Running 5 hp air compressor in shed electrical questions

rswojoFebruary 4, 2012

I have a shed approximately 150 feet from my house. It is fed by copper 6 gauge wire from a 200A sub panel in my house to a 100A breaker panel in the shed.

I want to install a Ingersoll Rand 5hp 240V single stage air compressor out there. I happen to have a 40A breaker handy.

Is the 6 gauge wire large enough to handle the load of the compressor? Will the 40 amp breaker work? What gauge wire should I use from the breaker to the compressor?

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Ron Natalie

Hopefully, there is some realistic breaker protecting the feeder to the shed. You're not going to draw 100A on #6.

40A is way oversized for a 5HP motor at 240V. #6 will feed enough for the 5HP, but what else is on the panel?

    Bookmark   February 4, 2012 at 6:39PM
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rswojo

I have a 400A main panel in the basement which feeds a 200A breaker panel. There are two 100A breakers in the 200A panel which in turn feed a 100A panel in the attached garage and a 100A panel in the shed.

The shed has some fluorescent lights, some outlets, a grinder, a table saw, that's about it. When I run the compressor it will be used for an impact wrench, at the moment I don't have any other plans for it. When I'm doing mechanical tasks I won't be using anything else (at least at the same time, well maybe a trouble lamp). I don't work out there at night.

My main concern was will the #6 wire handle the compressor without overheating. Thanks.

    Bookmark   February 4, 2012 at 7:15PM
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brickeyee

If it is a real 5 HP motor the full load current is 28 amps (Table 430.148).

It willl take significantly more than than at start-up.

The LRA code letter on the motor can be used to calculate starting current (Table 430.7B).

If the voltage drops during starting the current will go significantly higher and start time becomes longer.

Article 430 covers motor loads.

    Bookmark   February 4, 2012 at 7:44PM
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rswojo

This is the compressor.

http://www.aircompressorsdirect.com/Ingersoll-Rand-SS5L5-Air-Compressor/p689.html

I should contact IR for technical support on this, correct?
I don't want to spend hours poring over the NEC. I just need to know if the #6 wire will handle the load and what size breaker I will need to safely and properly run it.

I have a bulldozer with big, high torque bolts I want to break loose to remove the motor for a rebuild. The air wrench is an IR 431C and my portable compressor doesn't provide enough cfm to provide full power to the air wrench.

    Bookmark   February 4, 2012 at 9:02PM
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brickeyee

"I don't want to spend hours poring over the NEC. "

It is only one article.

If you do not want to find out how to do it correctly, hire the job out.

Luckily most homeowner insurance policies protect you form your own mistakes.

    Bookmark   February 5, 2012 at 10:59AM
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rswojo

It looks like I will have to go to the library and get a copy of the NEC.

From a little reading in the intrawebs:

1. It looks like my #6 wire will work.
2. Thermal protection should be provided by the motor itself.
3. Short circuit protection will be provided by a typical properly sized breaker.
3. A properly sized GFI will be needed as an outlet.
4. No switch will be required as the compressor will be just plugged in for use and unplugged when done.
5. No "lock out/tag out" disconnect will be needed as this not a factory where different people will be involved in working on this without knowledge of what one or the other has done or is doing.

Am I correct so far?

    Bookmark   February 5, 2012 at 1:04PM
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Ron Natalie

You're not going to find a 40A GFCI receptacle. If you need GFCI, you'll have to use a GFCI breaker. If you're going to do that, how about figuring out what the proper size is rather than just using the one you have lying around.

    Bookmark   February 5, 2012 at 2:27PM
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rswojo

I just mentioned I had one laying around and was inquiring if I could use it. Please forget I said that as I already got the idea several posts ago that it was not suited for what I need.

A GFCI breaker would simplify things, one less part to buy. Thanks. I didn't know such things were available, stupid me. The GFCI breaker would provide short and ground fault protection, right?

To sum things up again, all I should need is a properly sized GFCI breaker, properly sized wire to the 240V outlet, I'm thinking #8 and maybe a switch for the outlet.

Also I will need the motor specs and Article 430 of the NEC which I can get at my local library.

    Bookmark   February 5, 2012 at 3:01PM
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mike_kaiser_gw

From what I read, the compressor draws 21.5 amps. 125% for a motor circuit is 27 amps. A 30 amp circuit with #10 wire should work.

Of course, you may want to check the name plate on the compressor to confirm the amperage.

    Bookmark   February 5, 2012 at 6:31PM
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brickeyee

"A properly sized GFI will be needed as an outlet."

