I have what is probably a pretty basic electrical question for lots of you, but I’m stumped. I am putting in a new motion sensing flood light on my garage. Coming out of the wall I have a black, a white, and a red wire. Coming out of the light I have a black, a white, and a ground. What do I connect to what? Any help would be much appreciated.
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Are you replacing a fixture that was already there? It sounds like you are working off a 3 way switch. That would be one reason to have 3 wires on the switch. If this is an existing system and you are just changing out the light fixture, you should just be swapping black to black and white to white. If this is a whole new installation, have to know if it's a single pole switch or a 3 way.
On switches and plugs think of the hip hop scene, Black always love gold, and white get stuck with the silver. The red will go on the bottom of the switch, not the green, green is ground like grass.. 2+3=7
The other reason for having 3 wire at the switch is to supply both hot and switched hot.Or if it is a switch leg to have a neutral, becuase some X-10 and similar devices need a neutral.But a description of the existing switch and connections should make it clear.If he is just replacing the LIGHT with a light with a motion sensor then he needs to connect it how the existing light was connected.However, if he is replacing the switch with a motion sensor switch then he needs give more details of what he has at the switch.
The red wire at a light fixture might indicate a 3 way switch, like others are saying, but if that's the case I would not expect just one red, there would be two (junctioned in the box, not connected to the light). Red-black-white sounds like 240v to me.
I agree with BillHartman's idea that it is probably to give you the choice of either switched or unswitched power. You could take just a socket/bulb up there and hook the white on the socket to the white wire, then touch the other socket lead to first the black (have your wife try the switch), then the socket black to the red (try the switch again).
The red may have been roughed in for more options (such as connecting to another light).
There is also the possibility that a three wire cable was used simply because it was what was available at the time. I have done it myself a time or two. I run out of 12/2 and slip in a run or two of 12/3 to make it work. Nothing wrong with it except a little extra cost and you need to keep an eye on the box fill. Sometimes you have the right cable on hand. Sometimes not. Sometimes extra wires mean something. Sometimes not.If you don't hook up a wire cap it off, with a wirenut, tape if it seems necessary, and tuck it neatly into the bottom of the box. Resist the urge to cut it off. You never know when a spare wire may come in useful or really have some function that has yet to be discovered. Don't burn your bridges.
I agree 4Lorn,Also, sometimes the people later want to add another light (same switch) and if you pull the 12/3 life is easier. I wasn't saying cut it off and I like to cap (even tag) for the next guy.
It occurs to me that having an extra conductor would make installing a receptacle in the soffit pretty easy. This sort of receptacle is great for holiday lights. A stacked switch, two switches sharing a yoke, being substituted for the normal single unit indoors. A switched receptacle in the soffit is a fairly popular retrofit but it is one I seldom see even on high end custom homes. Saves the risky jury rigs I often see strung through windows or long distances up from the normal outdoor outlets.
4Lorn1,After I thought about it a little more my idea about another light option didn't make sense. We have installed the soffet outlets for xmas decorations too. That makes more sense. I took note the last time how the motion detector is hooked in to the floodlights. There's was a red wire involved in the one I was installing. Now if I can remember where I put that diagram...