Wiring an Air Compressor

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  #1  
Old 05-27-06, 09:29 AM
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Wiring an Air Compressor

Hi everyone. At an auction I bought a 60 gal upright air compressor and am going to be wiring it into my garage and need some advice. I bought 10/2 with ground wire and a 60 amp double pole circit breaker. The motor on the compressor is a 5hp, 230v, 60hz, 22amp, Phase 1 motor. It also listed an SF of 1.15 (not sure what this is or if it is necissary info). Any help in if I bought the right wire and breaker along with any other helpful information would be greatly appreciated. Thanks!
 
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  #2  
Old 05-27-06, 10:36 AM
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Return the 60 amp breaker, get a 30 amp one.

Also, you need a means of disconnect (i.e. a switch) for the air compressor if the panel is not located nearby.
 
  #3  
Old 05-27-06, 10:37 AM
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DO NOT put a 60 amp breaker on 10 gauge wire. The compressor probably calls for a 30 amp breaker ( check the installation manual) . The 10 gauge is appropriate for a 30 amp circuit.
 
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Old 05-27-06, 11:02 AM
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Originally Posted by racraft
Return the 60 amp breaker, get a 30 amp one.

Also, you need a means of disconnect (i.e. a switch) for the air compressor if the panel is not located nearby.
Why would the 60 not be acceptable per NEC 2005 table 430.52. chart listing is up to 55 amp but exceptions allow up to 88 amp.

OP: I presume this motor is rated continous use and if so it is required to have either internal thermal protection or if no internal protection is included then a seperate overload device responsive to motor current per NEC 2005 430.22
 
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Old 05-27-06, 02:23 PM
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SF is service factor, indicating how much "overload" the motor will handle. In this case 15%. I'd try a 50a breaker first and see if it trips on startup with a high head pressure. 60a, though legal, may be more than is required. Your wire size is fine, given a reasonable distance to the box.
 
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Old 05-27-06, 06:40 PM
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typical startup currents are 4-5 hundred percent of fla rating. The code specifiaclaly takes this into consideration when allowing the "oversized" breaker.

This is also why an internal thermal overload protection or seperate overload protection is additionally required. (motors over 1 hp).

I have a hard time recommending to someone to buy a breaker that is useless and non returnable once installed when it is very acceptable and legal to be used in this situation.

Code actually allows up to 400% of rating if starting currents are a problem.
 
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Old 05-27-06, 09:26 PM
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Originally Posted by racraft
Return the 60 amp breaker, get a 30 amp one.
In a motor circuit, table 430.52 allows the overcurrent device to have a rating of 250% of the FLA of the motor.

Originally Posted by 594tough
DO NOT put a 60 amp breaker on 10 gauge wire. The compressor probably calls for a 30 amp breaker ( check the installation manual) . The 10 gauge is appropriate for a 30 amp circuit.
#10 is usually associate with a 30 amp breaker. Not so in a motor circuit. The OC device can be 250% of the motor FLA.

Originally Posted by nap
Code actually allows up to 400% of rating if starting currents are a problem..
What section of the NEC allows 400 %?
 
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Old 05-27-06, 10:17 PM
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Ok.....well I had been hearing from a few sources that 60 amp was too big. But I thought a 30 amp could be too small.....it is 75-100 feet from the compressor to the circuit box and I don't want it tripping all the time. So I went and bought a 40 amp. Should I be ok with that? Thanks for the help everyone!
 
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Old 05-28-06, 06:38 AM
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Originally Posted by wareagle
I
What section of the NEC allows 400 %?
That would also be table 430.52 Exception No. 2 (c)
 
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Old 05-28-06, 06:45 AM
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Originally Posted by MushingJeeper
Ok.....well I had been hearing from a few sources that 60 amp was too big. But I thought a 30 amp could be too small.....it is 75-100 feet from the compressor to the circuit box and I don't want it tripping all the time. So I went and bought a 40 amp. Should I be ok with that? Thanks for the help everyone!
The only problem you may have is on start up. Motors draw much more than FLA ratings upon start up. This is why the NEC allows such a large breaker in a motor situation.

This is also why you are required to have either internal thermal protection or a seperate load sensitive protection (i.e. a motor starter with thermal overloads).

Does your motor have internal thermal protection?

Voltage drop with a 22 amp load and #10 wire at 100 feet should be fine.

voltage drop calculator
 
  #11  
Old 05-28-06, 07:17 AM
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I don't have the original owners manual and that since I bought it at an auction. But the original advertisement stickers on the compressor does say it has built in thermal protection. I am going to check the motor to see if it lists an OPC number which should give me the maximum breaker I can put it on. If not I will give the 40 a shot first and if I keep blowing it then we'll try the 60. A friend has an 80 gallon, 8hp peak compressor and his is wired to a 40 which is why I chose to try the 40. Thanks for all your help everyone!

