Need to enlarge hole in wood top plate: or use flex with minimum OD.

mysterylectric · 08-09-2011, 08:12 AM

Hi, I've got a legacy hole about 7/8 diameter which was drilled through a wooden bottom plate at an angle. It's ideally placed. I need at least 6/3+G through this hole, and would very much prefer a metallic conduit or metal clad cable.

I can just about pound a piece of 3/4" ENT through it but... messy.

I have plenty of 3/4" LFMC but it's a bit too large: may fit if I strip the jacket from that portion. Also messy.

So what's wanted is either to expand that hole to at least 1", or possibly go with a type of metallic flex conduit or cable with the smallest outside diameter in a 3/4" trade size.

The hole goes a good 4 inches or so through solid wood. In the past I've had trouble enlarging holes of this size under the best of conditions due mostly to difficulty in centering. These conditions are bad due to the steep angle, and immovable objects which give limited clearance.

Sooo... what kind of bit would be best? Ideally it would have a spade (playing card spade), spear-tip, or similar profile, with one or more twists. I'm hoping such a shape would center itself and "ream" through the existing hole while allowing some mis-match of drilling angle.

Seems to me I've seen these bits offered to the trade and may have used one, at least with a very long somewhat flexible shaft: but I can't remember details or manufacturers. A tapered reamer might work but I've never used one for wood, sounds like a headache. A "builders reamer" bit looks good but cost $100+.

Barring the ideal drill bit, what should I be looking for in terms of metallic flexible cable/conduit with the smallest OD? I've gotten a few specs from mfr on MC type cable, it's right on the edge. What else should I be looking at?

Thank you in advance!

-Jeff

brric · 08-09-2011, 08:27 AM

What is beyond the exit end of the existing hole? A 1" auger bit will work but often times they will grab and go too far. Good ones are pricey too.

Jim Port · 08-09-2011, 08:33 AM

Does your area not allow type NM-B cable? Are you doing this for physical protection?

Missouri Bound · 08-09-2011, 03:52 PM

You can enlarge a hole with a spade bit...it becomes the shape of a "Wankel" engine. It wobbles around in the hole. A long hole saw would be your best bet for a truer hole...not that you need it in this situation.