Do I really need a 7.5 hp compressor?

Kaizen · Jan 22, 2019

Presently have a 11 cfm husky 60 gallon unit that seriously held me up doing autobody stuff. Even a die grinder or body saw would get ten minutes of work only. Want to get a new one for new garage and thought I’d be happy with 5hp two stage 60 gal for about 1000-1500. But not really impressed by the cfm at 18. Will that keep up with my tools? Just me but also have a blast cabinet. Like the 7.5 hp cfms around 22 more but not the 2500 dollar tag.
Also some of this size comes with a magnetic starter single phase. Is this like when you run a 3 phase on single phase system?

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rlitman · Jan 22, 2019

I'm very happy with my 7.5HP, but I think that 18CFM would have been enough (just barely) for my purposes. I bought mine used from a body shop that had two guys working DA sanders with it.

The magnetic starter is so that the motor current doesn't destroy your pressure switch. It has nothing to do with phases. Ideally, you want a magnetic starter on a 5HP too, though since the pressure switches are often rated for 5HP, many makers skip this.

coljar · Jan 22, 2019

How often will you be needing that much cfm? I do at times, but not often enough to justify a 7.5. In my case, I do have the option of tying the compressor in the building next door in, but have rarely needed to.

Alexander · Jan 22, 2019

My shop used a 60gal $1000ish IR air compressor for about a decade, and it honestly kept up surprisingly well.

My take is it should keep up with you, but I'm not an expert as such. See below.

On and off, we use sanders, die grinders, blowers, nailers, hvlps, a demanding plastic spray system, a really demanding cnc router table, and have a multitude of air leaks throughout our system. Usually one, sometimes up to three high demand people at once. To be fair, nobody ever kept track of actual usage, but that system kept up with us pretty well ... until it eventually overheated for the last time because we were working it way too hard.

After it proved we were overdoing it, we moved that compressor to our backup building and bought an IR scroll air compressor that has been golden ever since. It's a UP6-7.5TAS-15W/D for those playing the home game.

The benefits include it being a reasonable noise level, and being able to keep up with anything we have thrown at it thus far. Downsides being that it's expensive, but I'm not the shop owner so I didn't pay for it.

Did I mention it's quiet?

71goldss · Jan 22, 2019

I can run my Dynabrade DA all day long with my 5hp 2-stage 60 gal Quincy QT-54. Never have I had to stop and let the compressor catch up. I don’t have a blast cabinet, so I can’t speak of how it would do with that.

diesel_dan · Jan 22, 2019

How patient are you? Ton of fabulous compressors come up for sale all the time. Used.

Some barely used and some abused... If you have the time to get educated on what is good, bad or GREAT, and also on how to tell if a compressor is what it's advertised to be...

Find a good used one, need more air? get a second good used one, or add more receivers (tanks) to the Hoss you bought the 1st time...

If your needs are immediate, open your wallet, WIDE...

sberry · Jan 22, 2019

5 HP runs the stuff.

navycryppie · Jan 22, 2019

Please define this unusual word "need". :-D

lbhsbz · Jan 22, 2019

I have a 5hp 2stage 80 gallon ingersoll and it will pretty much run whatever I want for as long as I want to. Sandblasting is no problem, DA sanders and die grinders are no problem. The only thing I have to remember is to occasionally let the compressor cool down. I was sandblasting a pile of parts for several hours and the compressor tank ended up almost too hot to touch.

thecj3man · Jan 22, 2019

I love my 707 Saylor Beall. It’s honestly more than I need 90% of the time, but those few times you need to use some air it is worth it.

navycryppie · Jan 22, 2019

Seriously though, I do have a 7.5HP 80 gallon Champion (3 phase). Picked it up off CL, and had already planned to run a phase converter in my shop (Miller CP-300). If you already have 3ph, deals on industrial compressors can be found, but it's probably not worth the added expense to put it in otherwise. There are 7.5 single phase Champions out there, but even more $. And single phase motors that big have HUGE startup currents, especially on a compressor.

Need? No. But opportunity knocked and it's always nice to have an abundance of any resource in the shop.

diesel_dan · Jan 22, 2019

thecj3man said:
I love my 707 Saylor Beall. It’s honestly more than I need 90% of the time, but those few times you need to use some air it is worth it.

