This has a factual answer, so I think GQ is the correct place.
A single-pole light switch needs to be replaced in my house. It has 3 wires: 2 for power, and 1 for ground. I marked the ground wire so I wouldn’t mistake it. The other 2 wires are black and red. Of course, I had to remove the switch to take it to the hardware store to buy the correct replacement…
Like an idiot, I didn’t mark whether the black wire came out of the “top” hole in the switch or the bottom hole. (The switch orientation: assume I’m holding the switch in an upright position, like it would be if it was already screwed into the wall. When the switch reads “off”, the switch is pointing down, like it would be if it were in the wall.)
My question: does the red or black wire go in the top hole?
Also, what should I expect to happen if I get the orientation wrong, and I turn on the power?
You mean this wouldn’t be an instance of “crossing the streams”?
The instructions I found on the internet (which conveniently didn’t answer my question) always said to replace one wire at a time: top to top, bottom to bottom, and ground to ground. Why doesn’t it matter?
With a single-pole switch, all you’re doing when you turn the switch on is connecting the top wire to the bottom wire, which completes the circuit. So it doesn’t matter which wire is connected where. (The ground wire does matter, that has to be connected to the green screw.)
Things are more complicated with three-way switches or double-pole switches (only used for things like furnace cutoffs.)
But your basic everyday single-pole, single-location switch is nearly impossible to screw up.
A red wire is usually the THIRD wire in a switch - almost always used for a two-way switch.
Those have two blacks and a red. A ground is either bare or GREEN.
It can be, but given that the OP brought the old switch to the hardware store to get an exact replacement, I think we can be confident that this is not the case, and it is in fact a single-pole switch. If this is a multiwire branch circuit, or they are simply using 14/3 romex to supply two different lighting circuits from the same box, then it would be perfectly normal to find a red wire on a single-pole switch.
Ok, now you’ve scared me. I’ve stopped, and I’ve turned the power back off.
Yes, there were 2 black wires and 1 red. I THOUGHT is was a single pole switch, but now that I think about it, there may be a second switch. (The switch controls my outside lights from my entry way. There may actually be an additional switch in the garage that does the same thing, making this a 3 way switch. Ooops…)
So I guess now I’m back to square one. I DID mark the wire that I thought was ground so I know where to put that wire when I go and get a correct 3 way switch.
That still leaves me with the question of where do the other 2 wires go? Black on top or red on top?
It also brings up another question. Am I replacing the correct switch? The switch in the entry hall was working fine until the day we started doing major arrangement and cleaning work in the garage (and possibly disturbing the switch in the garage). Then the entry way switch acted funny: If the switch was “on” the lights were on. If the switch was “off”, the lights were still on. If I played with the switch I could get it into a position in the middle where the lights actually turned off.
Thanks,
J.
p.s., kind of makes my comment about resourceful idiots more appropriate here, doesn’t it?
That’s almost certainly a single-pole switch then (although the modern convention is for the ground screw to be green.) A three-way switch will have four screws, two brass on opposite sides, a silver common, and a green ground.
There is the possibility that if this switch is very old, it’s an ungrounded three-way switch, in which case that silver screw is not the ground but is actually the common. However, since you said the two brass screws are on the same side, I think this is very unlikely.
Go ahead and make sure the circuit breaker is off and connect it up. The worst thing that will happen is the breaker will pop when you turn it back on. The second worst thing is that the switch won’t work, and you may need to call an electrician or knowledgeable friend with a multimeter to puzzle it out.
ETA: Another way you can tell if it’s a regular switch, if it’s the old toggle type and it has the words ON and OFF engraved on it, it is definitely a normal switch. If it’s blank, it may be a three-way.
Look at the old switch. Does the movable lever have the words on and off on it? Or is it blank? The first is a single pole switch the second a three pole switch.
Now go to the garage and look at the switch there. Does it have the on and off on it.
If a single pole switch is put in place of a three way switch is will sort of work.
The old switch does not have “on” or “off” on it, while the new switch I bought does. The old switch, then, IS a 3 way switch. Likewise, the garage switch also doesn’t have “on” or “off”.
There’s no evidence of handy work around these switches. They seem to be the original ones.
I’m still left with the question of does the black or red wire go on top (or bottom)? Is there any way to determine that by looking at the switch in the garage? For instance, should the (say) red wire be at the same location in the switch in the garage? Or should it be in the top on one switch and on the bottom in the other (assuming the same orientation of the switches)?
If you really have a three way switch, then if you turn the switch in the garage the other way, your switch you have hooked up now should cease to work - it should be off no matter what you do. If that is the case you have one traveler and the common hooked up to your single pole switch. So get a three way switch, and the one you don’t have connected/have connected to ground is definitely a traveler, and you have to take a max of two tries.
On your new three way switch Connect one of the wires you have on the brass screws on the regular switch to common and the other one to the regular side along with the wire you have not attached or on ground now. If both switches now work, you are set, if not just swap the terminals of the two wires that were attached to the regular switch before. It doesn’t matter which wire is on the top or bottom of the traveler side unless you care about how the switch looks - one way will make it that both switches have to be up or down to be on and off will be one switch up/one down, and swapped is the reverse of that.
Never underestimate the resourcefulness of idiots. 
