Is 7.5hp Single Phase a bad idea?

[Grandmaster]

I am shopping for what will hopefully be my last air compressor. Talking to some of the shops around town, it seems like a single phase 7.5hp motor might not be as dependable as the 5hp. Some dealers said there were problems in the past but they have been worked out...others say they "just had to replace a motor last week".

The cause according to them is that a 7.5hp single phase just pulls too many amps and either the magnetic starter or the motor burns out.

Seems to me there are a lot of 7.5hp single phase offerings out there, and if there were that many issues companies would not be offering them due to warranty liability.

Anybody else hear the same thing? Is it a bunch of BS?

Thanks,
Louis

 

[dnschmidt]

If you're using the proper wire gauge (normally 8 gauge for 40 amps. I used 6 gauge for mine since I'm a firm believer that more is better) there should be no issue. If you're trying to cheap out with the thinner wire it will have significant voltage drop and then the motor will burn out.

 

[American Locomotive]

Considering single phase motors are offered in sizes up to 15-20 HP, I can't see how a 7.5HP motor would be an issue at all.

As long as your wiring and motor-starter are properly sized for the currents involved, there should be no problems at all.

 

[ttpete]

Are you looking at motor HP or advertised HP? They can be very different. The best way is to look at the motor data plate for amperage draw. That's the factor to consider.

 

[md21722]

7.5HP calls for #6 NM-b (Romex) or #8 THHN (conduit).

My local dealer does not recommend a 10 HP single phase and basically won't sell them, but has no problem selling 7.5 HP single phase. Apparently they've had enough warranty issues with the 10 HP single phase they don't want to get their hands in it.

 

[Packard V8]

The problem is most real 7.5 hp air compressors are three phase. When walking past the big-box-store junk, their 7.5 hp rated single-phase motors are often about the size of a large loaf of bread and have two capacitors. Anyone who has any working relationship with electric motors can take one look and know those tiny sheet tin frame motors can't survive developing 7.5 hp on compressor duty.

If one must go single phase, in the past, the safest course was to stay away from the big box junk and go with a company which also makes industrial compressors. The problem today is some formerly dependable industrial compressor companies have begun whoring out the brand and putting their name on consumer-grade junk.

Bottom line - figure a dependable 7.5hp compressor is going to cost twice what the cheapo models are being advertised.

BTW, larger than 10hp single phase motors are very rare, very expensive and the power company won't like you using them. The reason that electric utilities cannot serve larger motors from single-phase lines comes from the high start-up current required by motors. For anything larger than 10 Hp on most lines, the initial starting current causes such a voltage drop there are blinking problems (voltage sags) for other customers on the line. Today's electronics are seriously affected by such voltage variations.

jack vines

 

[sberry]

The above is pretty accurate.

 

[helterskelter]

I've got a 7.5hp speedaire compressor that has been flawless. I've only had it for a year though, so long term durability isn't known. It's on a short run (~10 feet) of #6 romex. Really happy I went with the largest (in terms of CFM) I could afford and power reasonably.

 

[bastage]

I think what was said about properly sizing the wires is the ticket.. I have #6 ran in conduit for my 6hp compressor (all of like an 8 foot run from the breaker to the outlet where I used a 50a plug so I can move the compressor if needed without messing with the wiring). Complete overkill, but I dont have to worry about the voltage drop at all

 

[slodat]

bastage-- the code commandos will be quick to point out that unless you have the very expensive pin and sleeve style plug, you can't use a plug with that large of motor. The devices have to be rated for the motor HP to be used with motors..

I have a true 7.5HP single phase compressor (with a 15HP pump spinning half speed), powered with 8ga THHN, per manufacturer's specifications. It's a great machine and I expect it will last the rest of my life..



A quick tell on a 7.5hp single phase motor is all of th I have seen have lifting eyes because they weigh 100+ pounds.

 

[braidmeister]

My Ingersoll is 7.5 1ph. I've got 3ph, but the guy talked me out of it in case I ever wanted to sell it it would move faster than the 3ph one would.

I leave it on 24/7 with the auto drain. Kicks on a few times a day for 30 sec to make up air loss from the auto drain. Works flawlessly. I'd expect no less from Ingersoll...and a $2500 compressor.

 

[Grandmaster]

One of the shops I talked to said the problem was that the 7.5hp uses the same magnetic starter as the 5hp and the 7.5hp amp draw would just burn out the contactors in it. He said he would order me a 7.5hp on his next order if I really wanted him too, but that he strongly recommended against it based off of his experience with the 7.5hp motors.

