Electric Requirements for a New Compressor

[Evan(CA)]

There is no NEC allowance to use the next smaller size conductor for the motor supply conductors if the calculated amp size is not listed in the tables as is the case for 60C temp for #10 which is 30A, not 35A. Based on needing 35A you will need to use either #8 NM-b at 40A(60C) or #10 THHN at 35A(75C). If the devices in the panel are rated only at 60C then you use #8 at 60C.

I emailed the instructor at electrician math and got this reply

"Howdy. Thanks for your note. You are right to question the article number for
this claim. I went looking in about the 2002 NEC when this material was
generated. No success. You might want to check earlier editions of the NEC. I
would not have included that note if it was not a part of the code at the time
the material was written and typed. That could be 1999 or so. But with limited
time I can only rely on my memory for now. If I find the citation, I'll let you
know.

Anyway, there was a time when this was the approved calculations technique. No
more.

Meantime I have corrected the webpage by deleting that short paragraph.

Thanks for pointing out the error. I depend on my students in the classes I
instruct to do the same. And for sure the branch circuit conductor has to be at
least 125% of the motor FLC from the table for a standard motor. When it comes
to calculating other types of motors with the duty designation, then often there
is some different aspect to incorporate.

Cheers:>) David U. Larson "

So you are correct but do you recall this ever being code compliant in the past as he suggests or is he covering his ***?

 

[wyliesdiesels]

What about motors with rating "SPL"?

With an SPL rated motor, u look at the FLA rating on the compressor and then look at the NEC FLC tables and see how the nameplate FLA fits in with the table. So for example
Motor says SPL with an FLA of 16a. 3HP is listed as 17a FLC. So then u would wire the circuit based on a 3HP motor FLC rating.

I emailed the instructor at electrician math and got this reply

"Howdy. Thanks for your note. You are right to question the article number for
this claim. I went looking in about the 2002 NEC when this material was
generated. No success. You might want to check earlier editions of the NEC. I
would not have included that note if it was not a part of the code at the time
the material was written and typed. That could be 1999 or so. But with limited
time I can only rely on my memory for now. If I find the citation, I'll let you
know.

Anyway, there was a time when this was the approved calculations technique. No
more.

Meantime I have corrected the webpage by deleting that short paragraph.

Thanks for pointing out the error. I depend on my students in the classes I
instruct to do the same. And for sure the branch circuit conductor has to be at
least 125% of the motor FLC from the table for a standard motor. When it comes
to calculating other types of motors with the duty designation, then often there
is some different aspect to incorporate.

Cheers:>) David U. Larson "

So you are correct but do you recall this ever being code compliant in the past as he suggests or is he covering his ***?

I was gonna say that that code must have been long before i got into the trade....never heard of it.

Norcal or Speedy pete should be able to answer whether this was code back then.

This is just the reason why googling codes and then coming up with sites like his with no code reference is dangerous. I wonder how many people found his site and took it as current code?

 

[pattenp]

I'm no spring chicken and I honestly can't remember there being an exception that allows single motor supply conductors with an amp rating less than 125% of the FLC to be used.

 

[Chasbec]

Well, I did it! I bought the compressor and got it home last night. 80 gallon, 5 HP, 2 stage. Thanks to you guys for helping me sort through my electrical questions! Now to get it hooked up and running! I know a lot of people here are not big fans of Ingersol Rand, but, being on sale (at Tractor Supply), the price was right and, with the 4 year extended warranty and 1 year no interest to pay it off, I feel like I got a good deal. Now I need to get set up for sandblasting, the main reason I wanted to get a bigger compressor. Here's a pic of us unloading it last night...View media item 52522

 

[wyliesdiesels]

Nice! Thats a monster!

Is that a massie ferguson backhoe?

 

[Chasbec]

Nice! Thats a monster!

Is that a massie ferguson backhoe?

It's a '76 Allis Chalmers 715. Rusty as **** but it runs great. The backhoe is going to be my practice "canvas" for sandblasting and spray painting, something I've always wanted to do.

 

[bczygan]

Sizing electrical for a compressor has me all disoriented and confused.

One question is about how you determine what rating to use, FLC, FLA, nameplate rating, table from the NEC....???

And what's this about motors less than 1,200RPM's?

http://ecmweb.com/code-basics/motors-and-nec

I need a step by step for all the parts from the compressor motor to switch to disconnect, to wiring to breaker.

Bill

 

[LS6 Tommy]

Who said anything about that? We're talking about feeding the box/dc with nm and from there you'd use flex or seal tight to the compressor.

