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  #1   IP: 24.2.33.18
Old March 14th, 2012, 11:41 PM
iamjacob iamjacob is offline
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Default 220V Single Phase wiring

I'm in the process of making an air cleaner for my shop out of an old AC blower unit and some furnace filters.

The motor that I have is a 3/4hp, 3 speed, single phase, 220V motor and has a total 6 wires. Only 4 wires will be used, 2 go to the start capacitor and the other 2 are labeled "Line". The 2 unused wires are for the 2 unused speeds. The motor is also grounded to the shroud housing.

The thing that I want to know is, is it possible to power the motor using the hot lead from 2 separate 110V circuits?

I have 2 20A circuits that are dedicated to the shop area and the wiring is all 12/2. The label on the motor indicates it's only going to draw 4.6A so I would think that the 12/2 would be enough.

Do I need to just find a 110 motor or add a 220 circuit?

Thanks in advance!
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  #2   IP: 205.210.17.241
Old March 15th, 2012, 05:32 AM
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joed joed is offline
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It is possible if the two circuits are on opposite legs. Do not do it. It is a code violation.
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  #3   IP: 173.8.162.105
Old March 15th, 2012, 01:16 PM
iamjacob iamjacob is offline
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Thanks Joe,

Other than it being a code violation is it horrifically unsafe?

I installed the 2 shop circuits myself so I'm sure that there is probably other code violations that are present like the wires aren't stapled every 18". I will more than likely be removing the circuits if I ever move out so I'm not worried about it passing an inspection or anything, mostly I just want to make sure the house isn't going to burn down.

Can you explain how it is any different than an actual 220 circuit?

Thanks
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  #4   IP: 130.76.32.78
Old March 15th, 2012, 01:48 PM
suemarkp suemarkp is offline
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Half the answer depends on how you implement it, as ultimately a 240V circuit is just like 2 120V circuits side-by-side in the panel.

If you run 12-2 cable from the breaker panel and terminate each wire on its own 120V breaker (who are each on opposite legs of the service), it will work. All code would require is that you use a handle tied common trip breaker. This is required for all 240V devices, as it can be unsafe if the breaker on half the supply trips and the other does not.

A handle double pole breaker would be easy to retrofit if the 120V ones are side by side. You could probably keep the 120V circuits being fed from the same 2 pole breaker unless those 120V circuits are some of the few the code says can not serve things in other areas (e.g. kitchen counter circuit, laundry room receptacle circuit, bathroom receptacle circuit).

If you are somehow going to run an individual wire or use one conductor of a cable from separate 120V outlets, that violates the rules of all wires of a circuit being run in the same cable or raceway. In the old days of knob and tube wiring, the conductors were run separately can could be space feet or yards apart. If the wires go through ferrous metal enclosures, you can get inductive heating if they don't all take the same path (especially under fault conditions and in the knob-and-tube days there was no ground wire to clear a fault). It also can cause EMF field issues, but the jury is still out as to whether that is harmful or not.

Is it difficult to add a new circuit to this shop panel?
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  #5   IP: 173.8.162.105
Old March 15th, 2012, 02:32 PM
iamjacob iamjacob is offline
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When I wired circuits I put 4 separate 2 gang boxes around the shop. Each box contains a pair of outlets on each circuit, that way I could choose to use the circuit with the least load for what ever I wanted to plug in at the time. This means that the 12/2 wiring for both circuits pretty much run parallel to each other around the shop and my plan was to add a junction box to be able to tap into both hot leads.

Right now the breakers are half width 20A breakers that I would probably need to replace with full width so that they would go on different poles. I have no problem adding the bracket that ties the breakers together.

The panel is the main panel for the house and it has an unused 240 circuit for the previous owners spa. I could probably use the circuit and just add new wire but I was hoping that I would be able to use what I already have in place since it's such a small load. The 60A 240 circuit is a little overkill for the 4.6A motor.
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  #6   IP: 130.76.32.199
Old March 15th, 2012, 04:11 PM
suemarkp suemarkp is offline
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You can buy "mixed duplex" outlets that have a normal 15A 120V on the bottom, and a 240V 15A outlet on the top. They make these because it is easy to modify your circuits as you've wired them to this mixed 120/240 combination. Did you use a common neutral (12-3 cable) from the breaker and between each box, or two 12-2's? I'm hoping two 12-2's because of the tandem breaker. Ideally, you would have used 12-3 for a mixed 120/240V circuit (all wires of a circuit must be in the same cable jacket). But if both cables run together in the same knockout hole you should be OK.

Can you retrofit the receptacle feed so the first box gets fed with 12-3 and your 240V outlet goes there? If so, you could continue on with separate 12-2's to each outlet box as long as you don't want 240V in them. If you want 240V too, you should be using 12-3.

All you need to do is replace your twin breaker with a double pole breaker. Then you could replace one of your receptacles with the combo 120V/240V duplex type (it would need two hots and a neutral). If you don't have slots for this, you can get double pole twins for some panels.

One fly in the ointment is GFCI protection. Are your shop receptacles GFCI'd (or should they be)? If so, you can't use this mixed approach unless you buy a double pole 20A GFCI breaker (expensive) or use the LINE only side of the GFCI which means every outlet needs to be a GFCI type. If you are supposed to have GFCI protection, it may be easier to just run a separate 240V 20A circuit in the shop with receptacles in various locations. These are not required to be GFCI'd, and you only need 12-2.

You can't put a little 5 amp motor on a 60A circuit. It must be limited to 20A max.
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  #7   IP: 24.2.33.18
Old March 15th, 2012, 07:57 PM
iamjacob iamjacob is offline
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So at this point I'm a little confused.

You asked if I could retrofit the receptacle feed to the first box to run 12/3? I probably could but it would probably be just as easy to add the additional 12/2. Originally I ran 2 12/2 wires to each box. If I were to go the 12/3 route would I need a 20A 240V breaker instead of the 2-20A 120V breakers?

Ithink code requires GFCI in a garage but I didn't install any. The outlets are 48" off the floor and in the back section of the garage, the closest being 15' from the garage door (the double deep portion of the L shaped garage) so I'm not really concerned with any water issues.


As far as what your talking about as far as splitting the 240V to 120V, am I correct below?
1. I replace the 2 120V 20A breakers with a 240V 20A breaker.

2. I replace the 2 12/2 runs from the breaker box to the first receptacle with 12/3 and install a 240V outlet.

3. I split the 240 hots and add an additional common and ground to make 2 separate 120V circuits.

4. I connect the original outlets to the new 120V circuits.


Will the new 120V legs of the 240V circuit be able to run 20A? I have a few tools that run right at 15A so I would need the amperage.


I may just replace the 240v 50A breaker with a 20A breaker and run a dedicated 12/2 for the fan motor....

Thanks a ton for all the help!
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Last edited by iamjacob : March 15th, 2012 at 08:09 PM.
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  #8   IP: 50.135.128.55
Old March 15th, 2012, 09:24 PM
suemarkp suemarkp is offline
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What you propose would work and be code legal. However, as you point out, if you can change to 12-3 you might as well just run a new 240V circuit. Having the 240V separate is better because:
  • Using the blower will steal 4.6 amps from your 20A (and will steal from both circuits). You'd be overloading the circuits if you have some tools with 15A nameplate values and you run this 240V motor at the same time.
  • All 120V circuits in a garage are required to be GFCI protected. Your existing wiring is exactly right to implement that with just 2 GFCI receptacles. The reason for GFCI's is not water from the door, but the damp concrete floor.
  • You can buy more 240V tools since you're only using 5 of the 20 available amps.
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