Power rating

In electrical engineering and mechanical engineering, the power rating of equipment is the highest power input allowed to flow through particular equipment. According to the particular discipline, the term power may refer to electrical or mechanical power. A power rating can also involve average and maximum power, which may vary depending on the kind of equipment and its application.

Power rating limits are usually set as a guideline by the manufacturers, protecting the equipment, and simplifying the design of larger systems, by providing a level of operation under which the equipment will not be damaged while allowing for a certain safety margin.

In equipment that primarily dissipates electric power or converts it into mechanical power, such as resistors, and speakers, the power rating given is usually the maximum power that can be safely dissipated by the equipment. The usual reason for this limit is heat, although in certain electromechanical devices, particularly speakers, it is to prevent mechanical damage. When heat is the limiting factor, the power rating is easily calculated. First, the amount of heat that can be safely dissipated by the device, ${\displaystyle P_{D,max}}$, must be calculated. This is related to the maximum safe operating temperature, the ambient temperature or temperature range in which the device will be operated, and the method of cooling. If ${\displaystyle T_{D,max}}$ is the maximum safe operating temperature of the device, ${\displaystyle T_{A}}$ is the ambient temperature, and ${\displaystyle \theta _{DA}}$ is the total thermal resistance between the device and ambient, then the maximum heat dissipation is given by

If all power in a device is dissipated as heat, then this is also the power rating.

Equipment is generally rated by the power it will deliver, for example, at the shaft of an electric or hydraulic motor. The power input to the equipment will be greater owing to the less than 100% efficiency of the device. Efficiency of a device is often defined as the ratio of output power to the sum of output power and losses. In some types of equipment, it is possible to measure or calculate losses directly. This allows efficiency to be calculated with greater precision than the quotient of input power over output power, where relatively small measurement uncertainty will greatly affect the resulting calculated efficiency.

In devices that primarily convert between different forms of electric power, such as transformers, or transport it from one location to another, such as transmission lines, the power rating almost always refers to the maximum power flow through the device, not dissipation within it. The usual reason for the limit is heat, and the maximum heat dissipation is calculated as above.

Power ratings are usually given in watts for real power and volt-amperes for apparent power, although for devices intended for use in large power systems, both may be given in a per-unit system. Cables are usually rated by giving their maximum voltage and their ampacity. As the power rating depends on the method of cooling, different ratings may be specified for air cooling, water cooling, etc.

For AC-operated devices (e.g. coaxial cable, loudspeakers), there may even be two power ratings, a maximum (peak) power rating and an average power rating. For such devices, the peak power rating usually specifies the low frequency or pulse energy, while the average power rating limits high-frequency operation. Average power calculation rating depends on some assumptions about how the device is going to be used. For example, the EIA rating method for loudspeakers uses a shaped noise signal that simulates music and allows peak excursion of 6 dB, so an EIA rating of 50 Watts corresponds to 200 Watts peak rating.