Editing Power rating (section)

== Equipment types ==

=== Dissipative equipment ===
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In equipment that primarily dissipates [[electric power]] or converts it into mechanical power, such as [[resistor]]s, and [[Loudspeaker|speaker]]s, the power rating given is usually the maximum power that can be safely [[dissipation|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, <math>P_{D,max}</math>, 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 [[Thermal management of electronic devices and systems|cooling]].  If <math>T_{D,max}</math> is the maximum safe [[operating temperature]] of the device, <math>T_{A}</math> is the ambient temperature, and <math>\theta_{DA}</math> is the total [[thermal resistance]] between the device and ambient, then the maximum heat dissipation is given by
:<math>P_{D,max} = \frac{T_{D,max} - T_{A}}{\theta_{DA}}</math>
If all power in a device is dissipated as heat, then this is also the power rating.

=== Mechanical equipment ===
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.<ref name="AtkinsAtkins2013">{{cite book|author1=Anthony G. Atkins|author2=Tony Atkins|author3=Marcel Escudier|title=A Dictionary of Mechanical Engineering|url=https://books.google.com/books?id=0TjtKmSIL48C&pg=PA269|year=2013|publisher=[[Oxford University Press]]|isbn=978-0-19-958743-8|page=269}}</ref><ref name="Thumann2010">{{cite book|author=Albert Thumann|title=Plant Engineers and Managers Guide to Energy Conservation|url=https://books.google.com/books?id=D7xO-d6gLUgC&pg=PA320|year=2010|publisher=The Fairmont Press, Inc.|isbn=978-0-88173-657-1|page=320}}</ref><ref name="Eccles2008">{{cite book|author=William J. Eccles|title=Pragmatic Power|url=https://books.google.com/books?id=HWntzQ4vY3kC&pg=PA74|year=2008|publisher=Morgan & Claypool Publishers|isbn=978-1-59829-798-0|page=74}}</ref> 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.

=== Power converting equipment===
In devices that primarily [[Power converter|convert]] between different forms of electric power, such as [[transformer]]s, or transport it from one location to another, such as [[transmission line]]s, 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 [[watt]]s for [[real power]] and [[volt-ampere]]s 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]].<ref name="Patel2012">{{cite book|author=Mukund R. Patel|title=Introduction to Electrical Power and Power Electronics|url=https://books.google.com/books?id=Am5CI32itwAC&pg=PA54|year=2012|publisher=[[CRC Press]]|isbn=978-1-4665-5660-7|pages=54–55}}</ref> As the power rating depends on the method of cooling, different ratings may be specified for air cooling, water cooling, etc.<ref name="Patel2012"/>