****! 90% of the time???? If I had a 707 I'd run it slower and be happy with like 25CFM for me and all my neighbors...! :-D

Steve_P · Jan 22, 2019

A true 5 HP like a Quincy is enough unless you're pressure blasting. Then the 7.5 hp wins out. A good 5 hp is fine with a suction feed blast cabinet with a typical nozzle. Everything else is no issue but pressure blasting uses lots of air

Kev442 · Jan 22, 2019

5 HP 80 gallon here. Can count on one hand the amount of times I've had to wait a minute or two for it to catch up while blasting. Usually you have to stop the gun to reposition before running out of air.
When I first installed it, I was using the airgun to blow out the garage. When it caught up and shutoff while I was using it continuously, I knew I would be fine.

sberry · Jan 22, 2019

This is what they invented the 5 for. It became popular with the advent of the cut off tool and body work and when 100A service become normal. 3 used to be common for small garages with 60A, to run mechanic tools and for tire work including air in to pressure truck tires. They were 2 stage, but were a little small for continious sanding.

AreYaSerious · Jan 22, 2019

As many has already said, it depends on what your needs are.

Kaizen · Jan 22, 2019

diesel_dan said:
How patient are you? Ton of fabulous compressors come up for sale all the time. Used.

Some barely used and some abused... If you have the time to get educated on what is good, bad or GREAT, and also on how to tell if a compressor is what it's advertised to be...

Find a good used one, need more air? get a second good used one, or add more receivers (tanks) to the Hoss you bought the 1st time...

If your needs are immediate, open your wallet, WIDE...

I don’t have 3 phase and don’t want to deal with trying to make it work. I e been watching for deals around me in this realm and people are asking ridiculous prices for ten year old stuff. About the same price as new so I will just be going new

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TheLurker · Jan 22, 2019

If you have to ask...

you need it.

Kaizen · Jan 22, 2019

Thanks everyone. Sounds like the five with an 80 will suffice. Planning on boxing this in for noise reduction and had to figure this out so I can plan power. Thanks again

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Kaizen · Jan 22, 2019

TheLurker said:
If you have to ask...

you need it.

I expected far more answers like this. Trying to moderate so I can get a lift too!!!

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larryq · Jan 22, 2019

Kaizen said:
I expected far more answers like this. Trying to moderate so I can get a lift too!!!

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I think you'll be just fine with a 5 HP. 18 cfm is a lot of air and unless you're running a shop it's hard to see how it won't keep up with you, except on those rare occasions when you're using the blast cabinet with the trigger pulled for 30 min straight.

matt_i · Jan 23, 2019

My 5hp T30 compressor can *just keep up* (meaning motor running constantly with about a 20 second shutdown every 5-ish minutes) with my sandblast cabinet with the trigger pulled constantly. I can't quote nozzle sizes here, its a Trinco, its been too long since I checked or cared. I haven't done this often because I feel its abusive to the compressor and the wallet when the electric bill arrives (!)

I don't have any tools that use more air than that, I would guess a 7.5 would be better suited for multi-users or bigger blasting operations.

Jazz1 · Jan 23, 2019

I have a RK (Made in Canada)5hp 3 cylinder 18cfm @90 and suits me fine. I sandblast and no problem keeping pressure pot full.

sberry · Jan 23, 2019

matt_i said:
My 5hp T30 compressor can *just keep up* (meaning motor running constantly with about a 20 second shutdown every 5-ish minutes) with my sandblast cabinet with the trigger pulled constantly. I can't quote nozzle sizes here, its a Trinco, its been too long since I checked or cared. I haven't done this often because I feel its abusive to the compressor and the wallet when the electric bill arrives (!)

I don't have any tools that use more air than that, I would guess a 7.5 would be better suited for multi-users or bigger blasting operations.

I would size it up so if i was blasting steady the comp wasnt shutting off.

pcmeiners · Jan 23, 2019

5hp is sufficient for a shop, 7.5 is more then adequate without a blast cabinet... with a blast cabinet depends on the cfms needed.

Three phase is not is not what designates a magnetic starter, basically it is the current draw on single or mutiphase . A 5hp can run on a simple pressure switch, a magnetic starter would be better. Once you go to >5 hp, a magnetic starter is necessary. Magnetic starters generally have over current protection, a definite plus, the newest starters have electronic overload protection (even better).

