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I'm adding a couple of outlets mounted to the underside of the rafters in our attic but I'm not totally sure how to actually run the wire for the circuit through the attic. We've got lots of permitted/inspected circuits that are stapled to the top of the ceiling joists despite having no guard strips, and there's also at least one just kinda... strung above the collar ties w/ no real stapling?

The wire is going to run perpendicular to the rafters/joists for about 40'. Because these outlets are going to be mounted on the underside of the rafters I don't think running it across/through the ceiling joists makes sense. Is stapling to the edge of the rafters good enough? Should I add a running board along the bottom edge of the rafters and then staple the romex to that?

The entire attic will be a conditioned space (it's not currently, but that's changing very soon) so I'm not worried about the temperature up there. We're in Redmond, WA and it's a single story ranch house built in 1968.

Attic showing rafters and insulation

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  • Unrelated - why is your insulation inside plastic bags? Is this an in-progress photo ?
    – Criggie
    Oct 13 '21 at 2:12
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    @Criggie I assume that's how they came? It's very thin and fully encloses the batt so it's probably to prevent fibers/dust from flying everywhere?
    – Tivac
    Oct 13 '21 at 6:04
  • It might be different in your area, but that looks like fibreglass batts (locally called Pink Batts) and they come in a compressed bale of 10 or 20 for ease of handling. You'd slice the plastic open with a sharp knife and the batts puff up like a paperback book. Might pay to lift one up and see if it says anything about "do not cut" or anything. This might be a separate question all by itself.
    – Criggie
    Oct 13 '21 at 7:47
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    @Criggie they would be very heavy if compressed like that. Encasing the fiberglass in plastic is a fantastic idea, as the micro-fibers from fiberglass present health problems. Not as bad as asbestos, but it was after all the functional ingredient in the "itching powder" deployed by resistance against Nazis in WWII. Oct 14 '21 at 19:40
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    @Criggie these are regular fluffy batts, just enclosed in a very thing layer of plastic. I assume it's to avoid all the nonsense from fiberglass insulation by encasing it a bit.
    – Tivac
    Oct 14 '21 at 23:02
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tl;dr - If you want code compliance, drill holes in the rafters* to run the wiring through that (remember to drill holes in appropriate sizes and location in the rafters), or use a running board.


When running the wiring across the face of the rafters, you need a running board per the NEC, or you'll need to chase the wiring through holes drilled into the rafters. Many inspectors will approve wiring across rafters or joists without a running board (or just miss it), especially if inspecting an attic, since that requirement is typically for basements where someone might want to hang something like wet clothing from it, but it only takes an inspector requiring it one time when you haven't done it to screw you over and force you to redo it to code anyway.

Note: There's an allowance for running NM wiring unsupported if it's 7ft or higher above the floor surface (since it's less likely to snag something on it or hang something from it if it is that high, I suppose), but unfortunately from your photo it looks like your attic is not that tall, so that exception wouldn't apply.

Here's what I believe the relevant NEC is if you're working with NM cable (all excerpts from 2020 NEC):

334.15 Exposed Work:

In exposed work, except as provided in 300.11(B), cable shall be installed as specified in 334.15(A) through (C).

(A) To Follow Surface

Cable shall closely follow the surface of the building finish or of running boards.

(B) Protection From Physical Damage

Cable shall be protected from physical damage where necessary by rigid metal conduit, intermediate metal conduit, electrical metallic tubing, Schedule 80 PVC conduit, Type RTRC marked with the suffix -XW, or other approved means. Where passing through a floor, the cable shall be enclosed in rigid metal conduit, intermediate metal conduit, electrical metallic tubing, Schedule 80 PVC conduit, Type RTRC marked with the suffix -XW, or other approved means extending at least 150 mm (6 in.) above the floor. Type NMC cable installed in shallow chases or grooves in masonry, concrete, or adobe shall be protected in accordance with the requirements in 300.4(F) and covered with plaster, adobe, or similar finish.

(C) In Unfinished Basements and Crawl Spaces

Where cable is run at angles with joists in unfinished basements and crawl spaces, it shall be permissible to secure cables not smaller than two 6 AWG or three 8 AWG conductors directly to the lower edges of the joists. Smaller cables shall be run either through bored holes in joists or on running boards. Nonmetallic-sheathed cable installed on the wall of an unfinished basement shall be permitted to be installed in a listed conduit or tubing or shall be protected in accordance with 300.4. Conduit or tubing shall be provided with a suitable insulating bushing or adapter at the point the cable enters the raceway. The sheath of the nonmetallic-sheathed cable shall extend through the conduit or tubing and into the outlet or device box not less than 6 mm (1/4 in.). The cable shall be secured within 300 mm (12 in.) of the point where the cable enters the conduit or tubing. Metal conduit, tubing, and metal outlet boxes shall be connected to an equipment grounding conductor complying with the provisions of 250.86 and 250.148.

