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Old 03-02-2013, 08:52 PM   #1
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Ribbed white smooth black?


I just started working for a home improvement store and need to build some lighting displays. I noticed they have their displays connected to a pigtail and plugged into a wall of outlets. Their wiring was ribbed from the pigtail to black from the display and smooth to white and ground to green, but I have always used ribbed to white smooth to black. Which is correct and why do both ways work?

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Old 03-02-2013, 08:55 PM   #2
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Ribbed white smooth black?


The bulb does not care if the screw shell or center tab is hot. It will work either way. The center tab should be the hot to limit potential exposure to the hot conductor.

White = ribbed to screw shell.

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Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.
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Old 03-02-2013, 08:58 PM   #3
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Ribbed white smooth black?


@ Jim, what is meant by center tab? And screw shell is where the bulb screws in?
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Old 03-02-2013, 09:01 PM   #4
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Originally Posted by seangar78 View Post
@ Jim, what is meant by center tab? And screw shell is where the bulb screws in?
The "center tap" is the little metal tab/metal piece at the bottom of the inside of the lampholder. The screwshell is what the light bulb screws into.

As Jim said, it is usually ribbed to white and smooth to black.

If it is done backwards, the lamp will still work, only the screwshell will be hot/live instead of the center tap.
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Old 03-02-2013, 09:06 PM   #5
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Ribbed white smooth black?


@ Kyle, thank you and what would be the benefit the screwshell being hot/live instead of the center tab?
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Old 03-02-2013, 09:08 PM   #6
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Ribbed white smooth black?


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@ Kyle, thank you and what would be the benefit the screwshell being hot/live instead of the center tab?
There is no benefit. In reality you want the center tab to be hot, not the screw shell. There is a greater risk of electric shock if the screw shell is hot instead of the tab, because sometimes the shell protrudes from the socket.
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Old 03-02-2013, 10:02 PM   #7
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Ribbed white smooth black?


The code also addresses this issue.

200.10(C) Screw Shells. For devices with screw shells, the terminal for the grounded conductor shall be the one connected to the screw shell.

Basically, this means that the outer screw shell on a lampholder must be connected to the neutral, not the hot.

400.22(F) states that the ribbed wire of a flat cord is to be the neutral.

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Old 03-02-2013, 11:43 PM   #8
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Ribbed white smooth black?


Thanks micro.


Last edited by seangar78; 03-03-2013 at 12:38 AM.
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