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I'll be newly wiring a room on the second floor, in this case putting in outlets. The house is old, balloon frame 2x4s, with a double 2x4 top plate that the rafters rest on.

I have 250 feet of 12/2 mc cable that I was going to use to do the electrical that I got from a friend for free.

I am presently planning on running the cable along the rafter ties, down the rafter, through the top plate, and then back up and along to the next outlet. This means two holes in the top plate per outlet.

I'm just a bit uncertain about drilling through the double top plate. These are true 2x4s, but even so, the mc cable has an exterior diameter of around half an inch. That means 5/8 or 3/4 holes. Do two holes of either size present structural concerns that warrant a reinforcement of some kind? I know for notches in the top plate(s), you need to put steel reinforcement on, however I can't find a clear answer with holes of this size. Is there an optimum place to put the holes? In the middle, equidistant from eachother and from the studs?

Any assistance(or general advice) would be welcomed.

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  • have you ever worked with MC before?
    – RadioSpace
    Nov 29, 2020 at 16:17
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    @RadioSpace No. I haven't. I've used Romex before though, so I'm familiar with the general principles of wiring. I've familiarized myself with everything I can regarding MC cable, like fastening requirements, anti-short bushings, use of metal boxes, different fixtures to attach the cable to the box.
    – ConfusedToad
    Nov 29, 2020 at 17:10
  • oh cool. only thing i have is its very dirty. your gonna wanna get gloves. and protect any surfaces you'll be working around, it will mark walls and fabric. it's kind of a gray greasy residue. have fun
    – RadioSpace
    Nov 29, 2020 at 17:52
  • We’re you able to get your job completed? How did it turn out?
    – Ed Beal
    Jan 13, 2021 at 20:25

3 Answers 3

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I would be placing a junction box in the attic and only running 1 line down to each receptacle.

Running up and down is a waste of wire and if the walls are not open a lot more work fishing metal clad can be a pain in the back side especially if there is insulation. Having a 4x4 box makes a great junction box get a deep box with all 1/2 or 1/2 -3/4 TKO and you will only need 1 box for most rooms.

When I usually do this I many times add a raised single duplex cover and add a convince outlet it can be handy to have if there are no outlets up there.

With balloon framing you need to reduce the amount of damage (drilling) I don’t work with balloon as often as it is somewhat rare but you need to be aware additional holes will compromise the structure.

Make sure to use the insulating bushings that came with the MC inspectors look for this if your roll doesn't have a bag get some they are only a couple of $ for 50 or 100 I cant remember how small the bags come.

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  • I considered using a junction box, but I had some reservations: 1. There isn't really room in the attic to put a junction box. More accurately, there is room for the box, but once a ceiling is put in place, it would be totally inaccessible, as the attic area is much too small for me(or most adult males) to squeeze into. 2. I thought about getting some kind of junction box that sits on the wall, and doing the junction box there so it could be easily accessed. 3. The room requires 9 outlets. It would seem that the junction box would be super busy if done that way.
    – ConfusedToad
    Nov 29, 2020 at 17:26
  • With 9 receptacles I would use 2 boxes or go to larger boxes you can shoot 2 screws through the back of the box to mount it in the rafters or ceiling joists ether is fine but it needs to be accessible. #12 wire counts for 2.25 cubic inches count each hot and neutral and 1 for all the grounds. With the area being that tight I probably would not add the raised cover and receptacle. I mentioned the wire cubic size because depending on the size of the box you are limited to the number of wires. I use snap in connectors (no clamp inside the box) if a clamp inside the box 1A 2.25 cu inch adder.
    – Ed Beal
    Nov 29, 2020 at 17:49
  • I'll go with a larger box, but it is going to have to be wall mounted. I was debating between snap in connectors and Wago's lever nuts. I think I'm partial to the lever nuts, as they'd be easier to get out and fiddle with should changes be necessary. Wago 221 Lever Nuts Since those things only come with up to 5 ports, can I simply split each feed conductor using a 3 port, then go to 2 5 ports, and then to the outlets? As far as I can tell, nothing prevents this, as they're rated at 32 A, and it is on a 20 amp breaker.
    – ConfusedToad
    Nov 29, 2020 at 18:28
  • I do like the lever locks but after many push in stab failures I only use them on lighting (old habits). Yes they are rated for a 20 amp circuit. I was talking about the MC connectors , I strip 8” put a bushing on slip a snap on connector on the cable then put that in the box until it snaps in place. I don’t like the type that have the screw because I have found them over tightened two many times and not holding.
    – Ed Beal
    Nov 29, 2020 at 18:37
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The two codes that you'll need to mind in the US are the building code and the electrical code.

The electrical code wants your bored holes to be at least 1-1/4" away from the edge face of the studs so the cables are safe from drywall screws etc. If you need to bore a hole closer, you can use a protective plate, readily available at any hardware store, big box store, or electrical supply. But with 3/4" holes in a 4" wide plate, it's not an issue.

The building code has rules regarding boring holes in plates, but again 3/4" holes are nowhere near the limit so you're fine.

So boring those holes is a non issue, just make sure the holes are centered in the plate or close to it.

In another answer, @Ed Beal mentions putting junction boxes in the attics and dropping one cable to each box. This is the way to go if you need to conserve cable. It also gives you junction boxes to tap in the attic if need be in the future, and it can simplify feeding switches and ceiling lights.

On the other hand, it is more splicing, and every splice is a potential failure, so if you have cable to spare, you might do it as you planned, running one cable down and into the box, another out of the box and up.

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  • Actually the same number of splices because there is a splice in the box OR at the receptacle, but that’s not the issue if the walls are closed and fishing a cable MC is the toughest by far and can be almost impossible with fiberglass insulation as the ribs tend to collect the insulation and can stop a pull since a balloon frame there may also be fire brakes more often than other building methods again harder to navigate through with MC that is a big part of why I suggested the box(s) but I do this with remodels in most cases especially with closed walls.
    – Ed Beal
    Nov 30, 2020 at 17:54
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International Residential Code §R602.6 covers drilling & notching studs and top plates. No clue if that's the adopted building code in your area, but it's a good starting point.

You can drill or notch the top plate up to 50% of its width. That's more than enough for the MC.

If you ever need a bigger notch/hole for other projects (e.g. plumbing), the IRC allows bigger holes provided there is structural sheathing on that wall, or you install a galvanized metal tie: IRC R602.6.1 top plate framing to accommodate piping

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