I am building a 24'X30' garage that will have a guest space above. The service to the main building is 200Amp. The garage will include small appliances heat and a/c. There will be an efficiency washer dryer as well. It's not an apartment meant for full time use. How do I know if the service needs to be upgraded? I was under the impression from one electrician that there was no need. However the contractor is saying it may need to be upgraded. Any input would be greatly appreciated.
Generally we would go over 200A for a single family dwelling unless the house is over 3000 sq/ft, and even then we would run a calculation to see if a larger one is needed. When we see an addition to an existing building the code allows us to take the peak load of the existing building then add 25% to it then add the additional power you are installing in the new addition.
Now, where can you get your peak loads? If you look through your power bill for a year (2016). Somewhere in that mess they will give you the peak winter and peak summer usage, use the highest peak. Or you can call your utility company and they usually have that information available. It will be given to you in KVA or KW so you take that figure and divide it by .24 and that will give you you peak amperage for that year. If it is somewhere around 140A you need to consider upgrading your service before adding your garage.
If what I just said is confusing. You need to at least get some electrical professional to assist you with it.