hp vs. current rating

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clayton

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why on many contactors or switches, they give you a hp. rating and also the max. current. and the flc for a motor matching the hp rating is way lower than the max. current rating for the switch?

we surmise here that it is factoring in the start up current, but would like to know if someone knows for sure.

thanks in advance

clayton
 
That is correct. A contactor has essentially 2 ratings, a thermal rating, which is really a RESISTIVE switching rating and usually shown as just Max. Amps, plus an INDUCTIVE switching rating, often shown as HP or kW. The inductive rating takes three additional factors into account; Making Amps, Breaking Amps and Duty Cycle. Making Amps reflect on the contact's ability to withstand the instantaneous inrush current that occurs when magnetizing a coil, such as a transformer or motor winding, without damage. Breaking Amps reflects on the contact's ability to withstand the tendency to vaporize slightly when drawing an arc as it attempts to interrupt the flow of power to the motor, while at the same time the motor's back emf is feeding power back to the line. Duty Cycle refers to the contact's ability to dissipate the heat caused by starting current as the motor tries to accelerate and how often they can take getting heated up without becoming so hot that the contact material melts and welds.
 
http://www.mikeholt.com/code_forum/showthread.php?t=81293

Incandescent lamps have an extremely high (though very short duration) inrush current, because a cold filament has about 1/10th the resistance of a hot filament. Some contactors and switches will have yet another rating for incandescent lighting loads.

You will also find that plugs and receptacles have horsepower ratings, which are similarly lower than what you would expect from the amp rating of the receptacle. This can get fun if you want to install a receptacle for an air compressor, where the manufacturer has intentionally inflated the hp rating for marketing reasons.

-Jon
 
The horsepower rating is not just about being able to carry the motor inrush current. A horsepower rated device must be able to open (break) the motor locked rotor current. This rating assures that the switch can safely disconnect a motor that "does not start".
 
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