Knowledge is Power: A Guide to Blast Cabinet Air Compressor Horsepower

When most of us hear the word, “horsepower” we think of large powerful gas guzzling machines like an automobile engine. Electric motors, like the ones used to power air compressors, have horsepower ratings too. Why would someone shopping for an air compressor care about electric motor horsepower? Mainly because it relates to airflow output. Typically, a compressor motor with more horsepower means more airflow. That means better sandblasting.

That should make choosing a blast cabinet air compressor simple, right? Just look for a compressor with a motor boasting plenty of horsepower. Sort of, but things aren’t that cut and dry. Like many other aspects of air compressors, horsepower ratings can be convoluted. Furthermore, when choosing an air compressor there are more important specifications to consider. Don’t worry. We’ll do our best to help take some of the mystery out of it all.

There are two major considerations you need to make surrounding air compressor horsepower specifications. First, you need to be sure the horsepower listed is an actual horsepower rating and not a “peak” hp rating. Next, you need to ensure your electrical source can support the motor and its rated horsepower. Unfortunately, you’re not going to be powering a true 3 or 4 horsepower motor by plugging into your standard 110/120 volt electrical outlet. It just can’t supply enough power. Having a very basic understanding behind electricity and how it relates to hp in an electrical inductance motor will be a huge benefit here. We’ll help with all of that. Let’s get started!

Peak Horsepower Ratings for Compressors are Essentially Worthless

In the past, air compressor manufacturers would proudly list “peak” horsepower numbers on their compressors. Many other tools like wet/dry vacuums still wear peak HP as a badge of honor. These “peak”, “max developed” or similar specifications have little correlation to compressor performance in the real world.

For an electric motor, a “peak” horsepower rating is just that: the most horsepower it could produce. What’s the issue with marketing that? It is a bit misleading and not very useful to the end consumer.

These peak horsepower ratings don’t occur for more than a fraction of a second under normal conditions. They result from a set of carefully designed laboratory conditions specifically set up to calculate the most power a motor is capable of outputting for a very brief instant in time. Peak or max developed horsepower typically occurs in a condition known as a “locked rotor” state or when a motor has reached its stall torque value. Essentially, a motor is mechanically loaded to the point the shaft stops turning.

When testing for peak hp, running motors are often stopped from spinning using a brake or other high mechanical load. For a brief moment in time the straining motor will draw abnormally high current readings known as locked rotor current. Leave a motor in this state for too long and you’ll either draw enough current to trip any circuit overload protection or you’ll destroy the motor. The high current ratings measured at this brief instant are used to calculate the maximum horsepower output the motor is capable of.

Locked rotor current readings are very similar to the high current a motor draws at startup when the shaft isn’t moving and the start winding on the motor is energized. As a result terms like “startup current”, locked rotor amperage” and “inrush current” are used interchangeably. Depending on the motor and conditions they may not be the same thing, but for our purposes here they’re all similar enough.

At these points the motor draws very high current and would theoretically produce the most power it is capable of: peak horsepower. That’s great but who cares? The duration this happens in is so short it’s measured in hundredths or thousandths of a second. It may be “true” that the 115V rated motor output 5 horsepower for a fraction of a second while the start winding is energized at startup or while over-juiced and suddenly stopped in a lab. So what? It doesn’t tell us what we need to know: how much power an air compressor motor can put out continuously.

The bottom line is peak horsepower tells you little to nothing about what horsepower an electric motor can put out for a sustained period of time. These ratings are equally useless when comparing airflow output of two different compressors. Let’s say one motor puts out 5 hp for a few milliseconds and another makes 4.5 hp for a few milliseconds. If both only produce around 1 horsepower when running continuously, who cares?

Peak horsepower ratings are a little more important in something like woodworking tools. A table saw or router trying to cut through a knot in a board might be able to power through with something near its peak horsepower rating for a short moment. Peak ratings aren’t as useful when talking about air compressors for blast cabinets. We need to know what sort of power they make on a continual basis.