Looking at it again, I have NO idea why I even posted that...

Tommy

 

[wyliesdiesels]

Sizing electrical for a compressor has me all disoriented and confused.

One question is about how you determine what rating to use, FLC, FLA, nameplate rating, table from the NEC....???

And what's this about motors less than 1,200RPM's?

http://ecmweb.com/code-basics/motors-and-nec

I need a step by step for all the parts from the compressor motor to switch to disconnect, to wiring to breaker.

Bill

there is many threads where Charles and i have posted this but here goes:

Wire is sized @ 125% of NEC Table FLC based on motors nameplate HP.
If HP isnt marked or is "spl" then u take the FLA on the nameplate and figure out what the HP is using the NEC FLC tables. Then wire it as usual.

A disconnect is required if the compressor is not within sight of and more than 50' from the panel.

Breaker can be max 250% of FLC but u usually never need to go that high.

A HP rated outlet rated at the same or higher HP rating of the motor is required and standard NEMA outlets arent rated for more than about 3.5HP. Expensive Pin and sleeve connectors are available in higher HP ratings but be prepared to shell out some dough...If the correct rated HP receptacle isnt used then the motor needs to be hardwired...

If the PS(pressure switch) isnt rated for the same or higher HP rating as the motor, then a starter is needed...

Looking at it again, I have NO idea why I even posted that...

Tommy

Yeah i was a little lost when u said that...

 

[bczygan]

there is many threads where Charles and i have posted this but here goes:

Wire is sized @ 125% of NEC Table FLC based on motors nameplate HP.
If HP isnt marked or is "spl" then u take the FLA on the nameplate and figure out what the HP is using the NEC FLC tables. Then wire it as usual.

A disconnect is required if the compressor is not within sight of and more than 50' from the panel.

Breaker can be max 250% of FLC but u usually never need to go that high.

A HP rated outlet rated at the same or higher HP rating of the motor is required and standard NEMA outlets arent rated for more than about 3.5HP. Expensive Pin and sleeve connectors are available in higher HP ratings but be prepared to shell out some dough...If the correct rated HP receptacle isnt used then the motor needs to be hardwired...

If the PS(pressure switch) isnt rated for the same or higher HP rating as the motor, then a starter is needed...

Thank you for that description. It should be an electrical sticky.

One other question.

Outlet and pressure switch HP ratings......are the Running HP or the Max. developed HP?

Bill

 

[bczygan]

So let's take an example and see if I work it through properly.

Compressor says 7.0 Max. Developed H.P., 2.9 Running H.P. It also says 15A 240V.

2.9 is between 2 and 3HP on the NEC table 430,248 in the column marked 230V.

2HP is 12A and 3HP 17A. 2.9 would be 16.5A.

16.5 times 125% equals 20.625A.

Would I size the wire and breaker and disconnect for 30A rather than 20?

Obviously I could use a plug and outlet.

What about the cable from the plug to the switch?

Note that the cable from the switch to motor is 14ga 105C

BTW, I took the shroud off that covers the motor and all it has on it is 15A, no HP rating (This is a Craftsman compressor). The HP ratings given before are on a nameplate on the compressor.

And the pressure switch is rated (At 240V) 25FLA and 125LRA.

Why would the compressor say 15A?

Bill

 

[sberry]

Having switches plugs etc heavier than the ratings is a great idea in most cases. Now there are a 1000 details one could ponder and a lot of code issues but this is your own equipment and many of these things are not such a fine line, its not mission critical and we have seen a poster here that discovered 12 wire on a 7.5 that ran fine for 18 yrs.
That doesn't mean you should do that but back in the day we had a 3 hp on a 12/20 cable for a decade and never give it a thought.
In my own home it would be wired with the materials at hand, if I had a 20 would try it but if I had to buy a breaker it would be a 30, if I had a wire in hand and wanted to use it then it would be a 12, if I was adding a circuit, buying wire it would be a 10 as would everything else unless I scooped some bigger for free or I really had to have it.
You have the opportunity to wire as you go here, I wouldn't plan for every concievable scheme but when I needed another outlet wouldn't be scared to run it either.
If you are using small equipment a 6-50 is rated to 3 hp and fits welders. Small welders use 30, a 10/30 could easily run anything in a small garage, cheap enuf to run another one for a welder now or later but I suspect you are already short on breaker spaces??
.

