2 circuits on a 3 wire cable

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  #1  
Old 11-19-05, 08:55 AM
DIY ED
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2 circuits on a 3 wire cable

I would like to install 2 new 20 AMP circuits to my basement. The panel is in the garage, so I need to pull wire to the basement. I was wondering, could I safely use a 12 guage 3-wire w/ground cable, and use the white wire as a neutral for both circuits?
 
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Old 11-19-05, 09:07 AM
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What you are proposing is called a multi wire circuit. It is legal and saves wire as well as effort pulling the wire.

In order for this circuit to be legal and to be safe, the two hot wires must connect to opposite halves of the incoming 240 volt service. In this way the current on the neutral wire is the difference between the currents on the hot wires, and will be 20 amps at worst case. If you wire it wrong then the current on the neutral wire is the sum of the currents on the two hot wires, which could be as much as 40 amps in your case, which is a serious safety/fire hazard.

The easiest way to ensure proper connection in the panel is to use a 240 volt breaker. In certain (but not all cases) this type of breaker is required, so it's just easier to do this from the start.

With a multi wire circuit you need to be careful in regards to GFCI receptacles. Since you are talking about a basement, where GFCI protection is required if the basement is unfinished, it's a good idea to mention this. You can't attach a GFCI receptacle to a multi wire circuit and continue the multi wire circuit from the LOAD side of the receptacle.

If you do need or want GFCI protection then you either need to use a 240 volt GFCI breaker (expensive) or you need to make every receptacle a GFCI receptacle (using only LINE connections), or you need to split the circuit in the basement to two separate circuits using 12/2 for each.

One other caution. If you run the multi wire circuit through a receptacle you cannot run the neutral through a device, you must pigtail it to the device. This is to prevent the neutral from becoming open, which would possibly allow 240 volts on downstream receptacles.
 
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Old 11-19-05, 09:21 AM
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Yes you may, but there are special rules regarding 'multi-wire circuits'.

1) The black and red hots must originate from different legs of the incoming service. The easiest and safest way to do this is to use a double-pole 240V circuit breaker for the MWC.

2) If both hot legs of the MWC connect to the same device: receptacle, switch, etc; then you must use a double-pole breaker.

3) The shared neutral wire cannot be daisy chained through receptacles. It must be connected with wirenuts and pigtailed to the receptacles. This is to prevent the neutral connection from opening up if a receptacle fails. If the shared neutral opens, your plugged in appliances will be exposed to either low or high (up to 240V) voltage which will damage whatever is plugged in. Make sure the neutral connections are made well!

4) GFCI protection and multi-wire circuits do not always play well together. You cannot use the downstream protection feature of a GFCI receptacle on a MWC. There are a couple options, but it is usually a good idea to stick to seperate circuits if you need GFCI protection anywhere. Any receptacles in an unfinished portion of the basement need GFCI protection.

5) Multi-wire circuits can lead to confusion or appliance damage down the road. For most people the slight difference in running the initial wiring is not worth the hassle of getting the rules right for a MWC.

If running the wire is a challange, have you considered installing a subpanel for the basement? You could pull one feeder cable from the garage to a panel in the basement and run all of your new basement circuits to the basement panel.
 
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Old 11-19-05, 10:09 PM
DIY ED
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Originally Posted by ibpooks
Yes you may, but there are special rules regarding 'multi-wire circuits'.
<SNIP>
If running the wire is a challange, have you considered installing a subpanel for the basement? You could pull one feeder cable from the garage to a panel in the basement and run all of your new basement circuits to the basement panel.
Thanks for your reply.
Now I'm really not excited about starting this. LOL

The reason I was thinking of this, was because of the difficulty in getting any wiring into the basement. Pulling and fishing one cable would be easier, but I need at least 12 receptacles for a workshop and office.

I hadn't even considered the requirements of GFI, so thanks again for bringing that to my attention.

While installing a subpanel may be the better way to go, wouldn't I have to pull in a heavier cable to feed the subpanel?

Maybe I should just struggle with pulling in two 12/2 cables, so my brain doesn't hurt so much. LOL

Thanks again,
Ed
 
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Old 11-20-05, 06:45 AM
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If you are cionsidering a workshop then you probably want more than one or two circuits.

If you are going to pull one or two 12 gage cables, it probably isn't much harder to pull one larger cable, large enough for a sub panel.
 
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Old 11-21-05, 09:46 AM
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Originally Posted by DIY ED
I need at least 12 receptacles for a workshop and office
Although it would be legal to do so, I do not recommend putting an office on the same circuit as a workshop. Computer equipment and other office electronics should be on a circuit seperate from electric motors in power tools. After learning more about your situation, I think a basement subpanel is the way to go.

While installing a subpanel may be the better way to go, wouldn't I have to pull in a heavier cable to feed the subpanel?
Yes, a typical subpanel is 60A fed with 6/3 NM-B (Romex) cable. The 6/3 is certainly harder to work with than #12, but the upside is that you only have to do it once. After that, adding basement circuits is a breeze.

Maybe I should just struggle with pulling in two 12/2 cables, so my brain doesn't hurt so much.
That is also a valid solution. If you go this route, I suggest pulling in three 12/2 cables. Two for your workshop receptacles and one for your office receptacles plus lighting. If you put lighting on the same circuit as power tool receptacles, the lights will dim and flicker every time you switch on a saw.
 
  #7  
Old 11-22-05, 07:24 PM
the_fiddler
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I would agree you should either pull 3 12/2 wires and keep your office separate, or just pull th 6 ga and do a sub panel. I think in the long run the sub panel will be the easiest and most versatile way to go and I bet the larger cable will be easier to do than 2 or three 12 ga wires. If done right and neatly it will probably add much more value to your house in the mind of a prospective buyer too. I'd rather see a basement ready for anything than one that is already maxed out as is so often the case in older homes. My garage is annoying me this way right now.

F
 
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Old 11-22-05, 09:30 PM
DIY ED
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I really appreciate everyone's input. I have several things to consider.
I started pricing up the sub panel plan, and got a headache. LOL
I understand that it would be the proper way to do it.

My basement workshop is a small shop with light tools. The heaviest load comes from a 10" disc sander. Any heavy work is done in the garage, which is already wired.

Lots of thinking is happening.
 
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