Peak Confusion

Horsepower ratings for electric motors can be perplexing for the general consumer. The basic concept is simple: more is better. Air compressor and electric tool manufacturers have exploited this fact to gain a marketing advantage by slapping a peak horsepower rating on their product instead of the actual rated horsepower.

Peak horsepower ratings on electric motors can be many times higher than the actual rated horsepower a motor can produce for a sustained period while running. We’re talking at least 2 to 3 times higher and often much more. This power would only be produced for a split second. You can’t run a motor at the peak hp for long in your garage. If you do you’ll trip the circuit breaker or burn the motor up. That is what makes them so pointless to focus on. That didn’t stop manufacturers from using every trick in the book to be able to inflate the peak or maximum horsepower displayed to consumers. The practice reached comical levels of craziness in the 1990s.

I’ve still got a 120-volt, 15-amp 1990s Craftsman air compressor that proudly touts “5 HP” in large red letters on the plastic motor shroud. That’s odd because my standard residential 120-volt, 15-amp electrical outlet and wiring can’t safely provide the current needed to continuously power a 5 horsepower motor. (More on that later.)

So how did the manufacturer get away with their air compressor horsepower claim of 5 HP? It technically wasn’t a lie. The motor probably pulled high current for a few milliseconds on a test bench while mechanically loaded to the point the shaft stalled. This high current reading allowed the manufacturer to calculate a theoretical peak horsepower rating they could round up to a value of 5 horsepower. The big red “5 HP” is a peak “max developed horsepower” rating. It happens for well under a second. I’ll never see it continuously from my wall outlet which frankly makes the claim misleading to the average consumer.

In fact, there was a class action lawsuit about this and a settlement was reached back in 2004. As part of the terms of that settlement, the manufacturers involved agreed to stop the somewhat deceptive practice of marketing peak hp. They agreed to use more realistic ratings instead. One example is shaft output power that can actually be sustained without exceeding the rated operating conditions of the motor. The inflated air compressor horsepower nonsense has improved somewhat since then but it continues with many tools to this day.

Real Horsepower

What we care about is actual running horsepower. This would be the true running horsepower the motor is capable of running for a sustained period of time. Sometimes this horsepower rating is listed on the actual motor nameplate.

Let’s go back to my overconfident 1990s Craftsman compressor. Remove the plastic motor shroud boasting 5 HP and you might see a motor with a listed power of somewhere around 1 horsepower or less. Actually, you’ll often a find a motor with a metal nameplate that curiously has a blank space where the horsepower rating should be. If you’re lucky, the motor nameplate will list other information allowing you to calculate a reasonable estimate of the motor’s real horsepower.

Spoiler alert: if it has a standard 3-pronged 15-amp power cord, the horsepower output while running for sustained periods isn’t going to be anywhere near that 3, 4 or 5 Peak HP rating. The absolute best-case scenario in a perfect world would be somewhere around 2.41 hp. Even that is fantasy land. When you factor in real world scenarios, you’ll find most 120-volt air compressor motors are realistically probably going to be 1.5 to 2 hp or less. Why? The answer lies in electrical limitations.

Here in the United States of America, most residential electrical outlets are 120-volt 15-amp AC outlets on a 15-amp circuit. You might hear them referred to as 110-volts, 115-volts, 120, or even 125volts. They all refer to the same thing.

You’ll rarely get exactly 120-volts out of your outlet. In North America the utility company should provide you with a nominal voltage range of 120-volts +/- 5%. In very simple terms, the voltage should start out in the range of 114 to 126 volts. In most cases it will fall on the lower end of that range. Voltage drop between the source and your electrical outlet are very possible and very common. Let’s take a look at how that relates to air compressor horsepower.

Watt are You Getting At?

We need to briefly discuss electrical power. You’re probably familiar with the term “watt” when it comes to electricity. You’ve heard and used the term before, but what does it mean? What is a watt?

