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Old 11-12-2008, 11:26 PM   #1
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Plastic instead of metal...


I have this monster obsession against plastic device enclosures. Why or how in God's name was plastic ever approved? I have seen plastic j-boxes melted to the point of scorching the surrounding joyce space because some shmuck didn't tighten his connections tight enough on an electric heat airhandler.

When my sparky does any work for me he always uses metal enclosures.
He hates plastic too but can't tell me why it ok to use them, yet you see them in new construction all the time.

Can anybody clue me in on what the deal is?

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Old 11-12-2008, 11:42 PM   #2
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Plastic instead of metal...


I like plastic! I feel it is safer. And no grounding needed.

Also I like those big - say 12" x 12" plastic weatherproof junction boxes. Very easy to drill large holes in for conduit, etc.

I've not seen any problems with them melting as you describe. Maybe if I did, I would not like them so much!

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Old 11-13-2008, 05:44 AM   #3
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Plastic/PVC/fiber/etc boxes are approved because they meet a certain criteria set by folks who know something you and I don't.
If you are asking specifically why they are approved, I have no idea other than that. WHAT difference does it make???

hvaclover, I guess we would never work together because I hate metal boxes, at least for NM cable, and will NOT use them unless I had no other choice or some code dictated it.
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Old 11-13-2008, 05:48 AM   #4
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Plastic instead of metal...


EDIT: Wrong thread.
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Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
Answers based on the 2008 & 2011 NEC.

Last edited by Speedy Petey; 11-13-2008 at 02:13 PM.
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Old 11-13-2008, 06:45 AM   #5
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Plastic instead of metal...


I have used both plastic and metal, and each has their own place in the industry.
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Old 11-13-2008, 07:08 AM   #6
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Plastic instead of metal...


I like to use plastic for most nm installs. I don't like to install grounding clips all of the time!
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Old 11-13-2008, 07:18 AM   #7
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I have used both plastic and metal, and each has their own place in the industry.
Yeah, and usually that's Plastic in residential with NM wiring, and Metal in Commercial with Conduit or other metallic raceway material. I have used metal boxes in some residential work, actually most all the junction boxes I have installed have been metal; it can be a pain sometimes with the box connectors for NM though.
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Old 11-13-2008, 07:39 AM   #8
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I like to use plastic for most nm installs. I don't like to install grounding clips all of the time!
Grounding clips? Why not just use a grounding screw in the hole provided in the box?
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Old 11-13-2008, 07:42 AM   #9
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Plastic instead of metal...


I still prefer metal boxes for old work, but use plastic in special cases.
The problem I have found with plastic in old work, is that the cable comes into the box, and is not well clamped as it is with a metal box.
I know, the cable is supposed to be secured within a certain distance (the number slipped my mind) going into a box, but what do you do when the stud or joist isn't open when you do the work? How do you secure the cable?
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Old 11-13-2008, 07:47 AM   #10
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I know, the cable is supposed to be secured within a certain distance (the number slipped my mind) going into a box, but what do you do when the stud or joist isn't open when you do the work? How do you secure the cable?
I believe the cable must be secured within 12 inches of the box, it might be less. If you are installing an old work box where you have no access to the studs, securing the cable is not required. If you end up opening the wall cavity up later on, then you would be required to secure the cable as best you can. You shouldn't have to worry about the cable coming out of the box very easily, the plastic clip should hold onto the wire and the grip becomes tighter if the cable is pulled on outside of the box.
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Old 11-13-2008, 08:03 AM   #11
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Plastic instead of metal...


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Grounding clips? Why not just use a grounding screw in the hole provided in the box?
Six of one, half a dozen of the other. Either way is perfectly acceptable, and I see people do both.

As for plastic boxes, I'm a fan of them. There are a number of advantages to using them, but the biggest one I see is on multifamily residential construction. In fire-rated separation walls, the code requires a minimum of 24" horizontal offset between boxes facing opposing directions. Allied Moulded (one manufacturer) has had a number of their boxes tested and listed to reduce the offset from 24" to just inches. Their ceiling boxes can be placed a lot closer together in rated floor-ceiling assemblies as well. The only way to do that with a metal box is with intumescent putty pads, which run $10-$15 each. Many electricians do not have a firm grip on fire codes, and I've encountered closely offset boxes in a number of large scale projects...Once they understand the code, electricians are more than happy to install Allied's product in a few locations instead of a total re-design or hundreds of putty pads.

Another benefit of plastic is that you don't have the grounding requirement.

Most plastic boxes have integrated clamps for NM wire.

Most plastic boxes carry a 2hr fire rating. That isn't arbitrary. It is based on UL testing. If the box melts or fails prematurely, it doesn't get the listing.

Many people don't realize that switches that are installed in nonmetallic boxes must be grounded. "Well Mr. Inspector, how come they sell switches without ground screws then?" For metal boxes! I see this issue almost daily.
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Old 11-13-2008, 09:07 AM   #12
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Plastic instead of metal...


Well I got my answer, so thanks. You've help educate this HVAC guy just a little better.. Guess even metal can't protect any body from STUPID.

A hack job can make even metal dangerous.

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