None of these answers were very helpfull sooooo-
Here is what you do if you are a real Electrician.
*Using my 2011 Handbook (Check current code book but i doubt this has changed) Comment below if it has.
Look at *220.55 Note #4 and Note #5- note the phrase "household cooking appliances" not just "Ovens" and "cook tops" there is no hard definition that excludes micro. its states you can supply a cook top and two ovens max with one circuit . Period ! IT will be added together and the KW total will be seen as ONE range and then use your chart Column A B or C. Table 220.55.
SO NOTE 5= Microwave is a Household cooking appliance as well as it is a combo convection oven anyway which takes precedence anyhow.
the neutral load will be more than usual on the Micro wave but similar since the code compensates for the neutral by keeping it 30 amp min. and romex is full size N who cares !
So that's plenty for the micro and any low stetting oven neutral loads... as some ovens do apply a neutral load to the circuit in low setting apparently ...i did not know that .! Nor did i care since most feeder cables ran have a full size neutral. But some old houses may not have...
For example if you have 2 -3000 kw appliances then you go with =6000 @25 amps and the demand of 2 @ Column B is 65 %.(16.25amps) of that total could be used for sizing. feeder. and Breaker so a 20amp 240 volt circuit could handle all that ! Coool! Always run the neutral You never know what is coming out to the job or what could happen in the future if some decide to change it. IF I were doing it I would run 8/3 anyway and run the breaker at 30 maybe in the hypo above but you can do 20 and 20 amp wire if you want. Wire is a heat sink so you take more of a chance of burning up something like a wire nut or terminal connection if you use small wire. so think safety but dont over do it too much is my opinion.
*ARTICLE 210 .19 (A) 3 states that any appliance shall have an ampacity **(not less than) the rating of the branch circuit and not less that is the maximum load to be served.
*210 .19 (A) 3. Exception #1 talks about tap conductors being not less than 20 amp wire.... this can count for appliance conductors from factory and your flex whip if you need one to go from j box to micro or cook top . so those could be 20 amp min. but again size with your heat sink in mind ...don't always do the min required . if you get a loose connection it may save you if you have bigger wire. and 20 amp and 50 dont mix well either in a wire nut... seen those catch fire. two similar size wires twist better or use insulated lugs for a sure connection for two different size wires.
Neutral can not be ampacity of not less than 70 percent of branch circuit rating and not less than # 10= Exception #2
**so breaker maximum is not noted if you notice... any breaker will protect from a short ... the worry with an oven is a short not an overload since the element can only do so much. and people don't turn everything on and blast it for 3 hours at a time! Common sense is needed too here. its not like they are firing swords in there oven !!! cooking a turkey in the convection all day etc....if they do they deserve to have their house burn down ..LOL
Your Welcome!
You can run the oven all day if you want, heating element only cycles on as temp requires, which is also a consideration taken in setting up the demand factors.
If you open the oven door and leave it open then you possibly end up with a continuous load situation - if it happens for too long and you used demand factors to reduce conductor ampacity and overcurrent protection - then you could end up tripping overcurrent device after being in this condition for a while. Otherwise the longest element run cycle is usually during "preheat"