I am looking at Champion and Saylor Beall. Baldor, Leeson and Marathon motors seem to be the motors that come on them. They are all true 7.5hp motors per the nameplate. I have a 100amp subpanel coming off of my 200amp service. The main service is on the exterior wall. The subpanel is on the interior wall opposite of the main panel, and is currently empty. So maybe 3 feet of #1 copper powering the sub panel. The compressor will be placed within about 3 feet of the subpanel. My plan was to run either 6/3 SJOOW or #6 THHN though flexible conduit straight to a 40 or 50 amp breaker. So probably 6-10 feet from the compressor to subpanel. I have 242 volts across both legs, and with my wiring and lengths should have minimal voltage drop.

I am not an electrician but I think I should be supplying the compressor with plenty of power. Am I on the right path?

 

[braidmeister]

One of the shops I talked to said the problem was that the 7.5hp uses the same magnetic starter as the 5hp and the 7.5hp amp draw would just burn out the contactors in it. He said he would order me a 7.5hp on his next order if I really wanted him too, but that he strongly recommended against it based off of his experience with the 7.5hp motors.

I am looking at Champion and Saylor Beall. Baldor, Leeson and Marathon motors seem to be the motors that come on them. They are all true 7.5hp motors per the nameplate. I have a 100amp subpanel coming off of my 200amp service. The main service is on the exterior wall. The subpanel is on the interior wall opposite of the main panel, and is currently empty. So maybe 3 feet of #1 copper powering the sub panel. The compressor will be placed within about 3 feet of the subpanel. My plan was to run either 6/3 SJOOW or #6 THHN though flexible conduit straight to a 40 or 50 amp breaker. So probably 6-10 feet from the compressor to subpanel. I have 242 volts across both legs, and with my wiring and lengths should have minimal voltage drop.

I am not an electrician but I think I should be supplying the compressor with plenty of power. Am I on the right path?

Yup. Sounds exactly like my setup nearly word for word except I have 400A with a 100A single phase sub panel. Compressor also like 3' from the panel, but in reality it's more like 20 feet or so because it goes up, over and down the ceiling/walls. I'm running a 40A breaker on mine. I think it has a Leeson on it. Mine's been running 5yrs like this 24/7. No starter issues.

 

[malibu101]

bastage-- the code commandos will be quick to point out that unless you have the very expensive pin and sleeve style plug, you can't use a plug with that large of motor. The devices have to be rated for the motor HP to be used with motors..

I have a true 7.5HP single phase compressor (with a 15HP pump spinning half speed), powered with 8ga THHN, per manufacturer's specifications. It's a great machine and I expect it will last the rest of my life..



A quick tell on a 7.5hp single phase motor is all of th I have seen have lifting eyes because they weigh 100+ pounds.

What size tank is that?
That is the largest compressor on the smallest tank I've ever seen.

 

[slodat]

80 gallon tank.


Sent from my iPhone using Tapatalk

 

[sberry]

A little voltage drop isn't hurting these units.

 

[md21722]

A little voltage drop isn't hurting these units.

I agree, after 50' you need to start getting concerned. I run my 5HP Champion R-15's off 50' of #10 and get little to no noticeable voltage drop. #10 is allowed by code in my setup.

 

[sberry]

We had a thread here where someone came along and asked, is this wire right? His dad got an "electrician from work" to wire it up 18 years ago, number 12 to a 7.5. Worked fine, something eventually went wrong with the pump.

 

[matt_i]

You can probably pull 50A thru a #12 but it would melt the insulation off.

As others have said if you wire #8 THHN or #6 NM-b at a reasonable distance then I wouldn't worry. A contactor is a thing you can easily replace or upgrade if it ever goes bad.

 

[dnschmidt]

Why would anybody ever use an undersized wire? Screw that cheap $2,500.00 compressor how about my $400,000 house. The insurance company sees that you cheated on the wire and you're on your own buddy. People amaze me.

 

[malibu101]

80 gallon tank.


Sent from my iPhone using Tapatalk

Something must get lost in the picture.
No offense, but it looks stubby.
I have a 5HP R15 pump Champion with a 80gal circa 2003 and it seems to be a quite a bit taller than your pic.
Mine from floor to highest point on pump (filter) is 75"

 

[slodat]

It's short and squatty by design because of the size of the pump and motor.


