steel vs. plastic outlet boxes

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  #1  
Old 01-13-05, 08:45 AM
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steel vs. plastic outlet boxes

Just curious --

When is steel used vs. plastic outlet boxes? Are there any more hazards for steel due to arching, etc?

Also, is it safe to put electrical tape on outlet screws and at the end of hot wires? Will it be OK as long it remains there?
 
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  #2  
Old 01-13-05, 09:03 AM
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You use metal boxes with EMT, ridgid conduit or BX (flexible metal conduit). You use plastic boxes with NM-B or it's outdoor equivalent.
 
  #3  
Old 01-13-05, 01:14 PM
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Plastic or steel boxes.

Some city's will allow you to use plastic.
Some city's only allow steel.

People need to ask there city electrical inspector for city code requirements on electrical.
Some city's have there own requirements different from the NEC code book.
 
  #4  
Old 01-13-05, 01:40 PM
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In my opinion is never a good idea to use EMT with plastic boxes. It defeats the whole purpose of using EMT, and it may give a false sense of ground if somebody decides to use the pipe as a ground.

What exactly are you trying to do? If you can use NM-B cable why even bother with steel boxes?
 
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Old 01-14-05, 10:02 AM
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Unless your locality prohibits it, I would always prefer plastic with NM-B. I have on several occasions shorted one of the receptacle screws against a metal box. I hate when that happens.

Wrapping a good quality electrical tape (3M Super 33+ or equal) around your receptacle to cover the screws and bare wire ends is OK. I used to do that all the time, but after awhile I couldn't see any significant advantage to doing it so I quit.

Juice
 
  #6  
Old 01-14-05, 11:12 AM
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Originally Posted by JuiceHead
Unless your locality prohibits it, I would always prefer plastic with NM-B. I have on several occasions shorted one of the receptacle screws against a metal box. I hate when that happens.

Wrapping a good quality electrical tape (3M Super 33+ or equal) around your receptacle to cover the screws and bare wire ends is OK. I used to do that all the time, but after awhile I couldn't see any significant advantage to doing it so I quit.

Juice

Juice,
A signifigant advantage to wrapping the outlet w/ tape is to prevent shorting one of the receptacle screws to a metal box.
As far as metal boxes and NM...NM has been run to metal boxes for a very long time. Plastic boxes are a fairly recent addition in the history of wiring. IMHO, I prefer metal boxes. I think that they are better quality and allow for repeated device installation and removal without stripping out the mounting screws. The only downfall of metal boxes is that they must be bonded...an extra step.

As far as wrapping the end of a wire with tape, I would only do that on a very, very temporary basis. If the wire is to be left unconnected for anything longer than a little while, then cap it off with a wire nut.

If you are using conduit, then all fittings need to be rated for conduit. The plastic boxes I have seen make no provision for conduit entry, just cables. Therefore, to use them with conduit, you would have to alter them and therefore it would be a code violation.
 
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Old 01-14-05, 12:14 PM
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Scott,

I appreciate your response.

I think I went through my reply too quickly. I didn't see a significant advantage to taping the receptacles because I began using plastic boxes with NM-B. I always taped them before specifically because the screws can make contact with the metal boxes and short.

I rewired my garage all in pipe with metal boxes. Those receptacles and switches are taped.

Also, I when said taping "to cover the screws and bare wire ends is OK", I meant the bare ends of the wire that are hooked and properly terminated under the screws. I, too, would never use tape to cover a bare-ended, unconnected wire. Would always cap with wire nut and usually a neat wrap around the wire nut to ensure it didn't fall off.

Some non metallic (i.e., bakelite) boxes have knockouts to accept conduit. However, as it will make the grounding path of metallic conduit discontinuous, I would avoid mixing metallic conduit with non metallic boxes. Even if you run an equipment grounding conductor with your current-carrying conductors, somebody in the future may not realize there's an electrical discontinuity in the conduit system and rely on it as a ground. Just generally not good practice to mix, in my opinion.

Juice
 
  #8  
Old 01-14-05, 05:42 PM
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i agree whole heartedly.
 
  #9  
Old 01-14-05, 06:54 PM
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The KOs in plastic boxes are not really meant for conduit. They are meant for cable connectors, or pvc.
Metal conduit MUST be grounded. So if it is used with NM boxes bonding bushings must be used at every point. Crazy if you ask me. I'm almost sure it's not prohibited by code but it should be.
 
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Old 01-15-05, 02:08 AM
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Isn't arguing against electrically conductive boxes arguing against ground protection?

***

I started taping around when prepping for painting - I didn't want anyone stumbling into a dark room and cooking a finger. Then, later, I began to find terminals caked with paint , and I wondered why don't I see a melted brush and the remains of a thoughtless painter nearby, but I began to tape as a matter of course, because cover plates will be removed and not always in caution.
 
  #11  
Old 01-15-05, 05:15 AM
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Originally Posted by Kobuchi
Isn't arguing against electrically conductive boxes arguing against ground protection?
From a non-electrician's point of view...

Why would that be? The hot terminal can't short to the plastic, so what additional danger would there be as opposed to metal? If anything, I would think that metal would pose more opportunities for problems - since there has to be an electrical/physical bond to the box, that's just one more potential for mis-wiring a circuit for an uninformed homeowner.
 
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Old 01-15-05, 09:03 AM
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In areas where metal conduit is not used (which is most of the country) grounding is provided by a grounding wire and the box (metal or plastic) serves no purpose in the grounding system. So although metal boxes may be more rugged and more resistant to damage, they have the potential to make the installation electrically more dangerous. The electrical code recognizes this potential, and requires that the grounding wire be jumpered to the metal box to mitigate the danger.
 
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Old 01-15-05, 07:32 PM
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Clear, consise and to the point.
 
  #14  
Old 01-16-05, 06:10 AM
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In Massachusetts, the building code requires metal device boxes in garages to maintain fire rating in walls and ceilings adjacent to habitable space.
 
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