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I am replacing a bunch of switches and receptacles in my house.
While I understand the basics, this is pretty much my first electrical project, and I figure there must be some hidden "gotchas" or surprises that people run in to when doing this.
Any advice on what to watch out for?
Yes, aside from checking with a tester to make sure the circuit is actually off... there are several subtle issues that frequently catch novices.
Don't remove screws all the way - they are captive. After some distance they will start getting stiff. Stop there, don't force them.
Unless you're using the new "lever-nut" style receptacles or the dreaded backstabs, you need to torque the receptacle screws if they specify a torque. This requires a torque screwdriver; a recent requirement in NEC 2014 due to recent science that says it matters. (and some other field testing at trade shows that revealed pro electricians can't torque "by feel" any more accurately than their spouse, apprentice or other traveling companion.)
Most common North American style receptacles have 2 sockets and 2 attachment screws on each hot and neutral side. Between the screws is a "tab" of metal that can be flexed back and forth and snapped off.
Check the tabs. If your old receptacle has any tab(s) broken off, you must break them off on the new receptacle.
There's your tab. This $1 outlet was ruined, by soldering (!) and removing the screws.
If you break one off by accident, and the receptacle is valuable, that's fixable.
On receptacles, brass screws are for hot and silver wires are for neutral.
If your have an outlet with tab broken, and you are trying to replace it with a GFCI, USB or other specialty outlet, this is a specialty task. You'll need more research. Don't just cram the wires on anyway!
Take care to distinguish LINE from LOAD.
If you are replacing a plain outlet with GFCI, leave the warning tape on, and hook up only the LINE terminals (cap off spare wires). Power up the circuit and test the GFCI. After it works, shut off the breaker and consider adding wires to LOAD if you want to use this GFCI to protect that downstream part of the circuit, which is a cost-saver if you know what you're doing. Otherwise, add those wires to LINE. Most GFCIs support screw-to-clamp for 2 wires per screw; just read the instructions.
Anytime you see 3 or more screws (besides ground), watch out. 3-way switches are tricky, because screw position and wire colors are meaningless - they vary on every switch. The color of the screws matters. (or if uncolored: labeling).
I'm a big fan of buying a 5-pack of colored tape, and marking both travelers with yellow tape. (for a second 3-way: blue tape). This makes it a lot easier to tell what's going on in a busy box. There is no need to distinguish them from each other, so use the same color. Two same-colored wires in the same cable tells an experienced person these are travelers.
Marking is helpful because the native wire colors of travelers mean nothing. I've seen 3-way circuits in a how-to guide where 3 different pairs of colors were used in the same 3-way loop!
These switches have 2 brass and 2 black screws. Two pairs of travelers attach. One pair of travelers attach to the brass screws. The other pair of travelers attach to black. In most cases, it's easy to group them in pairs - any pair of travelers is always in the same cable or conduit. Just make sure that is so.
For marking, I typically mark all 4 travelers yellow, since you can easily see the grouping by looking at the cables.
Always:
It is very important that anything permanently installed - lights, receptacles, switches, etc. - be properly listed for your area. In the USA, this normally means UL listing. Beware of other marks - CE and many other marks (if legitimate!) do not actually mean the device was tested to meet verify that it meets safety requirements.
Check power before working on switches & receptacles. Turn off power using the method you think is correct - breaker or fuse - and then check again using a non-contact tester. If it lights up, you may have accidentally used the wrong breaker or fuse, or you may have more serious problems such as a circuit that is connected to two breakers.
Ordinary toggle switches are often replaced because they wear out or for decorative reasons. Straight replacement with an equivalent "dumb" switch is normally very easy as explained in Harper's answer.
However, if you are installing a timer, WiFi controlled switch or other smart switch, there is a potentially big complication: neutral
A smart switch needs to get power to run the electronics that make it smart. There are three ways that a smart switch can power itself:
The neutral wire is the simplest to design and, IMHO, the most reliable method. The switch effectively operates as a device (typically a very, very small computer) with one of the outputs being control of power to a light or other device.
The problem is that a "dumb" switch does not use neutral. Until recently, a typical switch box did not necessarily include a neutral. In some cases, that is easy to fix as the neutral is close by. In other cases, particularly switch loops, it is not so easy to fix. Current code in most areas requires a neutral in the switch box unless the wires are run in conduit (relatively easy to add a neutral) or certain other special cases. But if you have an older house, adding a neutral may be a significant added expense (time and money) to what otherwise would seem like a simple smart switch installation. Keep in mind that if your wires are run in cables (e.g., 12/2 or 14/2 - black, white and ground), you can't simply "add a neutral" - you must replace the entire cable.
This is a handy solution, when it works. This generally works well with incandescent bulbs - the little bit of current doesn't cause a noticeable glow in the light bulbs and everything works fine. However, with LED lights, often a very little bit of current is all that is needed to make it up. Even worse, some LED lights, particularly if they are not designed to work with a dimmer, will flash instead of glowing dimly, which is really annoying.
There are some switches which will use the ground as the return path for the current that powers the electronics. This is against the general policy of "nothing goes on ground except in an emergency", but it is allowed within certain limits regarding the amount of current and other design factors.
If you don't have neutral easily accessible in your switch boxes then do some research before buying any smart switches. The instruction manuals will usually make it clear whether neutral is needed or not, though it can be a little trickier figuring out how a switch that does not use neutral actually gets power.
The wrap direction of the wire on the screw terminal matters. It needs to follow the clockwise tightening, not go against it. Otherwise, the wire will tend to slip out of contact as you tighten it, possibly giving a bad connection.
Also realize that the NEC book is something like an inch thick, and that all those rules were written in fire. Are you sure you want to proceed using only the knowledge you get from these few answers instead?
