Is there an electrician in the house?

[The Blob]

I'm putting in wiring for a compressor. It will be a 5HP, 240V (obviously) motor. The manufacturer says it will pull 22.5 full load amps.

What size wire and breaker should I use?

I'm putting the compressor in a separate shed so the wire run will be about 60 feet. Except for about a 6 ft underground run the wire will not be in any conduit... it'll just run through the rafters.

The manufacturer (Ingersoll Rand) suggests 8 gauge wire for anything over 50 feet and a 60 amp breaker. I've got a bunch of 10 gauge on hand and really would rather not shell out the bucks for 8 gauge if I don't have to. And the 60 amp breaker doesn't, to this layman at least, seem like it would be protecting much. Does the motor really pull that much startup current?

I will do whatever is right... I just need to know what right is.

 

[whitehot84]

Voltage drop on AC ckts is calculated on 100' increments. The #10 should be more than enough. It would be wise to use a 40 amp breaker for startup purposes. This compressor will not run at a 100 per cent duty cycle unless you are planning on using multiple air tools at the same time.

 

[Junior Samples]

if your going to run a 40 amp you will need 8 gauge, 30 amp you can get away with 10 gauge, eventhough you would be running dedicated circuit and only pulling 23 amps, I would play it safe when it comes to Electricity

 

[John Larkin]

Right on, size the breaker (and the outlet) to the wire. EDIT: I would follow the manufacturers recommendations. For a little money, you will sleep better knowing there won't be a problem.

 

[Junior Samples]

Additional:

100 amp service into garage?

Somewhere on the compressor nameplate, there is "Maximum fuse size" or "Maximum OCP". thats the recommended breaker size, your breaker should be 250% of the current.

In the end you dont want to fry anything so use whats recomended to protect your investment.

 

[GregInAtl]

I'd recommend a #10 and a 30 amp.... we use 40 amp breakers on #8. I also would recommend going on and swapping your meter on your house to an extra capacity and running some direct burial "tri-plex" to your garage, w/ a 80-100 amp sub panel. (that is if you have a meter pack on the outside of the house ...main breaker in the outside meter box, not in the panel).
You have a compressor now, you'll have more toys/tools in there later, a little prep now saves heart ache later.
btw, if the 30 kicks as the compressor cycles, try a 35 double pole, then a 40 at last case...but I'd sleep fine w/ a #10 on a 35 a breaker personally.

 

[zeus87gn]

250% of 22.5A = 56.25A
Yes, you need the 60A breaker.
It will protect just right for your application.
The inrush should not be a concern, but sized correctly the CB will not give you nuisance tripping on startup.

The conductors should be rated 125% of the FLA.
125% of 22.5A = 28.125A (or 30A service)
Depending on what TYPE of wire it is, you may can use the 10awg.
The NEC rates only certain types of wire suitable at 10awg.
Typical non stranded copper conductor? Unless the box it's in designates the specific type and amperage rating, I would go with the 8awg and be done with it. All the 8awg ratings are over 30A.

Peace of mind is a good thing. You don't want to start a fire with overheated conductors. Insurance wouldn't cover it unless it was inspected and you submitted the report to the ins company. Not to mention you need to leave growing room. As previously mentioned, you may not run that compressor all the time, but you will be running it more and more once you get more air tools.

I deal with motor starting circuits everyday.
It's my job.

Z

 

[zeus87gn]

Right on, size the breaker (and the outlet) to the wire. EDIT: I would follow the manufacturers recommendations. For a little money, you will sleep better knowing there won't be a problem.

John,
The breaker, conductors (wire) and outlet of any circuit need to be sized for the load....regardless of what you are trying to get power to.

You are right tho, Mfg's typically know what they are doing.
Liabilities and all...

