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WoodworkerDave 04-04-2008 12:52 PM

Advice on basement shop wiring
Hi all,

I've been learning all I can about electrical wiring this past month in anticipation of installing a sub-panel and multiple receptacles in a basement woodworking shop I'm setting up. Here's a brief summary of what I plan to do:

Existing service panelboard has 200 amp main breaker and is full (actually is overfull by my count of circuits). My calculated current home electrical load is 87 amps. I plan to install a new 125 amp rated sub-panel with at least 16 to 24 spaces between the existing panel and the corner of the room (about 21" space to work with). I will feed it with a 70 or 80 amp double pole breaker from the main panel and will use 3 AWG wire for this feed (2 hots, 1 neutral, 1 ground) using 1 1/4" EMT.

Two walls of my shop are concrete basement walls. The existing service panel is surface mounted directly to the concrete. My understanding is that my new sub-panel should be mounted onto a plywood backer board which I should first mount onto the cement wall. I was planning on using 1/2" thick exterior grade plywood for this.

Does this sound ok so far? Is there any limitation on how close I can mount the sub-panel to the existing service panel? Any limitation on how close the new sub-panel can come to the adjacent room corner (another concrete wall)? Any recommendations on routing the EMT between the main service panel and the sub-panel? Can this just be a straight 3 or 4 inch shot between the sides of the panels or do I have to go to the top of the sub-panel? Thanks for any info. I can't find these answers online.

I'll have more questions to follow :thumbsup:

WoodworkerDave 04-05-2008 07:29 AM

No takers so far for my first questions. That's Ok, I have some more.

As part of my shop wiring, I plan to have multiple receptacles around the perimeter of the room to provide a flexible means of powering my equipment. I'm not quite sure how I will have my various power tools arranged (lathe, bandsaw, jointer, etc.) and want to be able to modify this arrangement in the future to accommodate changes. Incidentally, all my tools will be mounted on mobile bases so I can easily move them in my small shop.

My plan was to have two 240V circuits as well as a multiwire branch circuit (that will provide two 120V circuits feeding back on a common neutral) extending from the sub-panel in each direction around the room. I want the receptacles at a height of about 40 inches from the floor. This will be high enough to be above my mobile carts, workbench, etc. and low enough to be below my wall mounted cabinets and tool racks. These will all be 20 amp circuits and I plan on using 12 AWG THHN/THWN wire except for one of the 240V circuits, where I will use 10 gauge wire just in case I want to upgrade it to 30 amps in the future. I'll use a 10 gauge ECG wire to ground all the metal boxes and receptacles.

I plan to run these circuits within 3/4" surface mounted EMT which will run horizontally at a 40" height along the concrete walls. I will have several surface mounted 4 11/16 square boxes spaced along this conduit so that the conduit simply runs directly into the box and the next section of conduit picks up from the other side of the box.

When the conduit reaches my drywall covered stud wall at the corner, I will change over to flexible metal conduit and continue with the FMC now inside the wall and the boxes set flush with the wall surface.

My questions are:

1. What is the best way to change from EMT to FMC? Do I need to use a junction box or can I simply use an EMT connector similar to what you use to join two pieces of EMT together, or do they make something specifically for this connection (that screws onto the FMC and has a set screw for the EMT)? I know I would need the plastic protectors that prevent damage to the wires from the FMC edges.

2. Can I continue the FMC within the stud wall at the same 40" height from the floor, or do I have to bring it down to the usual 20" height that you use to run NM cable horizontally in stud walls and then bring it back up to 40" for the boxes?

3. Since I'm using multiwire branch circuits for my 120V receptacles, I think I will have to use individual GFCI outlets at each box. Otherwise, the common neutral will prevent the GFCI from operating correctly. Is this correct? I will need to use breakers that are physically tied together for the multiwire branch circuits.

4. Do you see any problems with what I am planning? Anything that you would do differently?

InPhase277 04-05-2008 10:04 AM

Everything you have said so far is right on. I will just say mind the clearance for your panels. 30" to the side and 36" in front. Also, they do make a changeover from EMT to flex, or you can use an EMT connector, a rigid coupling and a flex connector. Don't let any one section of flex be more than 6 feet long, and bond bond bond all your boxes.

In your MWBC, you will indeed need a separate GFCI at each location unless you run two neutrals.


J. V. 04-05-2008 11:14 AM

Which article applies to flexible conduit not to exceed 6 feet?

InPhase277 04-05-2008 12:12 PM


Originally Posted by J. V. (Post 113990)
Which article applies to flexible conduit not to exceed 6 feet?

Oops. Looking now it appears that smaller than 1/2" shall not be used in lengths over 6'. My bad.


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