GFCI protection is not required for 240 V loads.

"From what I read, the compressor draws 21.5 amps."

If the motor is horsepower rated in the nameplate you must use the NEC table to determine the running current, not the nameplate.
The table indicates 28 amps.

The wires must be 125% of the current, so 35 amps.
Depending on temperature rating of the conductors and connections you may need #8.

The breaker can be sized at up to 350% (inverse time breaker, AKA 'thermal magnetic') of the full load current to allow starting without tripping the breaker.
That means you can use a breaker as large as 98 amps (actually 100 would be allowed).

If the compressor has an 'unloader' for starting you might get away with 40 or 45 amps.
If you use it heavily (frequent motor starts) you might need to go even higher.
The KVA locked rotor code letter would indicate how large the starting draw is going to be.

    Bookmark   February 6, 2012 at 10:51AM
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rswojo

"40A is way oversized for a 5HP motor at 240V. #6 will feed enough for the 5HP, but what else is on the panel?"

"The breaker can be sized at up to 350% (inverse time breaker, AKA 'thermal magnetic') of the full load current to allow starting without tripping the breaker.
That means you can use a breaker as large as 98 amps (actually 100 would be allowed).

If the compressor has an 'unloader' for starting you might get away with 40 or 45 amps.
If you use it heavily (frequent motor starts) you might need to go even higher.
The KVA locked rotor code letter would indicate how large the starting draw is going to be.

So now my 40A breaker is looking a little more "usable"?

    Bookmark   February 6, 2012 at 6:29PM
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electricalkid

Keep in mind, his 6 AWG feeding the shed is protected by a 100A breaker.

    Bookmark   February 6, 2012 at 8:33PM
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brickeyee

"Keep in mind, his 6 AWG feeding the shed is protected by a 100A breaker."

While that is likely incorrect conductor sizing, it will not have any affect (beyond voltage drop at start-up).

The problem with long runs to induction motors is one of those 'try it' type things.
Depending on how often the motor is started you MAY need to increase the breaker size (and the NEC allows this).
Starting lower is at least marginally safer, but it can result in having to replace the breaker, or even replace it if the use of the motor changes).

Motor circuits are one of the things that the general rules about wire size an d breaker capacity no longer apply.

The OP has the correct idea that the motor protects itself against overload with built in thermal protection and thus cannot overload the feed, and all the breaker is doing is protecting the feed conductors against short circuit damage.

125% of full load motor current for the wires, and up to 350% of the full load current for the breaker.

So yes, you can see breakers significantly larger than what is typical for general use branch circuits based solely on wire size.

    Bookmark   February 7, 2012 at 11:16AM
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rswojo

No, it's not going to work. The 5 hp compressor is most likely a capacitor start/capacitor run motor with an LRA of 7.09. I would need about a 120A breaker I figure. If I got a 3 hp compressor it would work with a 100A breaker but I still wouldn't have maximum torque for my air wrench.

Those bulldozer bolts are torqued, I was bending a 1/2 inch breaker bar on them. I'm not going to spend the time, effort and money to attempt a one-time project so I will just haul the dozer somewhere and pay someone else to do it.

If I had buried the proper size wire to the shed it would be sure to work and I'd do it. But with the #6 it might work and it might not. If I spent mucho dinero on a 3 hp compressor and it didn't provide enough cfm for the air wrench I'd just have a nice compressor that wouldn't do the job I bought it to do. Too much of a gamble.

Thanks all.

    Bookmark   February 8, 2012 at 5:53PM
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lbpod

If your old compressor can get up to 150 psi,
then it can handle a 3/4 inch impact gun.
The problem is getting enough air to the gun.
you will need a 3/8 in. hose and fittings.
Depending on the size of the compressor's
air tank, it might not run the gun for a long
time, but maybe it'll break the bolts loose.
I'm sure you've used a good penetrating oil,
but maybe a 'hot wrench' would help.

    Bookmark   February 9, 2012 at 1:52PM
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brickeyee

"I would need about a 120A breaker I figure."

Thermal magnetic breakers allow a larger current to pass for shirt intervals for things like motor starting loads.

A 50 A breaker should allow 100 amps for a few seconds, and your motor should not take that long to start.
The undersized feed MAY cause problems though.

The LRA puts an upper limit on the peak starting current, but thermal-magnetic breakers are designed to deal with this.

If you could find a straight magnetic breaker for your panel, you would be allowed to use up to 700% of FLA since magnetic breakers trip VERY quickly on even short overloads.

    Bookmark   February 10, 2012 at 3:08PM
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