 

[GregInAtl]

I see the issue, it is state to state apparantly. I am in GA, electrician 10 yrs now, I'd do a 30a and 10 w/o hesitation. I agree it depends on the length of the pull, thats why I recommended a sub panel, and thus making the pull far shorter.
We'd get red tagged for 60a on an 8, no question. Our local inspectors allow/require (solid) 12awg to 20A, 0ver 20a to 30a is 10, over 30a to 40a is 8, and above 40a is 6(stranded). Funny to see how different states allow/red tag differently.

 

[zeus87gn]

We'd get red tagged for 60a on an 8, no question. Our local inspectors allow/require (solid) 12awg to 20A, 0ver 20a to 30a is 10, over 30a to 40a is 8, and above 40a is 6(stranded). Funny to see how different states allow/red tag differently.

Red tagged? As in not suitable for operation? Why?

Keep in mind I am looking at this strictly from a design standpoint....following NEC guidelines. If you have the practicing knowledge, please share.

 

[The Blob]

I'd recommend a #10 and a 30 amp.... we use 40 amp breakers on #8. I also would recommend going on and swapping your meter on your house to an extra capacity and running some direct burial "tri-plex" to your garage, w/ a 80-100 amp sub panel. (that is if you have a meter pack on the outside of the house ...main breaker in the outside meter box, not in the panel).
You have a compressor now, you'll have more toys/tools in there later, a little prep now saves heart ache later.
btw, if the 30 kicks as the compressor cycles, try a 35 double pole, then a 40 at last case...but I'd sleep fine w/ a #10 on a 35 a breaker personally.

The wiring from the house to the garage was done by an electrician. I've got 100 amps to the garage... all done to code.

Now I'm going from the garage panel to a shed which will require about 60' of wire. I'm going to put a little 70 amp panel in the shed and run the compressor circuit out of it.

Looking at ampacity tables, the 10 gauge wire would seem OK as you suggest. I'm a little confused about the breaker, though. If it trips a 30 at startup and I go to a 35, the safety valve (the breaker) is now above the wire rating so theoretically I have a problem in that if I developed a little short somewhere that pulled between 30 and 35 amps the wire would be the weak point.

I have the same question with IR's recommendation of 8 gauge with a 60 amp breaker so there is obviously something I don't know.

 

[salvageV6]

I stole this off an A/C site which deals with inrush currents.

Backs up a few previous statements. :smile:

" A/C condensers contain a hermetically sealed compressor motor as well as a fan to circulate air across the coils. The rules for protection to motor circuits are different than for circuits with simple resistive loads. When a motor first starts , it draws a much higher amount of current than it does after it is running. The high "inrush" current can exceed the rating of a breaker or fuse sized to protect the wire. The inrush current lasts typically only about 6 electrical cycles, or 1/10th of a second - less time than it would take to damage the wire or its insulation. However, if the overcurrent device is sized to protect the wire against overloads, the device might trip, and the machine would not be able to start.

Motor circuits get around this problem by dividing the two separate functions of an overcurrent protection device. "Overloads" are currents that can damage a circuit if allowed to continue for a sufficient time, whereas "short circuits" and "ground faults" are high currents that can cause immediate damage. Large motors and air conditioners separate these overcurrent functions. The breaker or fuse ahead of the air conditioner only protects against short circuits and ground faults. Overload protection is built into the compressor itself. The wire to a condensing unit must be large enough to allow the equipment to start. The device is therefore NOT sized to protect the wire against overloads. It is not uncommon to see a 50 Amp breaker on a #10 wire. The wire is protected against overloading by a separate thermal protection inside the compressor.

How do we know what the motor requires for wire size and proper fuse/breaker protection? It's all on the mfg. label attached to the appliance. If the label calls for fuses, there MUST be a fuse in the disconnecting means, not a circuit breaker. If the label calls for a "HACR" type breaker (Heating, Air Conditioning and Refrigeration), then that is the type of breaker that must be used. If the label simply states "maximum sized over current protection" then any form of overcurrent protection may be used. The HACR designation is not always visible on a circuit breaker once it is installed inside a panel or disconnect. "

I'd check out the mfg.'s labels and recommendations very closely before deciding on what to use.

