Hi
For AM/FM in USA, you can refer to FCC page.
MINIMUM POWER LEVELS for LICENSED BROADCAST OPERATION
The minimum power for a standard FM broadcast station which a construction permit applicant may request is 100 watts (0.100 kilowatts). Alternatively, if an effective radiated power of less than 100 watts is sought, the combination of the proposed effective radiated power and antenna height above average terrain must cause the distance to the predicted 1 millivolt per meter (mV/m) contour (or 60 dBu contour) to be greater than 6 km. See 47 C.F.R. Section 73.211 of the Commission's Rules. Please note that new FM stations will not be authorized to operate with less than these minimums. While there are a few Class D FM noncommercial educational stations (also called "10 watt stations") which are permitted to continue operations with smaller facilities, authorizations for new Class D FM stations were discontinued in 1978. No new Class D FM stations will be authorized outside the state of Alaska. See our Information Sheet about How To Apply For A Broadcast Station for additional information on application preparation.
Stations authorized in the new LPFM service will operate with effective radiated powers (ERP) between 1 watt (0.001 kW) and 100 watts (0.100 kW). In any case, the distance to the 1 mV/m (60 dBu) contour from an LPFM station or application will not be permitted to exceed a reference distance of 5.6 km. LPFM applications which would require operation with less than 1 watt ERP will not be accepted for filing.
The minimum power for an AM broadcast station which a construction permit applicant may request is 250 watts (0.250 kilowatts). Alternatively, if an effective radiated power of less than 250 watts is sought, an equivalent RMS value of at least 141 millivolts per meter (mV/m) at a distance of 1 km from the tower site must be proposed. See 47 CFR Section 73.21(a)(2) of the Commission's Rules. No AM broadcast station will be authorized with less than these minimums. See our Information Sheet about How To Apply For A Broadcast Station for additional information on application preparation.