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[sberry]

The guy didn't know any better. But it was about 35 A or so ins likely not continious. There are cases a 12 can run on a 50A circuit, not legal for that motor but for limited time won't burn the insulation off and the reason you got insurance is in case you do something stupid.

 

[larry4406]

I have a 7.5 Hp Puma 3 cylinder compressor with a Baldor motor. Been working fine for me.

 

[md21722]

One of the shops I talked to said the problem was that the 7.5hp uses the same magnetic starter as the 5hp and the 7.5hp amp draw would just burn out the contactors in it. He said he would order me a 7.5hp on his next order if I really wanted him too, but that he strongly recommended against it based off of his experience with the 7.5hp motors.

I am looking at Champion and Saylor Beall. Baldor, Leeson and Marathon motors seem to be the motors that come on them. They are all true 7.5hp motors per the nameplate. I have a 100amp subpanel coming off of my 200amp service. The main service is on the exterior wall. The subpanel is on the interior wall opposite of the main panel, and is currently empty. So maybe 3 feet of #1 copper powering the sub panel. The compressor will be placed within about 3 feet of the subpanel. My plan was to run either 6/3 SJOOW or #6 THHN though flexible conduit straight to a 40 or 50 amp breaker. So probably 6-10 feet from the compressor to subpanel. I have 242 volts across both legs, and with my wiring and lengths should have minimal voltage drop.

I am not an electrician but I think I should be supplying the compressor with plenty of power. Am I on the right path?

The #6 you are using is oversized, so it's fine.

For motor circuits, by code, you look at the nameplate for the HP rating and then follow the charts for current draw and wire sizing. 7.5 HP single per the charts is 40A FLC. Must be wired to 125%, so 40A X 125% = 50A. That's where the #6 NM-b (Romex) or #6 THHN (in conduit) comes in. The difference there is that Romex insulation is considered to be rated for 60C and THHN is good for 75C. Circuit breaker can be maximum 250% of FLC, so 40A x 250% = 100A to prevent nuisance trips since the motor or motor starter have thermal protection. In reality a 40-50A breaker should be fine. After 60A they get expensive.

A 7.5 HP single phase will draw 160-200A instantaneously on startup but the wire is not sized that way. The NEC is more concerned about improper wiring starting fires and whatever special interests they succumb to, like the manufacturers that want to sell you AFCI breakers or tamper resistant outlets.

 

[beakie]

The guy didn't know any better. But it was about 35 A or so ins likely not continious. There are cases a 12 can run on a 50A circuit, not legal for that motor but for limited time won't burn the insulation off and the reason you got insurance is in case you do something stupid.

no, that's the reason rates go up.

insurance is for unpredictable, unforseeable, unpreventable incidents... being and idiot isn't either of those. willful negligence/ignorance can be prevented.

 

[sberry]

No one is saying That it is a good idea but the blanket statement that you are on your own if you use the wrong wire is not accurate.
Second, lots of reasons places burn down, mostly human error, have never heard of a claim being denied from this and never heard of a fire caused by an under wired compressor or welder.
Insurance companies cover old ****, they cover electric heaters, they actually insure **** that should be condemed all the time.
Millions of faulty cars insured for liability every day. Millions of tires on the road shouldn't be there.

 

[Grandmaster]

The #6 you are using is oversized, so it's fine.

For motor circuits, by code, you look at the nameplate for the HP rating and then follow the charts for current draw and wire sizing. 7.5 HP single per the charts is 40A FLC. Must be wired to 125%, so 40A X 125% = 50A. That's where the #6 NM-b (Romex) or #6 THHN (in conduit) comes in. The difference there is that Romex insulation is considered to be rated for 60C and THHN is good for 75C. Circuit breaker can be maximum 250% of FLC, so 40A x 250% = 100A to prevent nuisance trips since the motor or motor starter have thermal protection. In reality a 40-50A breaker should be fine. After 60A they get expensive.

A 7.5 HP single phase will draw 160-200A instantaneously on startup but the wire is not sized that way. The NEC is more concerned about improper wiring starting fires and whatever special interests they succumb to, like the manufacturers that want to sell you AFCI breakers or tamper resistant outlets.

Those AFCI breakers are annoying but not nearly as annoying as those damn tamper resistant outlets. My new home has both and I am half tempted to replace them all. Supposedly if you insert all the prongs into the outlet evenly they are supposed to insert easily...apparently some require a lot more precision than I am able acheive!

I appreciate all the responses. I felt pretty confident that I had a decent handle on the circuit wiring and sizing.