 

[sberry]

Bill, I got a lot of stuff, I own 1 piece I don't even use that would overheat a number 10 wire. Get some 12 for 120V and 10 for 240.
I am a believer in code but I don't lose any sleep over the 1000's of fake 5 ho on a 10 cable 30 breaker circuits. There is likely sound reasoning behind not making an allowance, some of it is complicated and over my head, stuff we havnt even thought of, connection ratings, duty cycles etc.
Its important but not critical for your purpose. On short runs, under 60 ft lets say a wire a size larger than the cord on the machine is protected from thermal. The thermal on the motor will kick before it hurts the 14 on the machine which by default protects the 12 feeding it.
There are occasions the feed wire may be the same as on the machine,,, normally,,,,,,,,,, disclaimer for the uptights,,,, when you d this it is more closely calculated by the installer with limitations, may be single circuit in pipe etc or using different wire than a cable or a cord.
So, I have seen buzzboxes ran from a 10 cable, actually seen them on 12 but from a 10 till they were dam near a cinder on a 50A breaker and only marginally heat the wire.

 

[sberry]

Same for the 28A comps, world is full of them on 10/30, probably would see more breaker trips if they were on the right wire.
Most people don't realize most of the stuff they use is plugged in to a breaker with a higher rating than the wire they hold in their hand. The biggest normally is thru a 15 plug and cord landing on a 12 wire with a 20, in the case of a single user the breaker is rather irrelevant, its there to guard against multiple applied loads, not to protect the wire for any single load on a general circuit, this is the function of the equipment design.
This is one reason I harp a little on additional equipment installed on listed designs. We find it al the time in repairs, someone was smarter than the guy that invented it, it worked for 20 yrs but along comes mr maint electrician and decides it couldnt have possibly work, should be rewired and those guys that invented it were fools, one little fault and once its worked on never works right again.

 

[sberry]

Avoid the temptation to add a light circuit to remind yourself to turn it off to protect yourself against the HF drain valve to protect against rust, not that I does any good to start with. Most units need a wire to connect them, a hose from them and a hose to drain the tank on occasion depending on use.

 

[bczygan]

Thank you sberry!

I was afraid this thread was hopelessly dead.

And now you have come and anointed it with, count them, 4 stream of consciousness posts!

I'm going to have to get my mind right and adjusted, and dive into them.

Wish me luck!

Bill

 

[bczygan]

Well, it looks like I hijacked the OP's thread. Sorry!

Since this is an oiless compressor, and loud, and it will eventually go in a location out on a porch, I'll hook it up temporarily.

It already had 14 from motor to pressure switch and about 10' of 16 with a 20A plug.

I switched to a 30A locking plug.

I have a 30 A locking outlet in a plastic box, fed with #10 Romex, run about 50' to a 30A breaker.

We'll see how it works.

Maybe I'll change the cord to 14 or 12 later. Or even hard wire it.

Bill

 

[sberry]

It seems we went over his already but whats this changing the cord later on???? If there is 16 on this it aint sposed to be there, just because the last guy installed it doesn't make it right. Any wire leaving this machine needs to be 12 or better. You cant use a wire or cord smaller than the wires from the pressure switch to the motor, cant put smaller wires to bigger ones.

 

[bczygan]

It seems we went over his already but whats this changing the cord later on???? If there is 16 on this it aint sposed to be there, just because the last guy installed it doesn't make it right. Any wire leaving this machine needs to be 12 or better. You cant use a wire or cord smaller than the wires from the pressure switch to the motor, cant put smaller wires to bigger ones.

OK,
Looks like I'll be changing that first!

Thanks for post #8500!

Bill

 

[wyliesdiesels]

Thank you for that description. It should be an electrical sticky.

One other question.

Outlet and pressure switch HP ratings......are the Running HP or the Max. developed HP?

Bill

U go off of the running HP....max developed HP is a BS marketing term...

So let's take an example and see if I work it through properly.

Compressor says 7.0 Max. Developed H.P., 2.9 Running H.P. It also says 15A 240V.

2.9 is between 2 and 3HP on the NEC table 430,248 in the column marked 230V.

2HP is 12A and 3HP 17A. 2.9 would be 16.5A.

16.5 times 125% equals 20.625A.

Would I size the wire and breaker and disconnect for 30A rather than 20?

Obviously I could use a plug and outlet.

What about the cable from the plug to the switch?

Note that the cable from the switch to motor is 14ga 105C

BTW, I took the shroud off that covers the motor and all it has on it is 15A, no HP rating (This is a Craftsman compressor). The HP ratings given before are on a nameplate on the compressor.

And the pressure switch is rated (At 240V) 25FLA and 125LRA.

Why would the compressor say 15A?