According to Merriam-Webster, one watt is the rate at which electrical work is done when one ampere of current flows through an electrical potential difference of one volt:. ¹/₇₄₆ horsepower.

Why is this important? Stay with me here. There’s a point but we need to do some math.

Watts are power. We’ve already said most outlets in the U.S. are rated at around 125 volts and 15 amps electrical outlets. So how many watts can they provide?

We turn to Watt’s Law which gives us the handy formula:
P = V x I
(Where P is power, V is voltage and I is current in amps.)

For our standard 120V 15 Amp residential electrical outlet it looks like this:

120 volts x 15 amps = 1800 Watts

Remember, 1 horsepower is about 746 watts. So, the horsepower equivalent of 1800 watts would be calculated by dividing 1800 by 746 .

746 watts = 1 horsepower
1800/746 = 2.41 hp

So in theory, our 15 amp 120 volts outlet should be able to provide us with 1,800 watts which means it should be able to power a 2.41 horsepower motor. More on this later, but that’s best case scenario and not reality.

We’re splitting hairs here, but let’s go one “best case scenario” better. Let’s assume your power company consistently provides you with the upper end of the acceptable nominal voltage range I mentioned: 126 volts. In this perfect world you experience little to no voltage drop. You’ve got an electrical outlet that is rated for 125-volts so let’s assume that is the most you can safely get from the outlet.

125 volts x 15 amps = 1,875 watts
1,875 /746 = 2.51 hp

If you’re somehow still awake, you might be wondering about a 20-amp circuit. This is rather pointless because most 120-volt compressors are set up with a 15 amp motor wired with a standard 15 amp plug. Let’s ignore that and assume you have a 20 amp motor wired to safely run on a 20 amp dedicated circuit.

Wattage of 20 amp outlet:
120 x 20 =2400 watts
2400 / 746 =3.22 horsepower.

Let’s go further into fantasy land give you what would probably be the best-case scenario. Imagine your power company consistently sends you the upper end of the 120-volts + or – 5 % nominal voltage range: 126 volts. Brief fluctuations are permissible beyond that range but we’ll ignore that. We’re talking consistent power here. We’ll assume you’ve managed to set up a compressor with a motor that will safely run off of a 125-volt, 20-amp electrical outlet on a 20 amp circuit.

In theory you would see approximately 3-1/3 hp.

125 x 20 = 2500 watts
2500/746 = 3.35 hp

Back to Reality

So these numbers we’ve come up with in our calculations above aren’t so bad right? We’re talking 2.4 to 3.3 horsepower! Not too shabby. The problem is you aren’t going to see anywhere near the numbers we calculated above in the real world. The calculations we just went through are all theoretical. They make plenty of unrealistic assumptions. You know what they say about assuming, right?

Our math above assumes we’re getting constant voltage and full power to a 100% efficient motor with no losses. This is never going to happen in real life. These are unrealistic expectations that you aren’t going to see in your garage or shop. You’re not going to see full voltage. The electric motors found bolted to an affordable air compressor aren’t even close to 100% efficient. You’re more likely to find 50 to 75 percent efficiency.

Universal and induction AC electric motors like the ones powering air compressors are a complex topic. They are not all built the same. There are a variety of factors to consider such as efficiency, motor service factors, motor rpm and much more. I’m not an electrical engineer so all of this is probably over simplified. That doesn’t change physics or what we see in real life. Practically speaking, when we factor in real world scenarios, a 15-amp compressor motor on a 120-volt outlet probably isn’t going to get you more than about 1.5 continuously. Less than 1-1/2 horsepower is probably a safer bet yet.

Under ideal conditions, in an unusual scenario where you’ve got a 20 amp motor with a 20-amp circuit and outlet, you might be able to run about a 2 horsepower compressor motor. Either one means anemic compressor output for abrasive blasting. How do I know? Airflow. Stay with me. We’re almost done.

