I figured I'd tap into the collective knowledge out here

I need to get 220 into my garage for a compressor. I figure I can just run some conduit from an existing subpanel, do the wiring and then have an electrician come out and hook it up (~$50). When I looked at the subpanel, I thought it had plenty of space as there were a lot of "slots" that had not been used. I opened the thing and there does seem to be available space. I've done other wiring before, but nothing from the box and want to make sure I would be running the wire for nothing. Here is what the subpanel looks like:

http://img.photobucket.com/albums/v4...k/SubPanel.jpg

A close up of available space:

http://img.photobucket.com/albums/v4...bBob_Close.jpg

Am I OK? Is there anything else I should be concerned with? One compressor I am looking at specifies a 40 AMP Breaker. Would 12 gauge wire do it, or would I need something heavier.

Thanks!

 

There is room at the bottom right of your panel for a two pole breaker. If you choose a 40 amp breaker to fill this spot, you need to use 8 AWG wire. While article 430 might allow you to use 10 AWG for the running load amps of your motor, I believe there is a separate section of the NEC that limits 10 AWG branch circuits to no more than a 30 amp overcurrent protective device (and 12 AWG to 20 amps, and 14 AWG to 15 amps).

The 8 AWG wire will help with the starting load on the compressor motor.

 

Thats a cutler hammer CH series panel..... I work with those every day. Fefarms is right, you have just enough room on the bottom right. BTW, 12 gauge wire is only rated for 20 amps.... 14 gauge is rated for 15.... and so on. So 8 gauge would work well for you.

 

Section 240.4(D) is what normally limits 10AWG branch circuits to 30 amp breakers. But it contains a specific exemption for "motor circuits" (such as your compressor), and points to article 430 for sizing the branch circuit short circuit/ground fault protection. Depending on the running load amps of the compressor, and the overload protection embedded within it, you might be within the bounds of the NEC to use a 40 amp breaker and 10 AWG wire. For a short run, this is safe and should work fine. But I still wouldn't do it; I'd put in the 8AWG wire regardless. In my own shop, I used 6 AWG wire to hook up a 5 HP single phase compressor, but then I had a bunch of spools of it laying around.

 

like they said above you have room but you might want to come from your main panel as it looks like the sub only has a 60 amp feeder and the panel is full of 20 a breakers.you may or may not have enough capacity to start the compressor if the other loads are high or unbalanced.i'd also use #8
pete

 

Well, it sounds like 8 is the way to go. I was also concerned with the fact that the thing is nearly full, but to access the main would be a major effort. This is a slab home and the main panel is completely on the other side of the house. Besides a few breakers for lighting in the panel, the other ones are for: GFC for our whirlpool tub (nice to have but we have only used that thing about 3 times in the 10 years we have lived there); 2 for the furnaces (not high use items in Austin); 1 for the washer (this would get used). With that I'm thinking that should not be much of a problem.

From what I have read I should run a 3 wire (or 2+1). The compressor I'm leaning towards has hook-ups like:



I can understand the 2 hots, but it also references the 2 grounds. Would I need a 4 wire for this? I thought that 4 wire is only needed where there is a dual (110 & 220) power requirement.

Also, I'll need to put a switch up line of the compressor (these things don't have on/off switches ). Is that just a standard light switch, or is there some heavy duty switch that would be required.

One more question. It does seem that this should be hard wired rather than putting an outlet at the end. Although I'd prefer an outlet (I'm thinking I might get a welder in the future), my main priority is to do the compressor correctly. To wire the compressor to the junction box, I'd imagine I'll need a heavy duty cord (like a dryer cord). I'd also think that it would have to be 8 gauge also. Is that the case?

Thanks for all the information. I just want to make sure I have all the right stuff in place for when I get the electrician out there.

