Sketch Level: Midnight

[MrFreeze]

One of my two air compressors has had an audible leak from the bottom of the tank for the past few weeks. I finally got ambitious enough to disconnect it from everything, lay it on its side, and take a look. Found a pinhole with a rust streak around it. Just to see if I could, I "successfully" repaired it by drilling it out, tapping a hole (into about 3/16" of fairly clean metal), and screwing a 1/4"-20 bolt into it with part of a used bicycle tire as a gasket:



Believe it or not, this is actually holding 120 psi. Although I'm neither brave nor stupid enough to stand in front of it at that pressure. Yes, I am going to reinstall the compressor, but just temporarily. I figure if it lets go while upright, the worst it will do is put a dent in my floor.

The reason I'm posting this here is, I am looking to get a new compressor, and figured I would upgrade to a 2-stage while I was at it. Current compressor is wired to a 30A double pole breaker, which is about 3 ft of 10-3 wire away from a wall plug (NEMA 30A, I forget which configuration offhand). There is about 5 ft of 10-3 SJ cord from the plug to the compressor motor. Current motor FLA (nameplate) is 17.2 @ 240V.

The compressor I am considering says this in the specs:

• 21.5 Full Load Amps, Recommended Breaker Amp Size 40, Recommended Wire Gauge<50 ft. 8 Gauge

Not sure why a 21.5A FLA motor needs 8 gage wire, or a 40A breaker, but I also am not an electrician. Changing the breaker, outlet, and SJ cord is easy. Upsizing the wiring between the panel and the outlet would be "intrusive."

Since there seems to be an impressive amount of knowledge and experience on this forum, I thought I would ask for opinions. Can I run this compressor on a 30A circuit? Will it kill me and all the neighbors if I try? Do I have to cut a holes in my wall to install 8-3 wire if I want to run this compressor? Have you already concluded from my compressor repair that I am too dumb to be left in a garage full of tools unsupervised?

Appreciate all constructive input.

MrFreeze

 

[The Cobbler]

I did not read all of your post, and I commend you for fixing the tank, but , please
scrap it , it's a bomb waiting to go off

 

[MrFreeze]

...I agree. You should really read the rest of the post.

MrFreeze

 

[PCustoms]

...I agree. You should really read the rest of the post.

MrFreeze

I read the whole thing.

That tank is done, pressurizing it again is asking for an explosion. Your thinking of " the worst it will do is put a hole in my floor" is seriously flawed.

 

[MrFreeze]

Well, if I don't post an install about my new compressor, you can assume you were right, and I am dead. Try to think kindly of me.

MrFreeze

 

[PCustoms]

Well, if I don't post an install about my new compressor, you can assume you were right, and I am dead. Try to think kindly of me.

MrFreeze

Does anyone else, human or pet, live with you? Would the to see someone innocent get hurt.

11 posts in 4 years and 3 of them appear to be bragging about doing something stupid while acknowledging it's stupid.

 

[rlitman]

...Not sure why a 21.5A FLA motor needs 8 gage wire, or a 40A breaker, but I also am not an electrician. Changing the breaker, outlet, and SJ cord is easy. Upsizing the wiring between the panel and the outlet would be "intrusive."...

FLA is the current of the motor under full load, and that's a good representation of the "thermal" load. The LRA, or locked rotor amps are going to be MUCH higher than that, and that's what trips breakers on startup, so code allow for breakers to be oversized in these situations, and hopefully the motor comes with it's own thermal protection. That high inrush current also creates a large voltage drop, which isn't good for the motor, and that's why they recommend a larger wire gauge. The 50' is the distance from the outlet to your panel, not the SJ cord between the motor and the outlet.

And speaking of outlets, from the sounds of it, I'm guessing you're looking at a 7.5 HP motor. Nice. That's what I've got. It's also more horsepower than a plug is rated for, so you're going to need to ditch the outlet and hard wire it, preferably via a disconnect if the panel isn't in a direct line of sight.

For the record, my compressor gets power from a 30A breaker (it's what I had), but I wired it with 6 AWG (also something I had), and have never had it trip. Can you get away with your 30A breaker? Maybe, but that partly depends on how far the run home to the panel is.

As for the safety of that tank, I've seen worse. I'm not condoning what you did by any means - it's plumb stupid. But corrosion tends to form pits that leak and give you warnings like this before catastrophic failure. Especially on vertical tanks, because the end bells are both the thickest and strongest parts, and unlike the cylinder, they're not weakened by a weld line. If that hole was next to a weld, I'd be much more concerned, and if it were next to the linear weld on a horizontal tank, I'd say you're lucky to be alive.