As other mentions, used compressors can be a great buy... again, if your patient. I purchased a Quincy 325 pressure lubed unit, with about 25 hours on it for $1600 (good price for NYC)

rlitman · Jan 23, 2019

sberry said:
This is what they invented the 5 for. It became popular with the advent of the cut off tool and body work and when 100A service become normal. 3 used to be common for small garages with 60A, to run mechanic tools and for tire work including air in to pressure truck tires. They were 2 stage, but were a little small for continious sanding.

5 is kind of the "goldilocks" size for all the reasons you stated.

Kaizen · Jan 23, 2019

rlitman said:
5 is kind of the "goldilocks" size for all the reasons you stated.

Shopping around looking at belaire and IR and a decent five hp is only a couple hundred less then a 7.5. Both pretty comparable with cast iron cylinders. Confirmed I can get away without the big boy but for another three hundred I might get the last compressor I will ever buy.

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Kaizen · Jan 23, 2019

And what’s up with the voltages? Isn’t two hits of 120 = 220? Seeing 230 and 208 volts as choices?

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rlitman · Jan 23, 2019

Kaizen said:
Shopping around looking at belaire and IR and a decent five hp is only a couple hundred less then a 7.5. Both pretty comparable with cast iron cylinders. Confirmed I can get away without the big boy but for another three hundred I might get the last compressor I will ever buy.

Agreed. The compressors sold in 7.5HP models will generally be really good, and the 5HP versions of those premium models will not be much cheaper.

Kaizen said:
And what’s up with the voltages? Isn’t two hits of 120 = 220? Seeing 230 and 208 volts as choices?

FYI, 120+120=240. 208V is for three phase. The single phase motor will be rated at 230V, even though the electrical system is nominally 240V. And to really drive you nuts, the plug will say 250V, and the wires will either be 300V or 600V.

epmills · Jan 23, 2019

Not sure how much of a deal this is, but 5hp and 80 gallon. https://providence.craigslist.org/tls/d/shop-air-compressor-quincy/6788836359.html

Looks like 17.7CFM @90PSI per some googling.

DerekV · Jan 23, 2019

rlitman said:
208V is for three phase.

This is not always the case. 208V single phase is a real thing.

rlitman · Jan 23, 2019

DerekV said:
This is not always the case. 208V single phase is a real thing.

Real, as in anyone can always tap two poles from a three phase system. 208V "single" phase is still 120 degrees apart between the phases. Not like the 180 degrees in a true single phase system.

The Tool Tyrant · Jan 23, 2019

Another train of thought...

I am an avid believer in 'MORE STORAGE' rather than 'MORE HP'. I have a 5HP Quincy QR-325 in my five man fab / machine shop and it supplies ample air and in fact in the 15 years I've run it, we have never had to wait for air. :rocker:

I mounted the motor/ pump on a 120 gal. receiver, with a 240 gal. piggyback receiver mounted next to it for a total of 360 gallons. I have the pressure switch set to on @ 140 / off @ 175.

Another bonus is the fact that the motor cycles MUCH less frequently which adds to the life span of the motor.

Think of it this way...would you rather sit down with an 8 oz. beer or a 32 oz. beer? You're going to have to get up & down 3 more times to fill-up your 8 oz. glass, while I sit and relax with my 32 oz. mug! :beer:

DerekV · Jan 23, 2019

rlitman said:
Real, as in anyone can always tap two poles from a three phase system. 208V "single" phase is still 120 degrees apart between the phases. Not like the 180 degrees in a true single phase system.

Real as in some places measure 208v leg to leg rather than 240v, a very real and "true" (not sure what you meant by that) single phase. There is no need to get into the math on how voltage is transformed. Unless it's some kind of residential DG power source, all single phase is 3 phase somewhere upstream, whether that's transformed on a pole down the street or in a maintenance room inside your building. Doesn't matter. What matters is the service that is most-readily available to you that's suitable to get the job done.

