Wiring an AIr Compressor

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  #1  
Old 04-23-04, 04:03 PM
BackSplitRich
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Wiring an AIr Compressor

What I have is a 5HP motor rated at 240v 15amps.
I have a spare 2pole breaker in my panel which is only a couple of feet away from where the compressor will be mounted.
My question is, from the panel I would like to put a wire running to a box with a 240v 15amp receptacle. Is a 15amp receptacle enough? or should I go to a 20amp? Then from the compressor, run a small length of wire with an appropriate plug to plug into the receptacle. also, does the circuit beaker have to be matched to the receptacle rating...15amp breaker for a 15amp receptacle? Wire wise I guess I'm safe using a 12/3 which is good for 20 amps?
I hope I included enough info for someone to answer my question.

Thanks, Rich
 
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  #2  
Old 04-23-04, 05:06 PM
Chip Jenkins
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Grab your manual

I believe that the NEC calls for 20A outlets in a garage, no matter what. So, dont second guess that one, just use the 20. As far as the size on the breaker, it should be listed in the manual that came with the compressor. I ran conduit and wired outlets for the neigbor, including a dedicated circuit for his small 115V compressor and the breaker size was listed in the manual.

Chip
 
  #3  
Old 04-23-04, 06:09 PM
WFO
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Code stipifies that you load a circuit at 80% of its full rating, so, in theory, a 15 amp motor would be fine on a 20 amp breaker. However, motors generally pull anywhere from 4 to 6 times the running load when they start. So when you first turn the motor on, the inrush might trip the breaker. You can't just upgrade the breaker if you're using the 12/3 because then it wouldn't protect the wire. Go with the recommendations in the manual.
 
  #4  
Old 04-24-04, 06:04 AM
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You most likely need a 20 amp breaker and 12 gauge wire. However, review what the manual calls for on the breaker size, and size the wires based on the breaker.

You do not need 12-3 wire. This will provide an extra wire, the neutral, that you don't need, and won't have anywhere to connect to the compressor. 12-2 will have two conductors and a bare ground wire, which is all you need. However, you can use 12-3 (or whatever size is appropriate) and simply cap off the unused neutral wire.
 
  #5  
Old 04-24-04, 06:24 AM
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Motors fall under different rules than other apparatus, and I confess that while I have read the appropriate code section, I am in somewhat deep water on the topic.

Receptacles and switches used for motors must be horsepower rated as well as current rated. This is to accommodate the startup current and potential overload currents associated with motors. Even though your motor is a '15A' motor, you may be required to used a 60A receptacle. On the other side of the coin, you may be able to use wire that is smaller than you would expect for the breaker required. The devil is in the details on this subject.

Just as a point of reference: Leviton does not make a straight blade receptacle that is horsepower rated that is suitable for a 240V single phase 5 horsepower motor. The closest that they come is a 50A 277V receptacle. If you want to 'plug in' a 5 hp motor, then you probably need to go to the rather expensive 'pin and sleeve' connectors.

Questions:
Do you have a _motor_ or an appliance which _contains_ a motor? If you have a 'listed motor-operated appliance' with the motor full load current marked on the name-plate, then the current on the name-plate is what determines everything else. If you just have a motor, then you are required to calculate the full load current based upon the horsepower rating. I can only presume that this is because in the _appliance_ more is known about the mechanical load placed upon the motor.

What are the ratings on the nameplate itself? Many home power tools (lawn mowers, air compressors, vacuum cleaners) have horsepower ratings that are inflated by the marketing department. But the actual name-plate continuous values are much lower, because the marketing number is some sort of peak power rating, not reality. 15A at 240V is only 4.8 horsepower presuming 100% efficiency and 100% power factor. So either the '5 hp' number is bull or the '15A' number is bull.

Why do you want to plug and cord connect this rather than hard wiring it?

What does the manual say about the size of conductors and the circuit rating required?

-Jon
 
  #6  
Old 04-24-04, 10:29 AM
BackSplitRich
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Winnie-
The listing on the plate of the motor states its a 15amp motor...doesn't specify FLA or not...and like you said, I'm sure the motor is not rated at 100% efficiency. On the motor itself says 15 amps, in the "marketing" side, it says 6.5 peak HP.
I thought about it as well, I don't have to have a plug, I could just hardwire it directly to the panel, and minimize all the baloney. The compressor will only be about 4 feet from the panel.
About the manual, it specifies nothing about the breaker size, wire gauge or other relavant information about wiring it, it just states to have the unit wired by a qualified electrician.
Now, for the wire...I don't know what this wire is known as, but I call it "cabtire". Can this type of wire be directly wired to the panel? Or do I need "house wire" like a 10/3 rated for a dryer? I'd rather upsize the wire and use a 20 amp breaker.

Racraft- I was a little puzzled when you said I needed only a 12/2 wire but understand now...12/2 housewire, doesn't include the ground, just the conductors...cabtire 12/3...has a black, white and green, which is ground obviously, and was the type of wire I was planning on using.

