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Replacing dimmer. Old dimmer has two black wires attached to two white wires in box. There are two black wires in box not attached to dimmer but wire-nutted together. Is it ok to wire the new dimmer switch the same way?

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The quick answer is yes, it is ok however I'd advise to switch the wires. That is, wire nut the two white wires and connect the black wires through the dimmer switch, I'll explain.

You switch box is a hot box, any switch system has a hot box or hot lamp. If power is run directly to the lamp and only switch cables run to the switch you have a hot lamp. In this case you have a hot box, one set of those cable is live and the other set runs to the lamp outlet. On a hot box you want to bridge the neutral lines together(white) and switch the hot lines(black) that way your lamp isn't powered when change a light bulb (and the switch is turned off). Side note to complete: a hot lamp typically runs a black and white to the switch so you'd have one color on each side of the switch.

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Look to another question's answer for a illustration of the hot box. –  Jason Jan 25 '13 at 19:56
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You are assuming that the colors are correct (black-hot, white-neutral). Bad assumption since the switch was improperly wired. He should check which set of wires is hot using a non-contact tester. If the switch is breaking the neutral, then your answer is correct. But I would insist that he find out first. Also, on a hot lamp with a remote switch, you need to mark the white wire with black tape to show it is really a hot line. –  bib Jan 25 '13 at 20:45
    
Both the black wires, which are nutted together, are hot all the time. If the switch is on (light turned on), neither of the white wires is hot. If the switch is off (light turned off) one white wire is hot, the other white wire is not hot. (to repeat: the two white wires are nutted to the two black wires from the dimmer switch.) –  Bob Jan 25 '13 at 21:56
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Sorry Bob, I don't think you're measuring right, with the switch on, all wires have current. Off, only one wire has voltage. It's an honest mistake though. You do have a switched neutral situation and should follow Jason's hot box instructions. Each black to switch black (either way) and wirenut whites together. –  bcworkz Jan 26 '13 at 1:22
    
If the feed came through the light. And I don't know how the light was wired, would I still change to black/black? Or would it be best to stay with White/black which has been the case for 20 years? –  Bob Jan 26 '13 at 16:42
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