Electrical branch switching... mutually exclusive.

[Arthur.Marks]

I have two 30A 220V circuits that runs into my garage. One runs my welder. The other runs an electric heater in the winter. I am looking at getting a 60gal. 3HP air compressor soon. There are no slots left on my main box for another 220V line---and I wouldn't want the expense anyway. 99% of the time only one of those two circuits is running at a time. I would like to make a branch circuit hard-wired in line with the outlet used for the heater.

So the question: How to wire it so you have a (manual) switch that allows only one of those two connections to be live? I guess the ideal would be something of a disconnect box that has three settings: Off/disconnected, Line 1, Line 2. So if you had the box set on "Line 1", one connection is live and the other completely disconnected. This would eliminate both machines from ever running at once.

I'm just not familiar with what is available outside of simple, single line disconnect boxes. Is there such a thing that will do what I'm talking about above? What is it called? I've looked through McMaster's selection and couldn't find a match.

Any info is appreciated.

[bruto]

What you need is a double pole, double throw switch. I'm guessing you won't find one cheap.

Here's one: http://www.galesburgelectriclighting...productid=3878


As an alternative which might end up cheaper, if you can dispense with the center-off function (or put up with a separate master off switch), you could find two double pole single throw switches of the correct amperage, and mount them side by side in a single box. Mount one right side up, and the other upside down, and gang the toggles together.

edit...note: you'll often find the switch types abbreviated. What you'd be scanning for in listings is DPDT.

[darryl]

There was a similar request here not long ago. Guy wanted to have the water pump take precedence over the air compressor, with both having to run from the same feed. In your case, the air compressor would take precedence over the heater.

It can be done using a relay and not much else, except a housing, connections, etc. I'll see if I can find that thread.

[Black_Moons]

Need some of these: http://i205.photobucket.com/albums/b...switch_002.jpg
Just joking.

Unfortualy, past 20A you can't buy standard 'light switch' style switchs, and the price goes way up.

It might even be cheaper to buy a 30A relay with 120v coils, and just wire a light switch to the coils.

You could also wire it up so that the thermastat or pressure switch actualy controls the relay, So that either the heater or compressor can override the power as needed. (Make SURE you have a good popoff safty valve on the compressor if you do this, Relays are known to 'stick' sometimes, Of course, So are pressure switchs since they are not much more then a pressure activated relay)

[darryl]

Here's a link to the thread I was thinking about.

http://bbs.homeshopmachinist.net/showthread.php?t=44596

[macona]

They make breakers that serve two 120v circuits on one breaker. Get a couple of these and consolidate a couple of your 120 circuits and use the newly opened slots for a new 240 breaker.

Edit: Like these:

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&productId=100356518&langId=-1&catalogId=10053&ci_src=14110944&ci_sku=100356518&cm_mmc=shopping-_-googlebase-_-D27X-_-100356518&locStoreNum=4018&marketID=54

[Optics Curmudgeon]

Look into generator transfer switches, they are well interlocked to prevent both circuits from being connected at once.

[Arthur.Marks]

Hmmm... the switch that bruno linked to would be a great, simple solution. The spec's are a little confusing, though. Generally, it is listed as 30A. Then further down it specifies Max. Amp. as 24 and 2HP AC motor limit. See HERE. I did find THIS(DPDT 2 Pole Fusible Switch Box), but it is a bit like Black_Moons' joke... with possible asbestos content

The generator transfer switches sound good, but they're friggin' expensive!! $500+ ain't gonna happen. So far it sounds like the option of taking two switches wired opposite and then linked together is where I'm currently leaning:

As an alternative which might end up cheaper, if you can dispense with the center-off function (or put up with a separate master off switch), you could find two double pole single throw switches of the correct amperage, and mount them side by side in a single box. Mount one right side up, and the other upside down, and gang the toggles together.

[CCWKen]

Well, I did a search but couldn't find the model number and I just got back from the barn. (Not going back out again.) I have a 50A/240v switch that comes in a metal box and powers my compressor. That's how it's turned on and off. I'm pretty sure I bought it at Home Depot because that's were I did most of my shopping for boxes and wire. It's a simple slide switch in a 5x7 metal box. As I recall, they were less than $25.

You could use two of those but then you'd have to provide the "mutually exclusive" logic.

[Weston Bye]

Put a small 2-circuit breaker box at the end of the wire. Connect each device to it's own breaker, then design a mechanical blocker flag that, when in one position blocks one breaker and allows the other, and when in the other position does the opposite. Just make it so that both breakers can't be turned on at once.

I see you are in Chicago - don't know if such a contraption would pass the City of Chicago electrical code, though.

[mickeyf]

make it so that both breakers can't be turned on at once.

While this would work, my understanding is that breakers are not actually designed to be used as switches. In a typical lifetime they would have many orders of magnitude fewer cycles than a switch. You might want to compare specs and see if you feel comfortable with that approach first.

[Arthur.Marks]

Okay, I've done a little more investigating....

I think I am misleading myself WRT the original link to the DPDT easy solution by Bruto: http://www.galesburgelectriclighting...productid=3878
The switch is rated 30A. My breaker is 30A. The heater plug / outlet is rated 30A... and the motor on the "3.2HP" compressor is rated 15A. So who cares? Everything should work fine with that switch. I think I'll follow the simplest route.

Thanks again, guys! This has been very helpful.

[Arthur.Marks]

Then again, this is the same thing at 1/3rd the price, right?

http://www.alliedelec.com/search/productdetail.aspx?SKU=8700591

[lakeside53]

You need to make sure that any switch you use is rated for the HP (not the plate current) of the motor, and has a rating at least as high as the current of your heater. I don't see any HP rating on that switch. The Levitron link showed a max hp of 2... Of course, depends on the actual motor used on your "3.2hp" compressor. If it really is 3.2, just use a contactor driven by a low rated switch.

[bruto]

Circuit breakers that are switch rated, or carry the "SWD" designation, can be used as switches without problems.

As far as I know all the standard Square D breakers are suited to switching flourescent lights whether or not they bear the SWD, but I haven't been able to wade through the literature on line yet to see if that means that they are "switch rated" in general.

Square D does list among its accessories an interlock "QO2DTI" to do exactly what is called for here.

I've been using the Square D breakers in my shop as light switches for 22 years with no problems, but again, I don't know whether this would apply to other types of loads.