on the stove is on and the microwave is turned on the breaker goes off. An electrician was called and he said that the microwave should be on it's own circuit, best just to not use both at the same time. From my reading this does not seem to be a wrong way to wire a kitchen though perhaps not common. So why does the breaker shut off?
The electrician is right. The circuit is improperly sized for these loads.
It's very likely that the microwave was added as an afterthought without a moment of consideration for circuit loads. This is typical of work done by homeowners or appliance installers.
The reason is simple enough: installing a new circuit is way beyond the license of the installer. So he's confronted with the reality that he must violate Code... or cancel the sale and return with the brand new microwave in his truck, and the boss will fire him and get an installer who will install stuff.
- Anyway, by his logic, it's the homeowners' job to assure the necessary electrical service is provisioned for this appliance they know they are buying. It's not his job to pull service out of thin air. So really, the code violation is the homeowner's. He may have mentioned to the homeowner that this is temporary and the homeowner needed to have a circuit added. This news did not reach your ears.
So yeah. It was installed improperly and that is the mess you inherited. You need to run a separate circuit for this microwave. Besides, the circuit breaker that protects the range is not sized properly to protect the microwave. The microwave could pretty much catch on fire and the large range breaker would not care.