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#97841 - 04/05/06 03:14 PM Bonding a pullbox  
Tripp  Offline
Joined: Sep 2005
Posts: 99
Ah, bonding - my favorite topic. Here's my situation:

I have an outdoor run of RNC (PVC) which is interrupted at one point with a 6x6x4 metal raintight box, installed to accomodate the number of bends in the run and for ease of pulling. There will be no equipment or devices in this box; the conductors will merely be spliced together.

My question is this: What do I need to do to insure proper bonding/grounding of this box?

NEC 250.4 (A)(2)-(4) requires that (2)"Non-current-carrying conductive materials eclosing electrical conductors...shall be connected to earth..." [grounding] and that (3) "Non-current-carrying conductive materials enclosing electrical conductors...shall be connected together and to the electrical supply source..." [bonding].

Furthermore, 250.86 requires grounding (and bonding, according to editorial notes in the 2005 Handbook) of all metal enclosures.

My intent is to install a lug in the back of the box and attach the grounds there. But how does this satisfy the "shall be connected to earth" portion of the above-mentioned 250.4(3)? Furthermore, do I need to install a bonding bushing where each conduit enters the box? (remember: metal box, but NON-metallic conduit.)

Thanks for any and all input!

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#97842 - 04/05/06 03:39 PM Re: Bonding a pullbox  
mhulbert  Offline
Joined: Oct 2004
Posts: 132
Anaheim, CA USA
By bonding the box to your EGC, you are bonding it to earth. Remember that your EGC is connected to your Grounding Electrode at your service through your main bonding jumper. So, by bonding your box to the EGC, you are also tying it to your Grounding Electrode system.

The method you are planning on using should be fine. Don't forget to scrape the paint off under your lug to ensure a good connection! Sometimes I will use a small bus bar kit meant for a panel when I have a couple larger wires coming in. Don't have to use a split bolt, and you use less pieces = less labor.


#97843 - 04/05/06 04:13 PM Re: Bonding a pullbox  
Alan Nadon  Offline
Joined: Mar 2005
Posts: 399
Elkhart, IN. USA
The lug needs to be either in a machine screw threaded hole, (ya sure) or use a nut and bolt to fasten the lug, sheet metal screws are not acceptable.
Size the bond to the largest conductor or overcurrent device depending on tables 250.66 or 250.122 and if you increased the wires for voltage drop you must adjust the bonding conductor.
Bonding bushings on the conduits might look nice but will serve no practical purpose on PVC.

If it was easy, anyone could do it.

#97844 - 04/05/06 06:37 PM Re: Bonding a pullbox  
trekkie76  Offline
Joined: Jun 2004
Posts: 220
baileyville, maine, usa
You will need to put bushings on the PVC entering the box, but not bonding bushings 352-46(2002)

#97845 - 04/05/06 09:25 PM Re: Bonding a pullbox  
gfretwell  Online Content

Joined: Jul 2004
Posts: 9,126
You could use a lay in lug and just strip a short section of the EGC without cutting it.
10-32 self tapping screws will work too. It is just the course thread "sheet metal" screw that is prohibited.
If the wire is #10 or smaller you could just use a green S/T screw without a lug.

Greg Fretwell

#97846 - 04/06/06 08:49 AM Re: Bonding a pullbox  
luckyshadow  Offline
Joined: Jan 2005
Posts: 307
Maryland USA
Most weather proof boxes have that raised spot in them and include a ground screw, I usally do what gfretwell does and use a lay in lug with the supplied screw. By using a lay in lug you can pull the wiring straight through without cutting it.

#97847 - 04/06/06 05:14 PM Re: Bonding a pullbox  
earlydean  Offline
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
One point about drilling/tapping a hole in the box. When you scrape off the paint to ensure adequate bonding, you also destroy the corrosion resistance of the box! Be sure to apply a dab of oxide inhibiting compound beneath the lug, and after the lug is installed, and the wire attached, paint the box on both sides where the lug is installed with a corrosion type paint (galvanize).


#97848 - 04/06/06 07:34 PM Re: Bonding a pullbox  
mahlere  Offline
Joined: Nov 2005
Posts: 507
New Jersey
why not use a pvc box? especially if it's simply to meet code requirement for too many bends.

#97849 - 04/07/06 03:06 PM Re: Bonding a pullbox  
Tripp  Offline
Joined: Sep 2005
Posts: 99
Thanks for all the great replies! lots of good reminders there. As for the suggestion of using a PVC box: too late, metal is already installed. Maybe next time. Thanks again.

#97850 - 04/07/06 08:38 PM Re: Bonding a pullbox  
ShockMe77  Offline
Joined: Jun 2005
Posts: 821
Rahway, New Jersey
Be sure to apply a dab of oxide inhibiting compound beneath the lug, and after the lug is installed, and the wire attached, paint the box on both sides where the lug is installed with a corrosion type paint (galvanize).

That's a great idea, and fine workmanship. I'll keep this in mind next time I'm installing a lug in a metal box. Thanks for the tip!

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