So what’s the point?
Air Compressor Horsepower = Airflow

That brings us to our exciting conclusion. Why do we care about all of this horsepower nonsense anyway? In a round about way, horsepower means airflow. Airflow is what we care about most when it comes to abrasive blasting.

There is a general rule of thumb when it comes to true air compressor horsepower and airflow output:

This rule is the reason we care about air compressor motor horsepower in the first place. Horsepower relates to air flow. The more horsepower a motor provides, the more airflow the compressor should be able to provide.

Remember, we care about actual continuous air compressor horsepower here and not peak ratings. You aren’t going to be blasting for a few milliseconds so those peak horsepower ratings are pointless.

The general rule above is just a best guess of what you should expect to hopefully see. The horsepower rating of the electric motor, even if accurate, tells us nothing about the air compressor’s pump assembly. To know the true airflow output we would need to know a lot more about the compressor. That means the rated horsepower isn’t all that useful for sizing an air compressor. Knowing he air flow capacity or output rating is much more useful.

You Need More Than 120 Volts

Our horsepower discussion here is probably most beneficial for pointing out the electrical requirements for an air compressor that provides decent airflow. We’ve already said, under real world conditions a 120V/15A air compressor is going to have a motor that continually outputs around 1.5 hp at best. Consulting the chart above you’ll see a motor of this size typically only provides about 3-6 cubic feet per minute of airflow at 100 PSI. That isn’t enough for productive or continuous blasting.

We consider 10 CFM the absolute bare minimum and even then you’ll be wanting more airflow.
According to our rule above, air compressors best suited for an abrasive blast cabinet are going to have motors that continually produce 3 HP or more. That means a compressor with a motor running on a 240-volt setup.

Single Phase vs Three-Phase Power

You might encounter the terms “single phase” and “3-phase” when shopping for an air compressor. Most typical electrical service in the U.S. is single phase. You’ll often find 3 phase power in industrial settings. Why? Electric motors with higher horsepower usually require 3 phase power. Simply put, three phase power is generally better and more efficient when compared to single phase. Single phase power generally isn’t recommended for motors over 5 HP.

That doesn’t mean there aren’t larger horsepower air compressor motors that run on single phase. You’ll find single phase powered air compressors in the 5 to 7.5 horsepower range. However, single phase motors tend to be less efficient at higher horsepower ratings when compared to their 3 phase counterparts. Beyond 5 horsepower, 3 phase motors typically run more efficiently and more smoothly than single phase.

Gas Horsepower is Not the Same as Electrical

Many people without access to 240 volt electrical may turn to the world of gasoline powered air compressors. It is important to point out we’re talking about electrical motor horsepower here. Gas powered horsepower is a different animal entirely. Generally speaking, it takes double the horsepower for a gas compressor to do the same amount of work. Fore example if a 3 hp electric motor might gets you 9-12 cfm, you’d need around 6 hp gasoline powered motor for the same output.

Conclusion:

In conclusion, air compressor horsepower alone isn’t a very useful specification when shopping for a compressor. Peak horsepower ratings are even worse and are essentially useless. I’ll leave you with a summary of the main points. Here’s what you should take away from this post:

• Horsepower ratings give us a general idea of how much airflow output to expect.
• Typically, you can expect 3-4 cfm of airflow for every real horsepower.
• A 120V 15 or 20 amp electrical outlet is only going to power around 1.5 hp or less.
• For air compressors making 10 cfm or more you’ll need a real horsepower rating of around 3 hp  or better. That means a 230v compressor on a 240-volt circuit.

Need help choosing a blast cabinet air compressor?

This post is part of a series of posts to help you better understand air compressors and how they relate to abrasive blast cabinets. Choosing a proper air compressor is probably just as important as what blast equipment you choose. We’re here to help! Be sure to check out our blast cabinet air compressor buying guide. It features 10 key points to consider when choosing the best air compressor for your blast cabinet need. It is a great primer to get you started and will walk you through what specs to look for, what sort of budget you’ll need and more.

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