 

A pre-packaged compressor like the ones you are looking at usually does have a manual on-off switch, typically associated with the pressure switch. This lets you stop the compressor from cycling when you are not using it. With bigger models, it should trip the unloader, but not all of them do. It is always a good practice to wait for the compressor to fill up and shut itself off before turning off power to the compressor. This lets the unloader work so that the motor will not have to restart against the resistance of the compressed air.

You need a disconnecting means for the compressor and some sort of flexible electrical connection between the vibrating compressor and the stationary building.
There are a lot of ways to do this.

One way to do both at once is to go to Home Depot and buy a drier or range cord. Most of these are 4 wire now, but you may be able to buy a three wire, which is all you actually need. Hook the green wire to one of the two ground screws, black to L1 and Red to L2. The white (if present) is not used and the loose end should be insulated in the compressor connection box. The range cords will be rated for 40 to 50 amps, the drier cords for 30 to 40. Either is adequate for this particular compressor.

Get a matching receptacle from the next aisle over.

If the subpanel is "within sight" of the compressor, you could use the circuit breaker as the disconnect., and hard-wire the compressor with flex (armored) cable. This is the minimal code compliant solution.

Another option is a "molded case switch". Home Depot sells these pre-packaged in a weatherproof box for use as outdoor air conditioner compressor disconnects. They are fairly cheap, and usually rated for 50 or 60 amps. You can wire your air compressor directly to this with flex, or use it in addition to your drier cord.

The pull-out disconnects in the same aisle satisfy code and are a little cheaper, but these things always seem kinda cheesy to me.

 

You need a piece of 8-3 wire. This includes two hot wires (Black and red), a white wire and the ground wire. Basically the white wire IMHO is a waste of money, because it acts as a backup ground. So you could go with 8-2 wire, which includes 2 hots and the ground.

As far as the switches, I'm sure they have them somewhere. Your compressor doesn't have a switch built into it? Mine does, but doesn't work sometimes, so I shut the breaker off anyways.

 

Once again. Thanks for all of the information! What a fantastic resource these forums are. I'll be going with the dryer or range cord and an outlet. fefarm is probably right (why not he has been on everything else ) as is handyman. There probably is a power switch. I did not look for it in my initial look and did not see any reference to one in any documentation. I can do the same thing and use the breaker if necessary as the sub box is right next to the door. Hope to pick the thing up this weekend (along with an engine stand) so I can get started on my project!

 

As was pointed out by the "very knowledgable" responders to your post, most compressors that size do have some type of factory equiped switch, many times it is a "knife switch" in an enclosure and mounted some where on the skid but if you do not have a switch and are going to install one, as a suggestion as a minimum you might want to consider a "Double Pole" Switch (DPST) rated for the appropriate Amps, a furnace switch is similar to what you could use (heavy duty light switch). A better option in my humble opinion is a Box mounted Double pole knife switch. It's wise to steer clear of single pole switches that could be used but they leave one side of line hot even when it's off.

Good luck & Enjoy the "Fresh air".

 

A single pole switch would not be code compliant if part of the wiring installation for a 240v circuit.

 

I agree with midlf. That would not be up to NEC or safe for that matter. The only thing that MIGHT work is if you installed 2 single pole switches, one for each hot leg, but that's still not even all that safe.

Your air compressor should have the switch on it. There should be a 2 position switch (Auto and off.) If you didn't have that switch, then the compressor would probably run until it kicked out the safety valve.... which is NOT a good thing.

 

By the looks of your service panel there is only room left for a single pole breaker. You need a 2 pole breaker, so you'll have to look for some "mini's" to make some room for the 220 breaker. Some municipalities don't allow mini beakers, if you're having it inspected.

 

2fords- he does have room for the 2 pole breaker on the bottom right. It will get one leg from the 20 amp breaker thats across from it, and then the other leg is underneath.

 

Looks like you don't have a whole lot of room if you ever get into a welder etc. Do use a knife switch (DP) for the comp, not a good idea to use the breaker as a switch, and if you use a plug-in eventually the arc will ruin the socket. The use of a switch will allow you to shut off the comp easily when using the welder.