To be on the safe side ,I would recommend you call a certified electrician. You could end up with one hell of a fire if your main breaker is at fault. Electricity is one element you don't take chances with. One error and you can loose a LIFETIME. Enough said. Jack
As a couple of people said, you can install loads that are all smaller than the breaker they are connected to. But when you add up all the loads it can be larger than the main breaker.
Your original post said "a new compressor was just installed" If that means A/C was just installed when there was none before, or a larger A/C was installed, the total electrical load may now exceed the main breaker. This is easy to do with smaller services like 100A or less. In that case you need to increase the service size (relatively costly) or don't run major loads at the same time as the A/C. Major loads include electric drier, electric stove, electric water heater. When an electrician adds a major load they should determine if the existing service is large enough and at least warn you if it is not (it is a code violation if the NEC covers your area).
Else what is "new compressor"?
The furnace normally includes the thermostat and control circuit for the A/C. As several people said, if the furnace is on the lightning circuit you talk about, turning it off will also kill the A/C.
-- bud--
If you had a 100 amp service, with 40 - 15 amp branches, and each drawing 5 amps(well below their trip setpoints), 40 x 4 amps = 200 amps. The 100 main breaker seems like it's doing its job.
IMHO, get an electrican, and a load calc performed.
tom @
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.