334.23 In Accessible Attics

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

Which is:

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) Cables Run Across the Top of Floor Joists

Where run across the top of floor joists, or within 2.1 m (7 ft) of the floor or floor joists across the face of rafters or studding, the cable shall be protected by guard strips that are at least as high as the cable. Where this space is not accessible by permanently installed 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 ceiling or floor joists, neither guard strips nor running boards shall be required, and the installation shall also comply with 300.4(D).

And

334.30 Securing and Supporting

Nonmetallic-sheathed cable shall be supported and secured by staples, cable ties listed and identified for securement and support, or straps, hangers, or similar fittings designed and installed so as not to damage the cable, at intervals not exceeding 1.4 m (41/2 ft) and within 300 mm (12 in.) of every cable entry into enclosures such as outlet boxes, junction boxes, cabinets, or fittings. The cable length between the cable entry and the closest cable support shall not exceed 450 mm (18 in.). Flat cables shall not be stapled on edge. Sections of cable protected from physical damage by raceway shall not be required to be secured within the raceway.

(A) Horizontal Runs Through Holes and Notches

In other than vertical runs, cables installed in accordance with 300.4 shall be considered to be supported and secured where such support does not exceed 1.4-m (41/2-ft) intervals and the nonmetallic-sheathed cable is securely fastened in place by an approved means within 300 mm (12 in.) of each box, cabinet, conduit body, or other nonmetallic-sheathed cable termination. Informational Note: See 314.17(B)(1) for support where nonmetallic boxes are used.

(B) Unsupported Cables

Nonmetallic-sheathed cable shall be permitted to be unsupported where the cable:

  • Is fished between access points through concealed spaces in finished buildings or structures and supporting is impracticable.
  • Is not more than 1.4 m (41/2 ft) from the last point of cable support to the point of connection to a luminaire or other piece of electrical equipment and the cable and point of connection are within an accessible ceiling in one-, two-, or multifamily dwellings.

So you need to comply with 334.15. You also need to comply with 334.23, which says 320.23 applies to NM cables even though 320.23 only mentions AC cables. And 334.30 covers the general support requirements.

The entire attic will be a conditioned space (it's not currently, but that's changing very soon)

Okay, since you're air conditioning the space, that means you'll be putting some kind of insulation between the rafters, most likely. If you also put up drywall or some kind of board over the rafters (to create a "finished space"), then none of the above applies, because it wouldn't be "exposed work" anymore.

Of course, for wiring behind drywall, you typically drill a hole through the framing member (in an approved manner, of course), which is also an acceptable alternative to a running board for exposed work, too. If you ran the wiring across the edges of the rafters, then if you ever did want to put up drywall or some kind of 'ceiling' board, you'd have to provide additional protection for the wiring that's in the way (e.g. furring strips with metal plates over the gaps created for the wiring) or re-run the wiring through holes drilled into the joists.

So either way my recommendation is to drill holes, don't run wiring across the face. Future you will thank past you.


* - for any readers who have trusses instead of rafters, you typically are not allowed to drill holes through trusses. There are specific rules/restrictions for that which are not covered here.

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  • Great, detailed answer. Now I wonder if a folding staircase is considered a "permanently installed stair or ladder".
    – JPhi1618
    Oct 12 '21 at 20:27
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    @JPhi1618 Yes, it is something that is always there; you don't have to go get a ladder and prop it up against a wall or something to access it. A permanently affixed staircase or ladder, even a folding one, counts.
    – TylerH
    Oct 12 '21 at 21:29
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    This was incredible, thank you so much @TylerH. We'll be having the existing insulation removed and sprayfoam insulation against the bottom of the roof decking, but won't be drywalling against the rafters. Regardless, sounds like drilling between rafters is the right choice.
    – Tivac
    Oct 12 '21 at 21:33
  • @Tivac Happy to hear about that existing insulation... I didn't want to comment on it since it was irrelevant to the question you asked, but it is a hot mess! :-) Properly done, spray foam insulation is superior anyway.
    – TylerH
    Oct 12 '21 at 21:34
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    @Tivac PS, for the hole cutting, search for "boring and notching" (or vice versa) to find info on how to properly drill a hole through a framing member for electrical wiring, or for IRC 502.8. Here's the code, but it can be pretty technical: up.codes/s/irc-502-8-cutting-drilling-and-notching
    – TylerH
    Oct 12 '21 at 21:40

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