I have to anchor a bookcase to a wall to prevent it from tipping over.
When drilling into the wall, how do I make sure I can avoid drilling into the wires behind the wall?
I have bought a Stud finder which says it can detect wires, but I am not sure if it's that's the only thing I should rely on.
One of the Anchors will be on a stud, and the other will be on just dry wall.
Thanks.
This is something that you have minimal control over. Most electrical runs going horizontally are about 2 feet off the floor or very close to the ceiling. There are lots of reasons for this but one of the obvious ones is it limits the areas of concerns when drilling.
Also for horizontal runs there should be a little slack where an anchor would just push the wire out of the way. For vertical runs these are stapled to studs on one side or the other. So just hit the middle of the stud.
Really two things that help me investigate walls if I am worried are monkey hooks and my magnetic stud finder. With the monkey hooks you can prick a wall for a good two inches plus while causing almost no damage. You can easily "feel" something if you hit plumbing, electrical, insulation, whatever.
With the magnetic stud finder you can use the drywall screws to find your on center of your framing members. With a spare yardstick or 2x4 and a couple of screw points you can find the center of a stud the drywall is attached to. From there you can assume that there is a 1-2 inch "don't screw" zone on each side. As usually the only issues you will have with anchors is screwing into something tightly stapled. Even screwing into a wire ran through a bored hole would be hard because unless the hole is really small or there are a lot of wires the screw will push the wire out of the way.
Note: For a heavy bookcase I would rely on an L bracket using toggle anchors - pictured below - in the wall. My reasoning for toggle anchors is that other anchors could effectively strip or lose binding but in effect stay in place because the shelf hasn't moved enough. You don't want to find out that your anchor has stripped when you climb up to get something off a top shelf. The toggle anchor won't give you a snug connection but you don't need it to be hardcore snug, you need it to work. (also note that the toggle anchors usually don't have a sharp tip, therefore making it ultra hard to damage wiring)
+++ This answer was meant for anyone dealing with wood/metal framed walls. This would include most people in USA, Canada, Australia, New Zealand, parts of Mexico, parts of Asia, most dividers in large apartment buildings. The safe zones for drilling are usually the same no matter the country but the depth of the wiring and how it is secured is not. As for safety I would still offer that a prick test is better than any voltage meter I have ever used. If I was afraid of the prick going through the electrical conduit after going through a cinder block I would simply cover the tip (rubber, plastic, chewing gum).
I agree with the answer about using a stud finder with a voltage sensor. Also, bear in mind that while the horizontal wire runs are typically close to the floor or the ceiling, there are also cables running up studs, and they aren't necessarily stapled directly to the stud itself, either. Consider the NM cable standoffs (or "stack staples") in the pictures below, as well as the ad-hoc example with the cables tied up with a bit of extra NM. You really can't tell exactly what's in your walls without looking in there, although you can generally tell where this sort of thing might be an issue based on where you find things like light switches. But not always.
So, personally, if you're worried about it, I'd use a voltage sensor.
Unfortunately you can't be 100% certain but you can dramatically reduce your risk.
Checking local rules and practices for installing cables should be your first line of defense. For example in the UK we have "safe zones" where wires are normally run http://wiki.diyfaq.org.uk/index.php?title=Safe_zones_for_electric_cables . However it is not a guarantee, there may be exceptions to the rules (for example the UK rules allow running cables outside of the safe zones provided additional protective measures are taken) and there is no guarantee that the person who wired your house actually followed the rules.
Metal detectors and voltage detectors can be useful but again are not 100% certain and can suffer from both false positives and false negatives. Voltage detectors will not detect wires that are hidden behind/inside earthed metal.
P.S. Note that many posters on this site will post information on USA-specific rules and regulations without clearly indiciating them as such.
By code, when a hole is drilled to run a wire or pipe through a wood member (stud, plate, etc.), if there is less than 1.5" of wood between the face of stud and edge of hole, a nail plate (made of steel) must be used to protect the wire/pipe from unnecessarily long fasteners. Use a screw that will not penetrate into the wall more than two inches (1.5" of wood plus 1/2" drywall). As far as between the studs, drill as small hole only the depth of the drywall, then use a piece of wire or tip of a screwdriver to 'feel' for a wire or pipe directly behind the hole.
