posted 12-22-2008 09:45 AM ET (US)
If you have a 25-watt VHF Marine Band radio in transmit mode, the radio will produce approximately 25-watts of radio frequency (RF) power output. The transmitter in the radio is not perfectly efficient, and we can assume that it converts DC power to RF power with about a 50-percent efficiency. This means the radio will require an additional 50-watts of power during transmit, added to whatever its normal power consumption might be. In a system where the supply voltage is 12-volts, 50-watts of power represents a current flow of 50/12 = 4.2-amperes. If we allow about 1-ampere for the other circuitry in the radio, this implies a total current of about 5.2-amperes when transmitting.
In a circuit where a current flow of 5.2-amperes will occur under normal circumstances, use of an over-current protector that operates at 3-amperes is inappropriate. A current flow of 5.2-amperes represents 173-percent of the rated current. Most current protection devices will operate when the current flow in the circuit that is protected by them is at 173-percent of rating.
Fuses are generally made only in certain increments of current, so to protect a circuit where it is expected that the normal current flow will be about 5.2-amperes, a fuse rated for 6-amperes would be appropriate. A fuse rated for 3-amperes should blow rather rapidly after the transmitter is keyed.
ASIDE: In a transmitter using frequency modulation (FM), the carrier power is constant and does not vary with modulation. As soon as an FM transmitter is operated, its carrier power goes to the rated power and its amplitude does not vary with modulation.