I was more concerned that maybe there was something inherently wrong with 7.5hp motors that made them less dependable long term than a 5hp.

Thanks Guys, I feel more confident about getting a 7.5hp air compressor.

 

[sberry]

Yes, get the one you want or need. A bigger unit likely won't run as much and there is less temptation to want to cobble additional equipment on to make up for size. If this is a home/hobby environment or mechanics shop it won't get worn out anyway and if it really does repair is moot.

 

[theoldwizard1]

We had a thread here where someone came along and asked, is this wire right? His dad got an "electrician from work" to wire it up 18 years ago, number 12 to a 7.5. Worked fine, something eventually went wrong with the pump.

Likely because of duty cycle. Most compressor spend much more than 50% of the time powered OFF even when air is being used. ON time is typically only a few minutes, compared to OFF time, so overheating the wire is minimal.

 

[theoldwizard1]

It is clear that 3 phase power is much more efficient for running a motor. Any one have a feel for how much more "efficient" in terms of dollars and cents !

 

[bastage]

bastage-- the code commandos will be quick to point out that unless you have the very expensive pin and sleeve style plug, you can't use a plug with that large of motor. The devices have to be rated for the motor HP to be used with motors..

Well the code commandos can point out whatever they want. Doesn't change the fact that it's a 24a motor on a 50a plug and the point of code is safety and with as oversized as I made the plug and all the wiring back to the main panel I am in no danger at all of any of it melting and passing a safety risk. Also the electrician that did the work didn't even hesitate when I asked if it was ok to do it this way before he started working (not a hack job electrician either, but he is family so if there was a code issue he would have already known I wouldn't care unless it posed a safety risk).


On another note.. that champion you have there is one fine looking compressor and I love the oversized pump at half speed. I imagine it's pretty quiet for a air compressor too.

 

[sberry]

It is clear that 3 phase power is much more efficient for running a motor. Any one have a feel for how much more "efficient" in terms of dollars and cents !

Not much.

 

[ddawg16]

One of the biggest mistakes people make when selecting magnetic starters and disconnects, they look at the FLA instead of the HP rating. Just because a stater might be rated for the FLA, it may not be rated for the HP

 

[American Locomotive]

Not much.

Depends on the motor size and horsepower. 3-phase motors tend to be ~5% more efficient at the higher horsepower (say over 1 HP) level. However the difference grows in the fractional horsepower level.

 

[theoldwizard1]

Depends on the motor size and horsepower. 3-phase motors tend to be ~5% more efficient at the higher horsepower (say over 1 HP) level. However the difference grows in the fractional horsepower level.

I know most commercial HVAC blower are 3 phase. Same with blowers in large computers. These are not high horsepower motor, but the have a high duty cycle. Computer blower obviously 100%.

My buddy is a field service tech from back then. He said the installation instruction always said to check the fan rotation to make sure it was blowing and not sucking ! (Easy fix.)

 

[dnschmidt]

I would hope that the manufacturer of the air compressor would have somebody on staff that could properly size the magnetic starter. That's their job why should we, who buy their products, have to consider such things. Seems ridiculous

 

[manwithtools]

Well the code commandos can point out whatever they want. Doesn't change the fact that it's a 24a motor on a 50a plug and the point of code is safety and with as oversized as I made the plug and all the wiring back to the main panel I am in no danger at all of any of it melting and passing a safety risk.

Maybe not a problem for you , but when someone else decides to "unplug" the compressor while it's running (or worse yet, starting) - that's when the problem occurs - commonly referred to as arc flash.

There's usually a reason for code requirements, even though they sometimes seem to cover the remotest of possibilities. While there are some code requirements that are lobbied into existence by the manufacturers, this is not one of them.

 

[manwithtools]

One of the biggest mistakes people make when selecting magnetic starters and disconnects, they look at the FLA instead of the HP rating. Just because a stater might be rated for the FLA, it may not be rated for the HP

I'm confused? If you chose a control device by the FLA (NEC Tables 430-148, 149, and 150) and voltage for the motor it controls, you have specified it for the motor HP by design.

 

[manwithtools]

I would hope that the manufacturer of the air compressor would have somebody on staff that could properly size the magnetic starter. That's their job why should we, who buy their products, have to consider such things. Seems ridiculous

Agreed, they are not using a 5hp contactor on 7.5hp motors as was mentioned. Find another shop to buy your compressor from. There's no inherent difference in 5hp or 7.5hp single phase motor if you have adequate supply for both. Somebody's blowing smoke.