Bill

First off, u dont size the wire based on the FLA listed on the nameplate. U size based on the NEC FLC table.

Your motor is close to 3HP so size the wire based on 17a.

17a x 125% = 21.25a

#12 NM-b is rated max 20a so u would need either #10 NM-b or #12 THWN...

Breaker can be max 250% FLC...

The wire from the switch to the motor while only #14, has a higher temp rating (105* c) so it has a higher ampacity than building wire...

 

[FloydShine]

Wow! I had no idea how complicated all this was. Anyway, the bottom line of my post is to find out if my garage electrical service can handle the requirements of this compressor (with new 60 amp breaker and possibly some upgraded wire). It's on sale and I have to move quickly but want to make sure it's not something I'll be trying to unload on Craigslist in two weeks.

I guess what I'm seeing in all these postings is that it will work, without a problem. Being completely ignorant of all stuff electrical, my only concern is that I won't lose my lights and other equipment when the compressor turns on. Kinda like overwhelming the panel (is that even a thing?).

Thanks,
Chuck

And yet, early on you commented that you did not want to spend much money? If you don't understand, then spend some to have it done right, no sense in screwing up. Electricity can kill you!

 

[jrrushing]

Single phase motors generally pull between 2 & 3 times name plate amps during start up so that motor will pull between 44 - 65 amps starting that is probably why the call for 60 amp breakers and I would use a mag starter or you will eat up pressure switches

 

[Chasbec]

And yet, early on you commented that you did not want to spend much money? If you don't understand, then spend some to have it done right, no sense in screwing up. Electricity can kill you!

I don't recall saying anything about not wanting to spend a lot of money. I have no intention of doing the electrical myself. The entire point of this thread was to make sure my electrical service could support this compressor, before I bought it.

 

[BTMSUP]

I can tell you that I bought a very similar comp just last week and it runs just fine on a 30 amp double pull. It is what the electrician recommended who ran 220 to my garage.




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[AngryBird]

The 60 number is the frequency not amperage. 220V/1 phase/60hz. I thought the same thing about mine. Draws more like 21 amps. Run a neutral even if you terminate it in the box for now. You'll be glad you did later if you need to run anything that requires it.

I ran a 50A breaker with 6/3 wire and put a NEMA 14-50 range plug on my compressor(neutral blade isn't connected). I can unplug it and plug in my 50A RV if I want to run both A/Cs.

 

[wyliesdiesels]

The 60 number is the frequency not amperage. 220V/1 phase/60hz. I thought the same thing about mine. Draws more like 21 amps. Run a neutral even if you terminate it in the box for now. You'll be glad you did later if you need to run anything that requires it.

I ran a 50A breaker with 6/3 wire and put a dryer plug on my compressor(neutral blade isn't connected). I can unplug it and plug in my 50A RV if I want to run both A/Cs.

incorrect. If u read comment #3 u will see where the OP listed what the manu. called for which is a 60a breaker...

And your stunt with the dryer plug on the compressor has so many things wrong with it.

Are u sure its a dryer plug?

First off, a dryer outlet/plug is rated for ONLY 30 AMPS. U used a 50a breaker to protect it. Strike one.

Second, unless the dryer plug is HP rated for 5HP, then thats strike 2!

And IF u used a NEMA 10-30, then thats strike 3 cause theres no ground/EGC on a NEMA 10-30....

 

[AngryBird]

I was wrong its a range cord. I edited my post to reflect that. I used a NEMA 14-50 plug.

I didn't see that post. I wish my IR compressor had come with the specs for a breaker. It was not in the manual that came with it. The electrician / electrical engineer I work with said a 30 or 50 would be ok. She also recommended the 14-50 plug so I could run the RV too.

 

[wyliesdiesels]

I was wrong its a range cord. I edited my post to reflect that. I used a NEMA 14-50 plug.

I didn't see that post. I wish my IR compressor had come with the specs for a breaker. It was not in the manual that came with it. The electrician / electrical engineer I work with said a 30 or 50 would be ok. She also recommended the 14-50 plug so I could run the RV too.

Ok well the 14-50 r/p is most likely not rated for 5HP...

 

[sberry]

The 60 is the max breaker, you don't need it on this comp. It can go on a 60A circuit, two of them could.

 

[BTMSUP]

The 60 is the max breaker, you don't need it on this comp. It can go on a 60A circuit, two of them could.

Agreed. My comp and welder can run together on the same 60 amp circuit with no issues.


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[Jlarson]

Ok well the 14-50 r/p is most likely not rated for 5HP...

