Quick reference for running NM cable through joists

Bore, Don't Notch (When You Can)

NEC 300.4(A)(1) is the rule that gets cited in every rough-in inspection. Holes through wood joists must be at least 1-1/4 inches from the nearest edge. If you can't keep that setback, you protect the cable with a 1/16 inch steel plate or sleeve. No exceptions on dimensional lumber.

Notching is allowed under 300.4(A)(2), but the notch depth is capped and the cable still needs the 1/16 inch steel plate covering it. In practice, boring is faster, cleaner, and survives the next trade walking through with a nail gun. Save notches for situations where a hole isn't physically possible.

Carry a stack of 1-1/2 inch nail plates in your pouch. Cheaper than a callback when the drywaller hits your home run.

Hole Size and Joist Integrity

The NEC governs cable protection, but the IRC and the engineered lumber manufacturer govern what you're allowed to drill through. For solid sawn joists, the common rule of thumb is holes no larger than 1/3 the joist depth, kept out of the middle third of the span, and at least 2 inches from the top or bottom edge. Always defer to the structural engineer or the stamped print if there is one.

Engineered I-joists and LVLs have their own rules printed on the web or in the manufacturer's hole chart. You cannot drill the flanges. Period. Knockouts are sometimes pre-stamped on the web, and there is usually a clear zone near bearing points where no holes are allowed.

• Solid lumber: max hole 1/3 of joist depth, 2 inches from edges.
• I-joists: web only, never the flange, follow manufacturer chart.
• LVL and glulam: do not drill without engineer approval.
• Trusses: never cut, notch, or drill any chord or web member.

Cable Support and Securing

NEC 334.30 requires NM cable to be secured within 12 inches of every box and at intervals not exceeding 4-1/2 feet. When the cable runs through bored holes in framing, the holes themselves count as support, so you do not need a staple at every joist bay. That is one of the most common new-installer misunderstandings.

Where cable runs parallel to a joist on the side or bottom, it must be secured to the framing member. Running it loose along the bottom of joists in an unfinished basement is governed by 334.15(C), which sends you to 320.23 for protection rules. Smaller cables (two 8-2 or smaller, or three 6-2 or smaller) typically need a running board or guard strip if they cannot follow the framing.

Bundling, Stacking, and Ampacity

This is where a lot of installers get bitten on inspection. NEC 334.80 and 310.15(C)(1) require ampacity adjustment when more than nine current-carrying conductors are bundled together for more than 24 inches, or when NM cables are stacked through the same hole and pass through fire-rated sealant or insulation.

If you are running a panel feeder bundle from a basement panel up through a single bored hole into the floor above, count your conductors. Three 14-2 cables stacked through a foam-sealed top plate counts as six current-carrying conductors and you are still fine. Add a few more home runs and the math turns. Spread the cable across multiple bored holes when you can.

If you ever ask yourself "is this too many cables in one hole," the answer is yes. Drill another hole.

Bend Radius and Damage Protection

NEC 334.24 sets the bending radius at 5 times the diameter of the cable. For 12-2 NM that's roughly 2-1/2 inches of radius at the bend. Sharp 90-degree bends right out of a stack of cable on the truck are the fastest way to fail an inspection or strain the conductor jacket.

Cable run through joists in attic spaces below 7 feet of headroom needs guard strips per 320.23 if the cable runs across the top of joists. If it runs across the top of joists in any attic accessed by stairs or a permanent ladder, guard strips are required regardless of headroom. In accessible attics with only a scuttle hole, protection is only required within 6 feet of the access opening.

Field Workflow That Holds Up

Run a chalk line down the joist bay before you bore. Centered holes look professional, keep cables out of the way of plumbing and HVAC, and make the inspector's job quick. Use a ship auger or a self-feed bit sized for the number of cables you plan to pull, not just the first one.

Keep your home runs grouped and labeled at the panel end as you pull. Mark each cable at both ends with circuit purpose and room with a Sharpie before you stub into the box. The 30 seconds you spend now saves an hour during trim out.

1. Snap a line, bore centered holes, 1-1/4 inch from edge minimum.
2. Pull cable smooth, no sharp bends, watch for staples behind drywall.
3. Secure within 12 inches of every box, 4-1/2 feet between supports.
4. Plate any hole closer than 1-1/4 inch from the edge.
5. Label both ends before you leave the rough-in.