Can the wiring be run over the top of the ceiling joist, or must it be run through bored holes when running perpendicular to the joists?

This is a non-habitable space, will have an attic entrance only for the purpose of blown-in insulation and is not tall enough to stand in. There will be no junction boxes, lights or any other reason to enter this space.

  • 2
    For what it's worth, never ever bore holes through prefab roof trusses unless you get an engineer to approve it.
    – Kris
    Commented Oct 15, 2015 at 1:11

6 Answers 6


Accessible or not

While you might call this area inaccessible because it's small, dark, and difficult to enter. In this case, the codes definition of accessible is whether or not there's permanent stairs or a ladder.

Cable run across the top of joists

If the attic is accessible by permanent stairs or ladder, any cable running across the top of joists will have to be protected. Protecting the cable is as simple as installing "substantial guard strips", which can simply be a 1x2 furring strip on each side of the cable.

Cable protected by 1x2 furring strips

If there's no permanent stairs or ladder, this protection only has to be provided to cables within 6 ft. of the entrance of the attic.

National Electrical Code

Chapter 3 Wiring Methods and Materials

Article 334 Nonmetallic-Sheathed Cable: Types NM, NMC, and NMS

334.23 In Accessible Attics. The installation of cable in accessible attics or roof spaces shall also comply with 320.23.

Article 320 Armored Cable: Type AC

320.23 In Accessible Attics. Type AC cables in accessible attics or roof spaces shall be installed as specified in 320.23(A) and (B).

(A) Where Run Across the Top of Floor Joists. Where run across the top of floor joists, or within 2.1 m (7 ft) of floor or floor joists across the face of rafters or studding, in attics and roof spaces that are accessible, the cable shall be protected by substantial guard strips that are at least as high as the cable. Where this space is not accessible by permanent stairs or ladders, protection shall only be required within 1.8 m (6 ft) of the nearest edge of the scuttle hole or attic entrance.

(B) Cable Installed Parallel to Framing Members. Where the cable is installed parallel to the sides of rafters, studs, or floor joists, neither guard strips nor running boards shall be required, and the installation shall also comply with 300.4(D).

Cable run through bored holes

You could always run the cable(s) through holes bored in sanw lumber joists, but you'll have to make sure the edge of the hole is more than 1 1/4" from the nearest edge (2" according to IRC).

National Electrical Code 2014

Chapter 3 Wiring Methods and Materials

Article 300 Wiring Methods

300.4 Protection Against Physical Damage.

(A) Cables and Raceways Through Wood Members.

(1) Bored Holes. In both exposed and concealed locations, where a cable- or raceway-type wiring method is installed through bored holes in joists, rafters, or wood members, holes shall be bored so that the edge of the hole is not less than 32 mm (1 1/4 in.) from the nearest edge of the wood member. Where this distance cannot be maintained, the cable or raceway shall be protected from penetration by screws or nails by a steel plate(s) or bushing(s), at least 1.6 mm (1/16 in.) thick, and of appropriate length and width installed to cover the area of the wiring.

NOTE: When dealing with engineered structural members, boring holes may not be an option.

  • Can you run many wires between two guard strips of do you need a pair of guard strips per wire? Commented Jan 6, 2018 at 2:41
  • @ChrisMagnuson you can have multiple cables between the guard strips. Not sure if there's a limit to how many, or what the limit is (don't have the code handy).
    – Tester101
    Commented Jan 7, 2018 at 0:30
  • My AHJ only allows 2" but these only have to be as large as the cable so a 1/2" board ripped in 1" wide strips makes the inspector happy as code just states substantial. don't step on them they will break quite easily +. Your inspectors may want something stronger.
    – Ed Beal
    Commented Jul 18, 2018 at 21:18

There are two considerations for this.

  1. Attics without permanent access stairs/ladder.

    • You must protect cables located within 6' of the attic access hole.
  2. Attics with permanent access stairs/ladder.

    • You must protect all cables running perpendicular and atop the joists.

Ultimately, for type NM (Romex), this is directly referenced in the 2011 NEC 334.23 but only points you to comply with 320.23 (type AC), as does 330.23 (type MC). It is 320.23 (A) which specifies the requirements for "Cables Run Across the Top of Floor Joists" - "In Accessible Attics".

Yes, you can run cable through bored holes in the joists if that's what you prefer. Rarely have I worked on a job where the amount of cabling was substantial enough to warrant engineering/structural integrity concerns. Pipe - yes; typical Romex - no.

Having said that, you obviously can create such a situation if you do it excessively over a small span.

  • 4
    Another consideration is is the framing trusses or conventional framing. Trusses for the most part cannot be drilled. Commented Jan 12, 2015 at 21:59

Normally you would have two parallel runs that are perpendicular the ceiling joists. You want to space these runs over the load bearing walls on each side of the hosueb so that you can drop right in where needed. For most truss systems this will end up being close to on right on where your inverted V first hits floor on each side. The individual circuits/runs just need to be a couple inches apart if that.

This really does two things. First like I said earlier is that it allow you to drop in via top plate on you load bearing walls. Most importantly though is that it is orderly and keeps a clear walking path in your attic.

There is nothing wrong with running your electric on top of joists but you really should try to do it in areas that aren't going to be exposed to any foot traffic. If you need to go parallel then simply run them a few inches down on the side of a joist.

As for boring holes. I would almost always stay away from this in attics. There is no use in possibly hurting the integrity of your structure to run electric.


I always run a 2, strips of strapping along side my runs in a attic. Always 1ft for the tight sloped side of the roof to make sure nothing can end up damaging the wires.

  • 2
    Hello, and welcome to Home Improvement. It isn't completely clear what you mean; would you edit your answer to improve it? And, you should probably take our tour so you'll know how best to contribute here. Commented Nov 6, 2019 at 23:57

I run my wires on top as long as the height from top of joist to rafter is under 3 feet. If it is 3 feet or more and there is not a better way I will drill holes through the joist.


The "average" electrician will just leave them unsecured (obviously dangerous).

If you hire an expensive electrician, he will drill holes through the joists.

The proper method is to use running boards, which are two, parallel 1x2 boards that are nailed into the joist on either side of the wire. Electricians will not do this because they are not carpenters.

(Personally, I consider Romex to be strictly for hacks and always use thin wall EMT, so I don't even have problems like this.)


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