A few years ago, friend of mine leased a large room in a commercial building for his business. There were several small businesses occupying this building in the same manner, each having their own room(s). Every floor of the building had a utility room with a transformer and service panel. It was all 3 phase, but very few occupants had 3 phase running to their spaces. Most just had two of the three legs - leg to neutral was 120, leg to leg was 208. Any electrical additions or modifications were the responsibility of the tenants, so unless there was a need to run 3 phase, it wouldn't be the defacto standard option. There are lots of motors, welders, etc. that are designed to operate specifically on 208v single phase.

Kaizen · Jan 23, 2019

The Tool Tyrant said:
Another train of thought...

I am an avid believer in 'MORE STORAGE' rather than 'MORE HP'. I have a 5HP Quincy QR-325 in my five man fab / machine shop and it supplies ample air and in fact in the 15 years I've run it, we have never had to wait for air.

I mounted the motor/ pump on a 120 gal. receiver, with a 240 gal. piggyback receiver mounted next to it for a total of 360 gallons. I have the pressure switch set to on @ 140 / off @ 175.

Another bonus is the fact that the motor cycles MUCH less frequently which adds to the life span of the motor.

Think of it this way...would you rather sit down with an 8 oz. beer or a 32 oz. beer? You're going to have to get up & down 3 more times to fill-up your 8 oz. glass, while I sit and relax with my 32 oz. mug!

Yes still trying to decide if i want to add my current 60 to the new one for more storage. Its all inside the garage so it eats floorspace.

Kaizen · Jan 23, 2019

epmills said:
Not sure how much of a deal this is, but 5hp and 80 gallon. https://providence.craigslist.org/tls/d/shop-air-compressor-quincy/6788836359.html

Looks like 17.7CFM @90PSI per some googling.

Yea looks like a 2500 dollar unit new. I'm just not trusting enough. I know enough about car engines to make most sound great till the next oil change. Figure compressors are the same way. even if its perfect now who knows what the guy did to it. run it low on oil? overheat it? if it was 500 i might take the chance but i've seen too many videos of "bargains" where they have to get a new motor or new pump and it ends up a wash.

dnschmidt · Jan 23, 2019

Short answer: YES

There are always work arounds but they are work arounds. Get the biggest compressor you can and you'll be happy for the rest of your life. Get a smaller one and you'll be bitchin' for the rest of your life.

pcmeiners · Jan 23, 2019

I don't consider it such a hot deal, this is a lower end Quincy splash lube unit. With patience you can get a QR 325 ... large difference in build quality between the two models.

If you want to consider the QT 5, there is so little time on it (just by looks), you should not have an issue.

joe_padavano · Jan 24, 2019

I do media blasting, use a lot of air tools, and use HVLP spray guns. Upgrading to a 23 SCFM compressor was the best move I ever made. Get a used three phase compressor and replace the motor with a single phase. I got a new 7.5 HP Leeson on line for about $500. Used 3 ph compressors can be had in the $600 range.

Citation · Jan 24, 2019

My father did a lot of body shop type work, DA sanding, painting and had a blast cabinet. He had no issues with an 80 gallon, 5hp Champion (iirc) compressor. He mentioned that the sand blaster gave it a good workout but it was sufficient.

Reading through this thread I think 7.5 is overkill for your needs. However, the cost difference between a nicer 5hp (one with a mag starter) and a similar compressor with a 7.5hp motor is small. So long as you have the power capacity for it, why not?

If you are cheap like me then I would consider some alternatives.
1. Wait for a good deal on a used 5-7.5 hp compressor. You could probably find one under $1000.

2. If you have space, add a second compressor and tie them together. A second 3hp compressor would give you 20+ CFM total and double your tank size. If you found a deal on a used 5hp model you would be around 25cfm. This is the cheapest way to get more flow.

Tying to compressors together is easy but you need to do a few things. First, the cut in/cut out pressures need to be acceptable for both compressors. So if one is 135psi max and the other is 175 you may have to change the pressure switch settings. Also, for safety I would use the same psi blow-off valves in each compressor. A 175 psi unit might have a 200psi valve while the 135 unit has a 150 valve. In theory, setting the cut in/out pressure lower solves this problem but it's better to make sure both valves are set to the limit of the lower pressure compressor.

The cut in pressures should be slightly staggered so the two don't start at the same time. That means under light use one pump may do all the refill work. Under heavier use one then the other pump kicks in. Where shutting off you get similar behavior.

Do I really need a 7.5 hp compressor?

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