FCC Rules for ISM Band Wireless Equipment

In the USA, management of the electromagnetic spectrum is the responsibility of the Federal Communcations Commission (FCC). The FCC regulates not only the "intentional radiation" of radio transmitters, but also the "unintentional radiation" of noise from all sorts of electrical equipment. The FCC regulations appear in title 47 of the United States Code of Federal Regulations (47CFR), and radio spectrum issues are the subject of part 15 of the FCC rules, or 47CFR15 in brief.

The FCC website, http://www.fcc.gov/ has a lot of material online, including FCC rules (47CFR).

FCC Rules Applicable to ISM-SS Radios
(94 page PDF file of FCC rules part 15)
FCC 15.107Conducted Emissions including AC Line
FCC 15.109Unintentional radiation from ITE
FCC 15.205Restricted bands of operation
FCC 15.207Conducted emissions from intentional radiators
FCC 15.209Radiated emission limits, general requirements
FCC 15.247ISM Band Communication Equipment

Summary of Power Output Rules for ISM bands

The limits are three:
  1. Maximum transmitter output is 1W (30 dBm) (47CFR15.247.b.1)
  2. Maximum EIRP is 4W (36 dBm) (47CFR15.247.b.3) i.e. for every dB of antenna gain above 6dBi, transmitter output must be reduced by 1dBm; per this rule, a 24dBi antenna limits the output power to 12 dBm which is 16mW
  3. For fixed point to point operation in ISM2.4, peak output need only be reduced by 1 dBm for every 3 dBi of antenna gain above 6 (47CFR15.247.b.3.i) i.e. per this rule, a 24 dBi antenna may be fed by 24 dBm or 250 mW
  4. In ISM5.8, you can apply all the antenna gain you want (47CFR15.247.b.3.ii) with no reduction in output power.
The responsibility for staying within these power limits falls on the operator (or, if professionally installed, on the installer).