 

[gnxtc2]

Does the air compressor have a motor starter?

The 10ga wire feeding the compressor is fine. 22.5FLA x 125% = 28.12A. If you read the ampacity tables (NEC 310-16), you use 40A for calculations for 10ga copper wire if you terminating with 90*C terminals. If you terminating with 75*C terminals (most common) then you use 35A for 10ga copper wire for your calculations.

The breaker for the air compressor is 60A. 22.5FLA x 250% = 56.25A The breaker has to be rated HACR and make sure it's a 2 pole 60A breaker. On a side note, if you install an instantaneous trip breaker ($$$$$$) it would be 800% of the 22.5A=180A. Picture 10ga with on a 180A breaker (this was a learning lesson)


If you have a motor starter set the overloads to this:
-Motor has service factor of not less 1.15 ...125%x22.5A=28.12A
-Motor marked for temp rise not over 40*C...125%x22.5A=28.12A
-All other motors......................................115%x22.5A=25.87A
-If the motor has a thermal protector...........140%x22.5A=31.5A


You know much strain that motor is going to be under when trying to start the compressor with air in it. Put an amp meter on the line side of the motor and you'll see. If you under size the breaker, it'll keep tripping.

The problem with most people, they think branch circuits when sizing wire with breakers. This is a motor over 1 HP and those codes don't apply here.

Make sure you have a disconnecting means within sight of the air compressor. If the panel is within sight, it qualifies.

When you run the sub panel into the shed, make sure you pull 2 hots, one neutral and one ground, otherwise you will be driving 2 ground rods. Do NOT do both.

I'm licensed for 12yrs and in the electric field for 19yrs.

If you have any other questions, just ask.

Billy T.
gnxtc2@aol.com

 

[SloGN]

from what i have learned when running a/c circuits you need to know the start-up load on that circuit then work backwards. The pannel the feeds the the garage panel has a 100 amp breaker so you need to see if everything int he garage along with the 220 circuit your adding along with the new outbuilding panel does not exceed your 100 amps continues load if not your gonna over heat the panel and the breaker will fail over time. also does the panel that feed in the garage panel does it have 4 conductors in it 2 hots.nuetral and a ground.

Also when to add the other panel and 222 circuit the grounds need to be tied into the same place. Even if you have 2 diffrent grounding rods in the ground you can have a spike on the ground side due to a potenial diffrence in the grounds.

 

[whitehot84]

250% of 22.5A = 56.25A
Yes, you need the 60A breaker.
It will protect just right for your application.
The inrush should not be a concern, but sized correctly the CB will not give you nuisance tripping on startup.

The conductors should be rated 125% of the FLA.
125% of 22.5A = 28.125A (or 30A service)
Depending on what TYPE of wire it is, you may can use the 10awg.
The NEC rates only certain types of wire suitable at 10awg.
Typical non stranded copper conductor? Unless the box it's in designates the specific type and amperage rating, I would go with the 8awg and be done with it. All the 8awg ratings are over 30A.

Peace of mind is a good thing. You don't want to start a fire with overheated conductors. Insurance wouldn't cover it unless it was inspected and you submitted the report to the ins company. Not to mention you need to leave growing room. As previously mentioned, you may not run that compressor all the time, but you will be running it more and more once you get more air tools.

I deal with motor starting circuits everyday.
It's my job.

Z

Not to be argumentitive but the 250% you speak of is the maximum you can oversize motor protection. There are so many variables in motor ckts it comical. This particular application most likely is a capacitor start motor so it doesn't need a starter, just a disconnecting means. The capacitor keeps the motor from creating a surge when starting under locked rotor stuations so no need to go extremely large on the overcurrent protecgtion. A 40 amp breaker will allow the motor to start without tripping and the motor should have built in thermal protection.

By the way I have been an electrician for 40 years and build conveyor systems and control panels for a living.
Not to be argumentative mind you.