What are the code requirements for NM-cable (nonmetallic-sheathed cable or Romex®) in an attic?
Sunday, July 15, 2018
We often find damaged NM-cable in attics, especially where the cables are running near or across the scuttle opening, and here’s several ways that it happens:
- A homeowner pops up the access panel, sticks their head only barely into the attic, and slides boxes of Christmas decorations across the tops of the attic floor joists, scraping over the NM-cables around the opening. After many holiday seasons of sliding boxes back and forth, the sheathing and insulation get scraped away.
- Workmen entering and leaving the attic repeatedly walk on, or trip over, the cables running across where they step into the attic from the opening.
- A loose cable, like in the photo above, gets caught in the spring mechanism of a pull-down folding ladder at the attic opening and frayed.
Damaged cables can cause a short circuit or, even worse, arcing that will start a fire in the attic. For this reason, the NEC has protection requirements for NM-cables in an attic.
An attic that is “accessible”, which is defined by the NEC as having a permanently installed stair or ladder in place, must have protection for any cables that run across the top of the attic floor joists or within 7-feet where they run across the face of rafters or studs. A pull-down attic ladder does not count.
All other attics need protection within 6-feet of the attic opening, and this is the typical situation. The simple way to protect the cables is to run 1x2 or larger wood “guard strips” on both sides of the cable, as shown in the diagram below.
Where the cables are installed parallel to the sides of attic floor joists, rafters, or studs, no protection is necessary. Also, you can alternately choose to protect the cables by running them through bored holes in the side of the attic joists, but the bore holes must be sized and located to comply with requirements necessary to avoid structurally weakening the joists—which is a whole new can of worms. Plus, engineered structural members such as the bottom chord of roof trusses cannot be bored without an engineer’s approval. So we most often see guard strips installed.
One other way that NM-cables in an attic become damaged is when a homeowner lays down boards or pieces of plywood directly over the cables, compressing them against the top of attic floor joists. The edge of a board over a cable is especially problematic, because it tends to cut into the cable a little deeper each time the board is walked or crawled over. In the photo below, one cable is running under the plywood placed next to the attic scuttle opening and another one is located perfectly to be tripped over when entering the attic.
Whenever walk boards are installed, the joists should be notched just enough to create a pocket for the cables to sit before the walk boards are laid down—where notching of ceiling rafters is acceptable, but notching of the bottom chord of a manufactured roof truss is not allowed, except with specific instructions provided by the truss designer. As an alternative, guard strips can be run, or the cables can be routed around the area.
Also see our blog post What is the maximum spacing requirement for securing NM-cable (nonmetallic-sheathed cable)?
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