wiring issue with light fixture

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Old 04-30-09, 08:31 AM
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wiring issue with light fixture

I am trying to put up a new light fixture in my kitchen and having issues. Power comes to the fixture by way of a 14/2 wire, and there is another 14/2 wire that also comes in to the fixture's ceiling box, which goes to a 2-way switch on the wall nearby. I went to install my new fixture, wired all the whites together, wired all the blacks together, and the grounds. Turned on the breaker and the light came on, went to test the switch and when I hit the switch, the breaker tripped.

I know the switch can't be the issue because it was pre-existing and had a previous light off the same wire.

not sure what I did wrong here?
 
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Old 04-30-09, 08:45 AM
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You changed your wiring that went to a switch loop and created a dead short. What you should have done was to connect the new fixture to the same wires as the old one.

Now you will need to find out which wire is the power in. Turn off the circuit that controls this wiring. Open all your splices and let the ends hang in free air. Using a simple neon tester or volt meter determine which pair of wires has power. Turn the power off. Splices the black power wire to the white from the opposite cable. Connect the fixture black to the other black, connect fixture white to the white from the power cable. Connect all ground together and to the fixture mounting plate. Restore power and test.

If this is correct re-identify the white spliced to the black with a magic marker in a color like red, blue or black.
 
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Old 04-30-09, 06:00 PM
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That did it! You were spot on. Now, if you don't mind me asking, I'm trying to wrap my head around how the switch loop works and why it needed to be done that way, simply for my own self education. If you can briefly explain it, I'd be grateful. Thanks
 
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Old 04-30-09, 06:09 PM
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A switch loop carries the constant hot power to the switch. The black wire coming back from the switch is a switched hot. The white is used to supply the switch instead of the return from the switch so that you don't end up with 2 white wires attached to the fixture. This is also a Code requirement. To prevent your snafu the Code requires the white to be re-identified so that it does not get confused with a neutral.
 
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Old 04-30-09, 06:12 PM
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Two conductor cable is used for a switch loop. Two conductor cable in the US is normally black and white. You are supposed to recolor the the white cable black or red or whatever except green but often it isn't done.

Power may come in to the light fixture or switch box or a third J box (last is seldom done}. Where the power comes in is usually determined by easiest/shortest rout for the cable. Often it comes in at the light. When it comes in at the light you use a switch loop.
 

Last edited by ray2047; 04-30-09 at 06:28 PM.
 

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