 

[Plastikosmd]

As long as your airlines are pvc, you should be ok

(Joking, and normally I wouldn’t have to say “joking” but in this case it is warranted)

 

[mm08822]

21.5A @ 240vac single phase is about 4 hp. Not sure why mfr is stating #8 wire, but by NEC, #10 Cu would meet the requirements. Based on the short distances mentioned in the first post, I don't see any reason to change to #8.

The cb can be as large as 250% of FLAs, so up to a 50A cb could be used for this dedicated motor circuit.

 

[mm08822]

FLA is the current of the motor under full load, and that's a good representation of the "thermal" load. The LRA, or locked rotor amps are going to be MUCH higher than that, and that's what trips breakers on startup, so code allow for breakers to be oversized in these situations, and hopefully the motor comes with it's own thermal protection. That high inrush current also creates a large voltage drop, which isn't good for the motor, and that's why they recommend a larger wire gauge. The 50' is the distance from the outlet to your panel, not the SJ cord between the motor and the outlet.

And speaking of outlets, from the sounds of it, I'm guessing you're looking at a 7.5 HP motor. Nice. That's what I've got. It's also more horsepower than a plug is rated for, so you're going to need to ditch the outlet and hard wire it, preferably via a disconnect if the panel isn't in a direct line of sight.

For the record, my compressor gets power from a 30A breaker (it's what I had), but I wired it with 6 AWG (also something I had), and have never had it trip. Can you get away with your 30A breaker? Maybe, but that partly depends on how far the run home to the panel is.

As for the safety of that tank, I've seen worse. I'm not condoning what you did by any means - it's plumb stupid. But corrosion tends to form pits that leak and give you warnings like this before catastrophic failure. Especially on vertical tanks, because the end bells are both the thickest and strongest parts, and unlike the cylinder, they're not weakened by a weld line. If that hole was next to a weld, I'd be much more concerned, and if it were next to the linear weld on a horizontal tank, I'd say you're lucky to be alive.

only 7.5 hp if 3 phase.

 

[Norcal]

21.5A @ 240vac single phase is about 4 hp. Not sure why mfr is stating #8 wire, but by NEC, #10 Cu would meet the requirements. Based on the short distances mentioned in the first post, I don't see any reason to change to #8.

The cb can be as large a, s 250% of FLAs, so up to a 50A cb could be used for this dedicated motor circuit.

21.5A is in the range of a 5 HP 1Ø motor, the NEC requires that the table in art 430 be used to size the circuit & be 125% of the value which comes out at 35A, so if NM cable is used, 8 AWG is minimum since NM is sized by the 60° column, if THHN/THHW is is used then 10 AWG is permitted, since permission is given in Art 430 to use it at the 35A value.

 

[Snapped-off]

Drill some extra holes in the tank to render it useless when you're ready to scrap it.

 

[mm08822]

21.5A is in the range of a 5 HP 1Ø motor, the NEC requires that the table in art 430 be used to size the circuit & be 125% of the value which comes out at 35A, so if NM cable is used, 8 AWG is minimum since NM is sized by the 60° column, if THHN/THHW is is used then 10 AWG is permitted, since permission is given in Art 430 to use it at the 35A value.

Yeah, you're right, a few baldor motors' spec confirmed FLAs. Seemed too low to me to be a true 5 hp.

So with it truelly being a 5HP single phase motor, then the code requirements and math does work out for #8 NM-B or 10s @ 75C. The installation instructions over-simplified the #8 requirement.

 

[ArcReactorKC]

 

[niget2002]

I have a compressor in my shop with a tank on it that's older than I am. My grandfather 'fixed' it multiple times with a bolt and JB weld. My dad 'fixed' it twice with a bolt and JB weld. I 'fixed' it once with a bolt and JB weld. Compressor was used for probably 30 years with various holes plugged with bolts. It wasn't until I joined GJ that I even knew 'fixing' it was bad.

Will your compressor explode? Probably. But it may take a while before it does. Or it could explode tomorrow.

I have since replaced mine with newer equipment. When I stopped using it, I put a 1/2" hole in the bottom just in case I die and someone tries to use it. Maybe one day I'll find someone with a broken air compressor that has a good tank and I can move the pump and motor over to a newer tank. I don't really need the air compressor. I have a larger 25 gallon and a much larger 80 gallon at this point.

 

[wyliesdiesels]

Lets cut to the chase

Whats the HP rating as listed on the nameplate?

