Nominal power is a measurement of a mediumwave radio station's output used in the United States.
[2] Nominal power is ultimately a historical artifact of the regulatory regime employed by the FCC prior to the 1980s.
In the old system, rather than allowing licensees to choose any power level which would meet the efficiency and interference standards for their class, stations were restricted to a small set of power levels: 50, 100, 250, 500, 1000, 2500, 5000, 10000, 25000, and 50000 watts.
A station whose maximum coverage would otherwise be available at 4500 watts (given a specific directional pattern and antenna system efficiency) had a choice of either living with 2500 watts, or reducing the antenna efficiency to a level which would allow for 5 kW.
The resistor network exception was created to allow stations to reduce their antenna efficiency without having to modify the existing tower.