Four square junction box eccentric holes???

jaysgardenNovember 9, 2010

Couple questions:

1)Referring to a metal junction box say 4 square by 2.125 deep, When is a pigtail needed to be attached from the junction box to the equip grounding conductor? Is it only needed when more than one knockout on the junction box is used to run EMT to it?

2)WHy are some junction boxes knockouts concentric like pictured here from Home Depot:

3)And why are some junction box knockouts eccentric like pictured here:



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Usually, the CONcentric holes provide for two different sizes. That second one I think is the same way (two different hole sizes) but one if offset due to the screw?

    Bookmark   November 9, 2010 at 3:42PM
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The eccentric holes help hold the conduit nearer to the surface on which the box is mounted. Holes farther from the back of the box may require bending of offsets at the end of the conduit in order that the remainder of the conduit will lie tight to the surface- as required by code.

    Bookmark   November 9, 2010 at 3:47PM
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"When is a pigtail needed to be attached from the junction box to the equip grounding conductor? "

The better question is 'when is the box NOT required to have a wire ground connection?'

If the conductors pass through the box without splicing or cutting and no device is present (the box is empty except for the through conductors) there is no requirement for a pigtail.

The EMT provides the ground for the box.

    Bookmark   November 9, 2010 at 3:56PM
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Thanks you guys for looking at this topic but:

Look at the following picture of a 4 square drawn box. It has 1/2 and 3/4 inch knockouts. Each knockout can only be 1/2 or 3/4 inch. Here is the picture:

Now look at this following picture of a 4 square welded box with 1/2 and 3/4 knockouts. Notice that some of the 3/4 inck knockouts are "stamped or perforated" so that they could be a 1/2 knockout instead. Here is the picture:

What is the reason for the two different types of knockouts. Is there a reason to use one over the other?

    Bookmark   November 9, 2010 at 6:45PM
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Why does company A manufacture 17 different sizes of gloves while company B manufactures one labeled "ONE SIZE FITS ALL"?

Preference, I'd assume... What kind of mood the engineer was in that day.

If you're going to stock 300 boxes, concentric knockouts are probably a good idea since you don't know that far ahead of time which ones you're going to need. On the other hand, sometimes those concentric ones are a pain in the rear when you try to knock out the smaller one in a tight space and end up messing up the outer one too. That's a matter of preference and personal technique/habit.

    Bookmark   November 9, 2010 at 7:27PM
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you also don't need to pigtail a ground wire to the box if you don't pull a ground through the emt and it is used only for splicing. or even if there is a device, but the device is a self-grounding type or grounded through other acceptable means such as being cover mounted. also, if the ground wire is an isolated ground for an ig receptacle or piece of equipment it would be incorrect to attach is to the box.

    Bookmark   November 10, 2010 at 3:06AM
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If a ground wire is present and the box is metal, the ground needs to be connected to the box.

    Bookmark   November 10, 2010 at 8:21AM
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Ron Natalie

In the case of an isolated ground system you need two ground wires pulled. One to connect to the ground terminal on the receptacle and one to connect other things required to be grounded (or ground them through another acceptable means). There's no exception to leave the box ungrounded if it is metal just because you have an isolated ground device in it.

We used to pull the green wire (regular ground) and the green/yellow striped (isolated ground) when we were wiring up computer centers back in the day.

    Bookmark   November 10, 2010 at 9:17AM
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i was speaking of a scenario where only the isolated ground is pulled. it is completely unnecessary to pull an egc through emt. only preference. emt provides an adequate path for fault current back to the source.

    Bookmark   November 11, 2010 at 2:02AM
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So it does not matter how many knockouts for running EMT you use on a junction box there is no need to run a seperate equipment grounding conductor(green from the panel) or bonding jumper(green from the junction box to a grounding bushing where the EMT enters the junction box)? I have a book written by Rex Cauldwell and the way I interpret his one page is that if you use more than one knockout you should consider the bonding jumper.

    Bookmark   November 11, 2010 at 8:04AM
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