 

[Norcal]

I have a compressor in my shop with a tank on it that's older than I am. My grandfather 'fixed' it multiple times with a bolt and JB weld. My dad 'fixed' it twice with a bolt and JB weld. I 'fixed' it once with a bolt and JB weld. Compressor was used for probably 30 years with various holes plugged with bolts. It wasn't until I joined GJ that I even knew 'fixing' it was bad.

Will your compressor explode? Probably. But it may take a while before it does. Or it could explode tomorrow.

I have since replaced mine with newer equipment. When I stopped using it, I put a 1/2" hole in the bottom just in case I die and someone tries to use it. Maybe one day I'll find someone with a broken air compressor that has a good tank and I can move the pump and motor over to a newer tank. I don't really need the air compressor. I have a larger 25 gallon and a much larger 80 gallon at this point.

 

[MrFreeze]

Lots of good info here, and thanks for all the helpful responses.

First things first, I decided to err on the side of caution and removed my repair bolt. Then I reinstalled it with some red loctite. Permanent fix!

But more seriously, thanks for the education on the various governing codes. Clearly some very knowledgeable people here, which is why I asked. This is "nominally" a 5 hp motor, but I think we all know better than to take an air compressor rating at face value. Which is why I found the FLA and asked the question. Still not sure what to do here, but I will respond to some comments with more info (or questions) and try to reach a decision.

Best regards,
MrFreeze

 

[MrFreeze]

FLA is the current of the motor under full load, and that's a good representation of the "thermal" load. The LRA, or locked rotor amps are going to be MUCH higher than that, and that's what trips breakers on startup, so code allow for breakers to be oversized in these situations, and hopefully the motor comes with it's own thermal protection. That high inrush current also creates a large voltage drop, which isn't good for the motor, and that's why they recommend a larger wire gauge. The 50' is the distance from the outlet to your panel, not the SJ cord between the motor and the outlet.

I understand the LRA is going to be much higher, but I don't believe the LRA is used to size breakers or wiring, is it? Or is some percentage of it used? I notice LRA is not even listed on either of my two compressor motors.

MrFreeze

 

[MrFreeze]

Drill some extra holes in the tank to render it useless when you're ready to scrap it.

I'll probably cut the head off to see how bad it really is. If I find anything interesting, I'll post it for general amusement.

MrFreeze

 

[MrFreeze]

21.5A is in the range of a 5 HP 1Ø motor, the NEC requires that the table in art 430 be used to size the circuit & be 125% of the value which comes out at 35A, so if NM cable is used, 8 AWG is minimum since NM is sized by the 60° column, if THHN/THHW is is used then 10 AWG is permitted, since permission is given in Art 430 to use it at the 35A value.

Where did 35A come from? 125% of 21.5A is 27A. Unless you are looking at a nominal table value from the Art 430 you reference? Wiring from the panel (about 3 ft total) is 10-3 NM.

MrFreeze

 

[MrFreeze]

Will your compressor explode? Probably. But it may take a while before it does. Or it could explode tomorrow.

So could the sun! But your point is well taken...

MrFreeze

 

[MrFreeze]

Lets cut to the chase

Whats the HP rating as listed on the nameplate?

Haven't seen the nameplate, but manufacturer's literature refers to it as 5 hp. I had to dig deeper to find the 21.5 motor FLA.

MrFreeze

 

[mm08822]

Where did 35A come from? 125% of 21.5A is 27A. Unless you are looking at a nominal table value from the Art 430 you reference? Wiring from the panel (about 3 ft total) is 10-3 NM.

MrFreeze

"Years ago"' motor circuit conductors were sized from the actual nameplate FLAs. Then came high efficiency motors drawing lower currents.
Circuit conductors were sized smaller accordingly. Once those motors smoked, the next guy installed some cheap POS and it had higher flas. Circuit conductors were then undersized.
Hence, a new table in NEC article 430 giving a worst case design load current to size the wires.

Using the "old way" , 10s make the cut even if it was truelly older 60c NM. New NM-B has has 90c rated conductor insulation BUT it's forced down to the 60c rating.

Get the new compressor first!

 

[mm08822]

I'll probably cut the head off to see how bad it really is. If I find anything interesting, I'll post it for general amusement.

MrFreeze

Make a flower pot out of it or a lil weenie roaster.

 

[wyliesdiesels]

Lots of good info here, and thanks for all the helpful responses.

First things first, I decided to err on the side of caution and removed my repair bolt. Then I reinstalled it with some red loctite. Permanent fix!

But more seriously, thanks for the education on the various governing codes. Clearly some very knowledgeable people here, which is why I asked. This is "nominally" a 5 hp motor, but I think we all know better than to take an air compressor rating at face value. Which is why I found the FLA and asked the question. Still not sure what to do here, but I will respond to some comments with more info (or questions) and try to reach a decision.

