I want to run a 220 volt circuit from panel to an electric fireplace (12 amps) and an electric heater (15 amps). What size breaker should I use and what size wire?
add them together for at least 27 amps (for a 30 amps circuit)
split the circuit in a 15 for the 12 amps fireplace and a 20 for the 15 amps heater (to avoid the breaker tripping on the startup spike)
10 gauge wire is the largest I like to use in home use. Depending on who you talk to and what code book you are looking at, the amount of current a 10 gauge wire can handle changes. I tend to think it can safely handle about a 30 amp breaker (and I think most code books fall in line with this).
You can add up the amps (12+15=27 amps) to get the total amps possible. This would mean that a 30 amp breaker with 10 gauge wire should work fine. However, check your currents to make sure that is the max that will be pulled. Many items have a large start up current that can cause a breaker to trip if you aren't careful.
For things that pull this much current by themselves, it is generally better to split them into 2 different breakers. So for you I would go with a 15 amp and a 20 amp. Technically you can use #14 on the 15 amp breaker, but if it were me I would go ahead and run #12 in case you some day want to swap out your fireplace with something that pulls more power.