Yeah 14-50's are only rated 3hp line to line.

 

[sberry]

Agreed. My comp and welder can run together on the same 60 amp circuit with no issues.


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You may be able to but not every machine is rated to do this. The cords on most welders are sized for 50A circuits. In some cases such as the compact 200 wire feeders would be able to put 2 machines on a circuit, they draw 24 but are wired to allow them to run from a conventional welder outlet which is 50.

Agreed. My comp and welder can run together on the same 60 amp circuit with no issues.


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So I will repeat this simply trying to clarify, change the breaker to a 50 and have a number 6 wire or separate wire sufficient for each device and it would be legal.
Not all similar equipment of same current range and voltage could be combined. Some could be expressly be wired to go on a specific circuit, say a wood shop with multiple machines where it would be prohibitive to run a dedicated circuit for each one etc...... This is a generalization with boiler plate disclaimer that I am not qualified, for further clarity ask a couple experts here about specifics.
For most here and with new builds and general tools or tools on 120 which is a huge advantage in the fact they do not need dedicateds its simply as easy to add specific circuits for most equipment, nothing is as simple or safe as a wire from breaker to outlet.

 

[sberry]

I worked for a guy in a parking lot once for 2 months, had a 10 cable we ran to a 30A comp about 40 ft from a power pole in to a semi trailer where it sat and another 10 cable pulled around under the trailer to a welder recept for a buzzer sitting on a pallet with both wires tapped under the lugs on a 50 breaker.
I know a couple guys made a living from 60A service with a comp but most only rarely welded and it was easy enuf to do one thing at a time.

 

[sberry]

While not all code minimums make for the best design it doesn't mean they are not adequate and undersized. Many or most regarding ampacity were designed when voltage was 20 lower than today. A lot of machines list the same input wire for 208 thru 240.
I think the buzzer welder was listed when the V was 220 and hasn't changed. It actually uses 3 less amps than the day it was made,,, hahaha

 

[BTMSUP]

You may be able to but not every machine is rated to do this. The cords on most welders are sized for 50A circuits. In some cases such as the compact 200 wire feeders would be able to put 2 machines on a circuit, they draw 24 but are wired to allow them to run from a conventional welder outlet which is 50.

So I will repeat this simply trying to clarify, change the breaker to a 50 and have a number 6 wire or separate wire sufficient for each device and it would be legal.

Not all similar equipment of same current range and voltage could be combined. Some could be expressly be wired to go on a specific circuit, say a wood shop with multiple machines where it would be prohibitive to run a dedicated circuit for each one etc...... This is a generalization with boiler plate disclaimer that I am not qualified, for further clarity ask a couple experts here about specifics.

For most here and with new builds and general tools or tools on 120 which is a huge advantage in the fact they do not need dedicateds its simply as easy to add specific circuits for most equipment, nothing is as simple or safe as a wire from breaker to outlet.

Mine is all #6 wire. Everything was just installed last week by a master electrician per all requirements for all my machines.

No harm will come from him running his compressor of the dual pull 30's if he has the correct wire. The worst that can happen is, he will pop a breaker and then know he needs to upgrade.


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[sberry]

I agree no harm is likely to come to him.

 

[BTMSUP]

[emoji106]


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[sberry]

There is a bigger is better theory and it isn't always true or you have to be willing to put up with the cost. I am a farmer and I rely heavily on shop but I really don't use a thing that would overheat a 10 wire.
The longer I go the more I give to simplicity and if I had to do it all over would stay within a convenience guideline so to speak if I could. I would give 50A to what I had to and if I could help it the rest would run from a 10 cord which is pretty much what I do on the rare occasion something has to leave home.

 

[Chasbec]

Mine is all #6 wire. Everything was just installed last week by a master electrician per all requirements for all my machines.

No harm will come from him running his compressor of the dual pull 30's if he has the correct wire. The worst that can happen is, he will pop a breaker and then know he needs to upgrade.

If I'm having an electrician come to connect the wiring anyway (which I am), is it really going to cost that much more to change out the breakers and abide by the manufacturer's recommendations? From what I'm seeing online, they are not very expensive at all http://www.homedepot.com/b/2-Pole/N-5yc1vZbwo5oZ25ecodZbm1e/Ntk-Extended/Ntt-60%2Bamp%2Bdual%2Bpole%2Bbreaker?Ntx=mode+matchpartialmax&NCNI-5

 

[BTMSUP]

Not expensive at all. Just unnecessary. I also wonder where that 60A came from because mine suggests 30A.


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