Best regards,
MrFreeze

what is the HP rating listed on the nameplate?

 

[wyliesdiesels]

I understand the LRA is going to be much higher, but I don't believe the LRA is used to size breakers or wiring, is it? Or is some percentage of it used? I notice LRA is not even listed on either of my two compressor motors.

MrFreeze

nope the LRA isnt used to size a motor circuit but the HP and corresponding FLC values in the tables is

 

[wyliesdiesels]

Where did 35A come from? 125% of 21.5A is 27A. Unless you are looking at a nominal table value from the Art 430 you reference? Wiring from the panel (about 3 ft total) is 10-3 NM.

MrFreeze

you dont size the wire by the FLA on the nameplate. you size it by the FLC tables in 430.248 (or .250) based on the nameplate HP

10/3 is not code permissible for 35a rating.

 

[mm08822]

I understand the LRA is going to be much higher, but I don't believe the LRA is used to size breakers or wiring, is it? Or is some percentage of it used? I notice LRA is not even listed on either of my two compressor motors.

MrFreeze

The motor probably has a code letter on its nameplate and would provide a lookup table value. Whether you actually reach that level of inrush depends on many things beyond the motor itself. The code allows you to increase the motor circuit cb value from 30 thru 50A. This range will cover your needs.

 

[Norcal]

IMHO, it’s foolish to repair a pressure tank the risks of catastrophic damage is too great. There was photo posted here of the aftermath of a compressor tank rupture in a car wash & the damage done to the CMU wall is scary, the power of compressed gases is immense.

 

[wyliesdiesels]

wheres the photo?

 

[PCustoms]

wheres the photo?

Of?

 

[rlitman]

... I notice LRA is not even listed on either of my two compressor motors...

Most compressor motors are made to go with the compressor. Since they're not sold as stand-alone motors, they frequently have missing data and too often just show "special" under various fields. It can make deciphering what the motor's heritage is difficult.

 

[Norcal]

wheres the photo?

I merely commented that there was a photo out there, was hoping that someone would remember it & post it.

 

[wyliesdiesels]

IMHO, it’s foolish to repair a pressure tank the risks of catastrophic damage is too great. There was photo posted here of the aftermath of a compressor tank rupture in a car wash & the damage done to the CMU wall is scary, the power of compressed gases is immense.

Of?

the CMU wall Norcal mentioned above

 

[wyliesdiesels]

I merely commented that there was a photo out there, was hoping that someone would remember it & post it.

oh you said "posted here" thought you meant on GJ.... my bad

 

[Shiftless]

I found this picture of a CMU wall that was mostly destroyed by an exploding compressor tank. Is this the one referenced above?

 

[RPH]

I found this picture of a CMU wall that was mostly destroyed by an exploding compressor tank. Is this the one referenced above?

Impressive!

 

[Joemctag]

FLA is the current of the motor under full load, and that's a good representation of the "thermal" load. The LRA, or locked rotor amps are going to be MUCH higher than that, and that's what trips breakers on startup, so code allow for breakers to be oversized in these situations, and hopefully the motor comes with it's own thermal protection. That high inrush current also creates a large voltage drop, which isn't good for the motor, and that's why they recommend a larger wire gauge. The 50' is the distance from the outlet to your panel, not the SJ cord between the motor and the outlet.

And speaking of outlets, from the sounds of it, I'm guessing you're looking at a 7.5 HP motor. Nice. That's what I've got. It's also more horsepower than a plug is rated for, so you're going to need to ditch the outlet and hard wire it, preferably via a disconnect if the panel isn't in a direct line of sight.

For the record, my compressor gets power from a 30A breaker (it's what I had), but I wired it with 6 AWG (also something I had), and have never had it trip. Can you get away with your 30A breaker? Maybe, but that partly depends on how far the run home to the panel is.

As for the safety of that tank, I've seen worse. I'm not condoning what you did by any means - it's plumb stupid. But corrosion tends to form pits that leak and give you warnings like this before catastrophic failure. Especially on vertical tanks, because the end bells are both the thickest and strongest parts, and unlike the cylinder, they're not weakened by a weld line. If that hole was next to a weld, I'd be much more concerned, and if it were next to the linear weld on a horizontal tank, I'd say you're lucky to be alive.

Appreciate your typically good, level-headed advice, rlitman.

 

[PCustoms]

I found this picture of a CMU wall that was mostly destroyed by an exploding compressor tank. Is this the one referenced above?

I bet it left just a small dent in the floor though.