What is the proper way to run wires along/across basement ceiling joists?

Question

I want to add an electrical outlet in my unfinished basement. It makes sense to run the wire along the ceiling because that's where all the other wires are. What is the proper way to install NM cable either parallel to or perpendicular to the ceiling joists, given that the ceiling is not insulated and the joists are exposed?

The current situation is a bit mixed. Parallel wires are sometimes stapled to the edge of the joist and sometimes to the side, but not generally as far as 19 mm (3/4 in) from the edge. Perpendicular wires are mostly stapled to each joist and hanging freely between, but sometimes stapled to a board.

I live in Massachusetts, and therefore are subject to NEC 2008 with amendments.

Answer 1

Like Mike Powell says, I think it's against code to staple wire to the bottom of joists. Regardless of that, there are many reasons not to do that:

• If you ever want to put up drywall, the wires are in the way
• It's ugly

If you're running parallel to joists, staple the wire to the inside of the joist every couple feet. The wire should be at least an inch away from the bottom edge (basically, if you ever cover it with drywall or some other ceiling, you don't want people putting nails etc into the cable).

If you're perpendicular to the joists, drill holes (I generally use a 1/2" spade bit for 14AWG NM), and then run the cable through those.

You should also have a staple within 12" of any bend or entrance to a box.

Answer 2

Tester101 indicated not to drill too many holes within the joist, as it could weaken it and cause structural damage. I thought I should clarify something... The principal force that is applied to a floor joist is moment. With that, you get tensile along the bottom edge, and compression along the top edge. The other force would be shear, however moment is your main one. Being that it is primarily a moment force, the relative force implied to the joist would zero-out the closer you are to the centre (width). In other words, the centre (width) of the joist is your pivot point. That being said, as long as you drill in the centre, or as close to the centre as possible, you won't significantly reduce the joist's load-bearing capacity. (Just think of a truss joist, or an I-joist... that will give you an idea of how little the force is in the centre). So for a 2x10 joist, you would drill about 4.25" from the bottom or top, as that should be smack in the centre. The amount of holes doesn't matter, as long as you don't touch the edges. Of course, this is within reason, if you drill out 90% of the entire centre you would leave very little shear resistance. Coincidentally, this is also why an I-beam is shaped that way. There's relatively little load in the centre (the thin part), whereas most of the loading is tension along the bottom and compression along the top. I should also note that if the joist is cantilevered, than the whole thing reverses on the cantilevered section (compress along bottom, tension along top).

As a structural engineer, I can tell you that the worst thing you can do is "notch" the joist. By that I mean cutting out a section of the bottom or top edge of the joist. If you cut out a section of the bottom edge, which becomes under tension when a moment is applied, the joist would be significantly weakened. Similarly, if you cut out a section of the top edge, which would be under compression when a moment is applied, the joist would be significantly weakened.

Sorry for the long answer... it's a slow day. But hopefully this knowledge comes to good use for someone.

Answer 3

Here is what New York State says

• Where the cable is run at angles with joists in unfinished basements, cable assemblies containing two or more conductors of sizes 6 AWG and larger and assemblies containing three or more conductors of sizes 8 AWG and larger shall not require additional protection where attached directly to the bottom of the joists. Smaller cables shall be run either through bored holes in joists or on running boards.

• Where run parallel with the framing member, the wiring shall be 1.25 inches from the edge of a framing member such as a joist, rafter or stud or shall be physically protected.

• Bored holes in studs and vertical framing members for wiring shall be located 1.25 inches from the edge or shall be protected with a minimum 0.0625-inch steel plate or sleeve or other physical protection.

• Where installed in grooves, to be covered by wallboard, siding, paneling, carpeting, or similar finish, wiring methods shall be protected by 0.0625-inch-thick steel plate, sleeve, or equivalent or by not less than 1.25-inch free space for the full length of the groove in which the cable or raceway is installed.

• Bored holes in joists, rafters, beams and other horizontal framing members shall be 2 inches from the edge of the structural framing member.

• Securely fastened bushings or grommets shall be provided to protect wiring run through openings in metal framing members.

• The maximum number of 90-degree bends shall not exceed four between junction boxes.

• Bushings shall be provided where entering a box, fitting or enclosure unless the box or fitting is designed to afford equivalent protection.

• Ends of raceways shall be reamed to remove rough edges.

Basically staple all parallel runs 1.25" away from the joist edge. All perpendicular runs should pass through a bored hole 2" from the edge of the joist. Support wire every 4-6' with either staple or through a bored hole. Staple wire within 12" of entering a box or bend. And make sure all boxes have proper wire clamps.

NEC 2008

300.4 Protection Against Physical Damage. Where subject to physical damage, conductors shall be protected.

(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.

Exception No. 1: Steel plates shall not be required to protect rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, or electrical metallic tubing.

Exception No. 2: A listed and marked steel plate less than 1.6 mm (1⁄16 in.) thick that provides equal or better protection against nail or screw penetration shall be permitted

(2) Notches in Wood. Where there is no objection because of weakening the building structure, in both exposed and concealed locations, cables or raceways shall be permitted to be laid in notches in wood studs, joists, rafters, or other wood members where the cable or raceway at those points is protected against nails or screws by a steel plate at least 1.6 mm (1⁄16 in.) thick, and of appropriate length and width, installed to cover the area of the wiring. The steel plate shall be installed before the building finish is applied.

Exception No. 1: Steel plates shall not be required to protect rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, or electrical metallic tubing.

Exception No. 2: A listed and marked steel plate less than 1.6 mm (1⁄16 in.) thick that provides equal or better protection against nail or screw penetration shall be permitted.

334.15 Exposed Work. In exposed work, except as provided in 300.11(A), 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, 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, 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(E) 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. NM 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 NM cable sheath 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.

334.17 Through or Parallel to Framing Members. Types NM, NMC, or NMS cable shall be protected in accordance with 300.4 where installed through or parallel to framing members. Grommets used as required in 300.4(B)(1) shall remain in place and be listed for the purpose of cable protection.

334.30 Securing and Supporting. Nonmetallic-sheathed cable shall be supported and secured by staples, cable ties, straps, hangers, or similar fittings designed and installed so as not to damage the cable, at intervals not exceeding 1.4 m (4 1⁄2 ft) and within 300 mm (12 in.) of every outlet box, junction box, cabinet, or fitting. 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 (4 1⁄2-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 nonmetallicsheathed cable termination.

Answer 4

This question is pretty old, but another alternative would be to run Electrical metallic tubing (EMT) which could be hanged across joists. Plus it looks so much better for exposed work. It's a little bit more labor but is definitely worth it.