Thanks all for the input. Rich
 
  #7  
Old 04-24-04, 12:46 PM
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Okay, you didn't explicitly answer if you have a _motor_ or an _appliance_ with a motor. The below assumes that you bought a compressor with the motor attached, and the name-plate is for the _appliance_, and specifies the current for the appliance. The below is _not correct_ if you bought a motor and a compressor and connected the motor to the compressor.

Since the appliance has a listed current of 15A, it requires a 20A circuit. If you wish to cord and plug connect this, then you need a 20A plug.

If you do not cord and plug connect this appliance, then it is still a good idea to use a _short_ length of flexible cable to provide vibration isolation. The flexible cable would be the 'cab-tire' that you are considering. It is also called 'SO' cord.

You should not be placing the compressor in front of your panel. This is because your panel should remain un-obstructed.

Near the location of the compressor you should place a junction box. This is were you will mount the receptacle _or_ connect the SO cord. Between the panel and the compressor you cannot use SO cord. You must use proper building wire, and it must be protected properly (eg. if you use Romex it needs to be protected in the wall, or you could use MC cable, or single wires in conduit, etc.)

At the junction box you would mount your receptacle, or if you simply hard wire with SO cord, you would mount the SO cord using a strain relief fitting. The SO cord would be a _short_ loop that goes right to a strain relief fitting on the air compressor terminal box.

If you use a receptacle, I would recommend that you use an 'L6-20' locking receptacle and cord cap. These just feel right for large power tools

You are right about how conductor numbering is screwy on cords. 12-2 NM cable has 3 conductors. 12-2 SO cable only has 2 conductors.

-Jon
 
  #8  
Old 04-24-04, 12:47 PM
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12/2 NM-B is the name of a cable with black/white/bare.
12/3 NM-B is the name of a cable with black/red/white/bare.
 
  #9  
Old 04-24-04, 01:00 PM
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12/3 NM-B has black/red/white/bare
12/3 SO has black/white/green

It was very annoying when I found this out first hand

-Jon
 
  #10  
Old 04-24-04, 02:45 PM
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The full-load current of a 5 HP , 230 volt single-phase motor is 28 amps.

The required ampacity of the Branch-Circuit conductors is 30 (approx) x 1.25 = 38 amps. The conductor size is #8.

The rating of the circuit-breaker is 30 X 2.5 = 70 amps (approx).

"If it's not there, it can't fail"---- this applies to the proposed cord-plug & receptacle connection. I suggest you connect the motor supply cord directly to the Branch-Circuit conductors in a junction-box.

The C-B will serve as the motor Dis-connecting means because it is "with-in sight" of the motor.

Good Luck and Enjoy the Experience!!!!
 
  #11  
Old 04-24-04, 06:51 PM
WFO
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Pattbaa.....I'm curious as to how you arrived at the 28 amp value for the 5 hp motor. Seems like 20 would be a closer value, and that's using the old 1 kva per Hp method (which usually comes out a little high anyway). Also, are you suggesting he put a 70 amp breaker on a #8 AWG wire?
 
  #12  
Old 04-24-04, 10:21 PM
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The numbers that Pattbaa is suggesting come right out of the NEC, in article 430. Basically the NEC says 'for this horsepower motor, at this supply voltage, you have to size the conductors presuming this load current'. The current which the NEC requires you to presume is considerably greater than the full load ampere rating as determined by the manufacturer, and is necessary because motors can draw considerably more than rated FLA under normal overload conditions.

Once you have sized the conductors sufficiently for the expected overloads on the motor, it is presumed that the motor overload protection (eg thermal breaker, overload relay, etc.) will be able to properly protect the _wire_ from overload. Since the motor is protecting the _wire_ from overload, the circuit breaker is there to provide _short circuit_ protection. So the circuit breaker is permitted to be oversized relative to the wire

So a 5 hp 240V motor, which might consume 20A when producing 5 shaft horsepower will be treated as though it has 28A current requirements and placed on conductors with a minimum ampacity of 28A * 1.25, but the circuit breaker can be larger still.

Pattbaa, I think that you made an error in saying 'The rating of the circuit-breaker is 30 X 2.5 = 70 amps (approx).' As I understand that section of code, you are _permitted_ (rather than _required) to use a breaker of up to this size to protect the conductors feeding this motor. However a smaller breaker can be used if it is sufficient for starting the motor.

Finally, I am quite certain that the motor in question is _not_ a 5 Hp in the sense that the NEC describes something as a 5 hp motor.

-Jon
 
  #13  
Old 04-25-04, 11:15 AM
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I was remiss in not considering the possibilty that this may be a 3-phase motor.

I would select the maximum permissible rating for the Branch Circuit Overcurrent Protective devise to avoid and possible motor-starting problems.

We did not mention a controller- possibly it could be the Circuit-Breaker.

A motor over-load protective device should also be considered for maximum protection of the motor.
 
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