Field tested: running NM cable through joists

Hole placement in the joist

The hole is where most jobs go sideways. NEC 300.4(A)(1) is clear: bored holes in wood members must keep the nearest edge of the hole at least 1 1/4 inches from the nearest edge of the stud or joist. If you cannot hit that distance, the cable has to be protected by a 1/16 inch steel plate or bushing.

Center the hole in the depth of the joist when you can. Staying in the middle third keeps you clear of the top and bottom tension zones the framer cares about, and it keeps you clear of drywall screws and subfloor nails. On a 2x10, that is roughly 3 inches up from the bottom edge.

• Minimum 1 1/4 inch setback from the nearest edge, per 300.4(A)(1).
• Use a 3/4 inch or 7/8 inch auger for 12/2 and 14/2 NM, 1 inch for 10/2 or doubled runs.
• Keep holes in line with each other so the cable does not snake up and down between bays.

Notching vs boring

Boring is almost always the right answer. Notches are allowed under 300.4(A)(2) but they require a 1/16 inch steel plate over the notch, and most framers and inspectors hate seeing them in joists. Engineered I-joists and LVLs are a different animal entirely, see the manufacturer's hole chart before you touch them.

For sawn lumber joists, IRC limits notches to the outer third of the span and caps notch depth at 1/6 the joist depth. If the notch is in the middle third of the span, it is not allowed at all. Bore instead.

If the GC tells you to notch a 2x10 in the middle of a 14 foot span so he can land his HVAC trunk, get it in writing. That notch is a structural problem, not yours, but your cable is the thing that gets blamed when the floor squeaks.

Securing and supporting the run

NEC 334.30 requires NM to be secured within 12 inches of every box and supported at intervals not exceeding 4.5 feet. When the cable passes through bored holes in framing, the holes themselves count as support, so you do not need a staple at every joist, just at the terminations and any point where the cable leaves the framing.

Parallel runs along the side of a joist need to be stapled flat. Stacked or bundled NM triggers the ampacity adjustment in 334.80 once you have more than two cables bundled together for more than 24 inches, so keep your fill reasonable when you are ganging home runs out of a panel.

• Within 12 inches of every box, per 334.30.
• Every 4.5 feet on open runs.
• Flat against the framing member, never on edge.
• Watch bundle fill to avoid 334.80 derating.

Crossing plumbing and HVAC

The plumber is going to drill the same joists you are. Get there first when you can. When you cannot, keep your cable on the opposite side of the joist bay from copper and PEX so a future leak does not soak your NM, and keep it well away from hot flue pipes and uninsulated hot water lines.

If your cable has to share a bay with a B-vent or furnace flue, maintain the clearance the appliance listing calls for, usually 1 inch for a double wall B-vent. NM jacket is rated 60C and the conductors are 90C terminated at 60C, so radiant heat from a flue will cook it fast.

Fire blocks, top plates, and penetrations

When you drop down from a joist bay through a top plate into a wall, 300.4(D) still applies: keep the cable 1 1/4 inches back from the face of the plate, or protect with a steel plate. Double top plates are two chances to nick the cable with a drywall screw, so stack your holes and feed carefully.

Fire blocking under IRC R302.11 has to be restored any time you penetrate it. Use an approved fire caulk or putty pad rated for the assembly. In a two family or multifamily job you are almost certainly in a rated assembly, check the UL listing for the penetration method before you close it up.

1. Drill the top plate centered in the wall, 1 1/4 inches minimum from each face.
2. Feed the cable without kinking it on the plate edge.
3. Staple within 12 inches of the box below.
4. Fire caulk the penetration if the assembly is rated.

Inspector-ready checklist

Before you call for rough inspection, walk the run. Look for daylight through any hole where the cable is closer than 1 1/4 inches to the edge, check that every staple is seated flat without crushing the jacket, and verify the cable is not running across the bottom of joists where future work will hit it.

Most red tags on NM rough-ins come from three things: missing staples near boxes, unprotected cables within 1 1/4 inches of a framing edge, and bundled home runs without derating considered. Fix those three and you are clean.

Take photos of every penetration through a rated assembly before the drywallers show up. When the AHJ questions your fire caulk six months later, that phone photo is the only evidence you have.

• 334.30 support and securing verified.
• 300.4(A) and 300.4(D) clearances honored or protected.
• 334.80 derating checked on bundled runs.
• Fire blocks restored per R302.11.