The rules of routing wire

What are the rules for drilling holes into the structure?
Wood-frame houses are not all built the same way, but it's typical to see 2" x 4" wall studs, 2" x 10" floor joists and 2" x 6" (or larger) ceiling joists. Typically, these are spaced 16 inches apart, center to center. In some new homes, spacing for joists may be 24 inches, center to center. The architect sizes the wooden structural members to compensate for holes accommodating wires and pipes.

You should make yourself familiar with the terms used to describe the structure of your house. Local building and fire codes incorporate the general principles we're going to talk about, but some cities, counties and states restrict and amend these principles because of the risk of earthquakes, hurricanes, snowfall, or tornadoes. Talk to a local building inspector if you have questions about building or fire code. Or, if your house is being renovated or is still under construction, you can ask your builder or contractor for advice.

In non-bearing walls (left) hole diameter can be up to 60% of the stud width, and notch depth can be up to 40%. In a load-bearing wall (right) holes can't exceed 40% and notches can't exceed 25%.

Studs that support joists are called "bearing" or "load-bearing," and are often (though not exclusively) found in exterior walls. A non-bearing stud is typically found in some interior walls that divide one room from another. Non-bearing stud walls are your preferred wire routes, since they can have larger holes drilled into them. Of course, treat a stud as load-bearing when in doubt.

Here are some general rules:

• Studs — Drill holes in the center of each stud to better avoid nails. If you notch a stud, use a nail plate.
• In a non-bearing wall, hole diameter can be up to 60 percent of the stud width, and notch depth can be up to 40 percent. In a load-bearing wall, holes can't exceed 40 percent and notches can't exceed 25 percent.
• Joists — Keep all holes centered vertically in the joist. Don't allow a hole to be drilled within 2" of the top or bottom of the joist. The extreme ends and the middle third of the span of the joist carry the load, so avoid making holes there if possible.
  You are limited to a hole that is a third of the measured depth of the joist. A 2" x 6" joist actually measures 5-1/2" deep, so you can drill a 1-1/2" hole. You can drill multiple holes rather than one very large hole when many wires need to go one way.
  Engineered roof and floor trusses or wooden "I" beams often have open "web" space for wiring so that you don't need to make holes. Consult your builder for maximum hole sizes, if you need to drill. Generally, for such "TJI®" (or "truss-joist") trusses, smaller holes are allowed near the ends, and larger holes are allowed near the middle.

(A) Load-bearing walls intersect directly with the floor above. You'll see the wall run continuously up to the ceiling. (B) Non-bearing walls don't intersect directly with the floor above. In this image, there's enough room for a heating duct to run between the wall and the ceiling.

• "Glue lams" or headers — You cannot make any holes in laminated support beams (glue lams) or headers (the supports over doors, windows, or arches). Your wire routes must avoid these structural members at all costs, even if it means a large detour.

Glue lams are laminated support beams consisting of many thin pieces of wood glued together. You cannot drill or notch these beams.

• For your wires to pull easily, the diameter of a hole should be about twice as big as the total diameter of all the wires you plan to pull through it. Since hole sizes are limited, you may have to plan your wire routes to use multiple holes. For example, you might drill a 1-1/2" hole to accommodate a 3/4" bundle of cables.

To ensure the best performance from your system, be careful not to pinch or sharply bend cables.

• Don't bend the cable as you run it past the intersections of joists and studs; instead, create a smooth, gradual curve. Also, be careful when using plastic wire ties — they should be snug, but not tight enough to pinch the cable.
• Don't run it through holes occupied by other cables, unless they're also low-voltage wires, such as security or phone lines, and there's plenty of room to pull you cables.
• You'll need to install an open-backed junction box (or "J-box") or plaster ring ("P-ring") near your receiver. This is where the wire will exit the wall. The J-box or P-ring will also need a face plate — either with a hole that allows the cable to pass through the wall, or with connectors that link the in-wall cable to the out-of-wall cable.

What if your house is steel framed?
Steel framing is usually found in commercial structures. It normally makes wiring go faster, since steel frames and joists often have large pre-made openings for wire.

You can buy a punch to make more openings. All holes must be in the center of any steel member and cannot be any closer than 1-1/2" to another hole. Holes can be larger than wood framing. However, punching limits you to a fairly small hole size. If you must make new holes, ask your builder to approve a maximum hole size.

The edges of pre-made or punched openings can be very sharp. You'll need plastic grommets from an electrical supply store to fit inside the openings to protect the wire.