Edit: Well....no OPC number that I can find. Guess I just shoot for the 40 first. Thanks everyone for the help over the days!
 

Last edited by MushingJeeper; 05-28-06 at 07:49 AM.
  #12  
Old 05-28-06, 07:49 AM
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most newer two pole breakers are hacr rated. a 30 amp hacr will handle the starting surge for this motor.

you can only up the breaker size if you have seperate overload protection.

If you upsize the breaker for the starting surge you still have to upsize the wire to match the breaker rating. At least as far as the starter or thermal protection for the motor.
 
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Old 05-28-06, 08:23 AM
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[QUOTE]
Originally Posted by jwhite

you can only up the breaker size if you have seperate overload protection.
it is a requirement to have either thermal over load preotection, whether it be internal or seperate in this case.so it's covered.
If you upsize the breaker for the starting surge you still have to upsize the wire to match the breaker rating. At least as far as the starter or thermal protection for the motor.
could you quote the code section that requires the wire upsize?

I believe 430.62 and directives within specifically removes that requirement. The breaker in this case only acts as a short circuit interupting device and not an overload. That is why the overload protection is required in the motor controller or as internal thermal protection.

as far as the hacr rating, could you point me in the right direction in the code that allows this. I can't seem to find the hacr rating defined or accepted in the code.

Now I don't want to give the impression that a large breaker is required, as a matter of fact, the smaller the breaker, the more protection is afforded within reason and limits. If a 30 or 40 can be used without nuisance trips, then by all means use it. The code simply allows adjustment for different reasons and motor startup has always been one of those reasons. I also cannot recommend a person buying several breakers with no other use to them. (i.e. wasted money). I generally have access to most any of these breakers involved here at all times so it is easy for me to swap one out when it is a nuisance.
 

Last edited by nap; 05-28-06 at 08:35 AM.
  #14  
Old 05-28-06, 08:45 AM
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thanks nap I stand corrected on the wire size.

as for the breaker it is not addressed in the nec because you will not need to up size the wire or breaker. they are hvac breakers mostly, but they do take a good starting surge.
 
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Old 05-28-06, 09:03 AM
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Originally Posted by jwhite
thanks nap I stand corrected on the wire size.

as for the breaker it is not addressed in the nec because you will not need to up size the wire or breaker. they are hvac breakers mostly, but they do take a good starting surge.
I was actually hoping you could turn me on as to what the NEC does in defintion or actual spec's usage. I have seen these and understand what they are for but maybe the NEC has not really addressed them as of yet. It does sound like a good usage for the HACR rating though.

If they prove to be what they claim to be, they ultimately could cause rewriting sections such as the motor section for just the reason you stated. Ultimately this would be a good thing. Just as with the SWD and HID rated breakers and their usage.
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I think I'm going to include this as my signature line. I come across as being arrogant and argumentative, even when I am not trying to be so:


I'm not being argumentative, just anal
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Old 05-28-06, 09:46 AM
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Originally Posted by nap
I'm not being argumentative, just anal
Nap
Tks for being anal, you forced me to do some needed review of the code today
 
  #17  
Old 05-28-06, 10:03 AM
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I'd have to put in with NAP on this one. The intent of the charts and tables is to select conductors capable of handling the full load running current of the motor while at the same time being able to select the smallest overcurrent device that will allow reliable starting of the motor.

There would be almost no point of having a special article on motors if you installed wire installed based on the tables, then had to remove it because starting current tripped the breaker and required up-sizing.

The code is quite clear that in a motor branch circuit/feeder the protection is only for a fault condition - overload is to be handled by one of several means at the motor.

Some compressers start under load and others have an unloader that lets the motor come up to speed before placing a load on it. Some motors have very high starting currents. If you are lucky enough to get an old repulsion start/induction run motor in good shape the starting current will be quite low even when starting under load compared to a capacitor start type.

It is for these differences in motor types and load characteristics that the ability to adjust the size of the OCPD used for circuit fault only protection is provided.

The OP needs to post ALL the data on the motor label to determine what is allowed/required.

Finally, with all the above, I have a 5 horse compressor that runs just fine on #10 with a 30 AMP breaker, about a 40 foot run. My neighbor needed a 50 AMP on his - different make of motor and compressor.
 
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Old 05-28-06, 10:13 AM
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Originally Posted by jwhite
Tks for being anal, you forced me to do some needed review of the code today
That is one thing I have found since I have come here as well. It gets me back into the code book and actually helps in the day to day things I tend to forget or take for granted (and sometimes am wrong about) ,,,,,,and that's a good thing
 
  #19  
Old 05-31-06, 07:29 PM
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Some air compressors allow for startup without being under load by keeping the compressor's valves open until the motor gets up to speed. A unit like this may only need a 30 amp breaker. Manufacturer should provide reccommendations.
 
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