Compressor horsepower

I am looking at purchasing a new air compressor. I have a question as to how the horsepower is rated. I have found similar compressors with equal CFM air supply but one will state a HP and one will state a running HP. In this case the running HP is much lower than the HP stated. Is there a formula that can be used to compare the two for compressors in the 5 to 7 HP range?

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Compressor horsepower

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Jan 18, 2008
 Article answering your question I hope
by: Anonymous

http://www.truetex.com/aircompressors.htm

this was the article I used to base my decision.

True 1 HP should yield 4 CFM @ 100 PSI, at standard atmosphere, temperature, and humidity.

2 HP is the most you can get with 115 volts and 20 amps.

Peak HP can be a bit higher, but not for long before a breaker blows.

Get the one with the running HP.
Jan 18, 2008
 New
by: Anonymous

Don't forget the standard wire in a house will only feed 15 amp (max.) to an air compressor or about 1/2 HP motor capacity before you start blowing fuses. I suggest you talk to an electrician if you are in the 7HP range
Jan 07, 2008
 "Start" vs "Run" Horsepower
by: Bill - Publisher

Thanks for the question.

Some compressor manufacturers may suggest that their unit's peak horsepower at start is what you can expect as that compressor's ongoing run horsepower.

When a compressor kicks in, it takes a surge of electrical power to start the motor, and for a brief spell, that motor will sometimes triple it's HP output.

After the motor is running, the electrical surge is no longer required, and the motor HP output drops back to it's normal "run" horsepower.

The "running" horsepower will always be below the starting horsepower, and if the spec doesn't differentiate between the two, bet that it's the starting horsepower and not the run horsepower that they are showing.

Because motor efficiencies vary, pulleys have different diameters, different compressor heads have larger or smaller bore sizes, it's very hard to give an absolute as to what you can expect from a given motor/compressor.

You need to find out from the manufacturer what that particular compressor will deliver in discharge CFM at the pressure you need it.

Don't ask for a formula, ask what the unit will deliver in usable compressed air.

Here's more info on compressor sizing.

Regards,

Bill

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