Although you can't just trust a voltage sensor you can improve your chances significantly: Find and follow the wires leaving a socket. This will tell you the range over which it can detect voltage and will allow you to adjust the sensitivity if that's an option. If you can't follow the wires away from a socket, it's no use for what you're doing this time.
By following the wires from all nearby sockets, you can get an idea of how they run, and whwther they get near your target area.
Wiring should be roughly centered in the stud. Keep your anchor penetration to an inch or so and you should be safe. It's not common practice to actually locate wiring for this type of thing, though you're wise to be mindful of it. Also consider whether you have plumbing in the wall. Obviously pipes are larger than wires and can be closer to the surface.
Varies depending on your location and building codes there, but as a rule of thumb wires will go vertically or horizontally. Wires should not go diagonally through a wall. So don't drill in the column or row of a power point or switch.
Do remember walls have two sides, check the other side for switches and plugs and fuse boards.
Another option is to push through your gib/drywall with something non-conductive, in the spot where you want the fastener to be anyway. It doesn't take a lot of force to put a hole in drywall linings. An old insulated electricians screwdriver works well.
Once there's a small hole you can use a stiff wire through to sweep around and find anything by contact, although by this stage your hole is probably fine to use for the fastener.
If you want to spend money, a borescope can tell you what's inside the wall, but they need a hole of at least a centimetre to get the camera through, which is probably too big for your fastener.
Another option is to pop off the faceplate of any plug/switch fitting in the immediate area of where you're drilling, and use a torch/flashlight to see in what direction the fixed wiring goes. Naturally you want the mains supply to the circuit turned off to do this, and test socket before removing from wall.
If you have access into the roof space or under the floor then have an explore and see if there are power cables coming through the top/bottom plates of the wall and into the floor/roof cavities. Again, access is the deciding factor (along with your dimensions, flexibility, and overall interest :)
I've found keeping a cheap boroscope in the toolbox helps immensely with this. Small holes next to baseboards are always easy to spackle and are rarely noticeable after being fixed.
Many stud finders now have a voltage sensor home depot has several. Some are cheaper this is the first one that came up. Use this to find your stud then check for power. No power No worries. This is the safest route.
Using a Monkey Hook (as suggested by DMoore) is a great idea to start a hole to probe around behind the wall board.
Use an electronic stud-finder to locate studs but follow up using a magnet to locate the drywall screws into the studs. If the stud happens to be toward the middle of the drywall sheet, the drywall screws will be at or near the center of the stud. Run a string/plumb line down, in combination with the magnet to find additional drywall screws.
Be careful, if the stud is at the joint of two pieces of drywall, the drywall screws will likely be arranged in two vertical columns, each to the left and right of center. Often the screws will be staggered, one screw to the left of center and the next screw down will be to the right of center. Run a string/plumb line down, in combination with the magnet to find additional drywall screws, and to determine if the screws line up in 1 or 2 columns. The only way to tell is to be careful to check around for all the screws you can find. If you find you are at a joint of two pieces of drywall with two distinct columns of screws (3/8 to 1/2 inch apart), you can position your screws in between the two columns but if possible avoid positioning your screws directly next to an existing drywall screw.
To anchor a bookshelf, especially a tall or heavy one, I would recommend that you DON'T anchor it to the drywall. Instead, mount a piece of wood like a 1 x 4, between two studs (or between 3 studs if the bookshelf is wide enough), with two screws into each stud. Then anchor the bookshelf to the 1 x 4. This is more secure, and gives you lots of leeway up / down, and left / right to anchor the bookshelf at multiple convenient points.
Others have mentioned to use a stud finder with a voltage sense feature to check for wires behind the drywall. IF you are anchoring directly to the drywall, this, in combination with using the monkey hook as a probe is a great idea.
