Box Fill

[wireday]

I have a steel switch/device box. 2X3X3.5 it is listed as 18 CI. this box comes with two cable clamps at the back of the box. Is the 18 CI figured with these left in?Most likely I will just remove and discard these clamps. I am using a knock out in the back,with a MC speed lock connector. I am using 5 #10 conductors and one single yoke receptacle in this box.

 

[Dennis Alwon]

No, you have to deduct for the clamps if you use them

 

[wireday]

Im not going to be using them,I have a MC connector outside the box,wanted to know if the 18 CI was with the clamps accounted for.My fill calculates to 17.5 I most likely will just remove the two clips,was just wondering if the manufacturer calls this box 18 CI with these left in place.

 

[infinity]

No, the 18 cu in is without clamps, if you use one or two then you need to add one conductor to your fill.

 

[wireday]

Thank you Rob, I guess its common practice to just remove them if not being used. This box for me is very close to being full. Doing the math on this box I get 21 CI,NEC must derate to 80 percent or something.

 

[infinity]

Thank you Rob, I guess its common practice to just remove them if not being used. This box for me is very close to being full. Doing the math on this box I get 21 CI,NEC must derate to 80 percent or something.

You have 7 "conductors" without the clamps.

7 * 2.5 = 17.5 cu in.
18 cu in box good.

 

[Dennis Alwon]

Im not going to be using them,I have a MC connector outside the box,wanted to know if the 18 CI was with the clamps accounted for.My fill calculates to 17.5 I most likely will just remove the two clips,was just wondering if the manufacturer calls this box 18 CI with these left in place.

This is an issue that came up with plastic boxes and the internal clamps. According to the manufacture we have to deduct for the clamps which I have never seen anyone do it like that. You would think with the molded boxes that the cu in is with the clamps but apparently it is not.

 

[curt swartz]

This is an issue that came up with plastic boxes and the internal clamps. According to the manufacture we have to deduct for the clamps which I have never seen anyone do it like that. You would think with the molded boxes that the cu in is with the clamps but apparently it is not.

I remember a long discussion here about that. I'm still not convinced that I believe the manufacture. I think it may be a carryover from when all plastic boxes had separate removable clamps similar to metal boxes. The hard plastic boxes (Allied Molded & Union/T&B) still use separate clamps but with all of the pvc boxes the clamps are part of the mold.

 

[infinity]

I remember a long discussion here about that. I'm still not convinced that I believe the manufacture. I think it may be a carryover from when all plastic boxes had separate removable clamps similar to metal boxes. The hard plastic boxes (Allied Molded & Union/T&B) still use separate clamps but with all of the pvc boxes the clamps are part of the mold.

I agree, why stamp the number of conductors allowed into the box and then say you need to use a different calculation.

 

[jumper]

I remember a long discussion here about that. I'm still not convinced that I believe the manufacture. I think it may be a carryover from when all plastic boxes had separate removable clamps similar to metal boxes. The hard plastic boxes (Allied Molded & Union/T&B) still use separate clamps but with all of the pvc boxes the clamps are part of the mold.

I agree, why stamp the number of conductors allowed into the box and then say you need to use a different calculation.

The reason is that UL calculates the allowable fill without the clamps and the CMP backs up UL.

 

[infinity]

The reason is that UL calculates the allowable fill without the clamps and the CMP backs up UL.

I don't know if we're referring to the same thing I meant these clamps:

 

[jumper]

I don't know if we're referring to the same thing I meant these clamps:

Yeppers, the same thing.

I used a round one in the previous thread on this subject.

The general consensous was that we would just ignore the rule.

 

[jumper]

And here is that thread:

http://forums.mikeholt.com/showthread.php?t=139162

 

[infinity]

Yeppers, the same thing.

I used a round one in the previous thread on this subject.

The general consensus was that we would just ignore the rule.

 

[Dennis Alwon]

I agree, why stamp the number of conductors allowed into the box and then say you need to use a different calculation.

How is it any different from a metal box? My reaction was the same as yours but then I realized if you have to count them in a metal then you should count them in the plastic. The difference that I see is one clamp in a metal box takes up way more space then the clamps in a plastic box-- at least it seems that way. I have not known an inspector that enforces it.

 

[curt swartz]

Dennis, with a metal box the clamps actually take up room in the box when they are installed. With a molded plastic box the clamps don't take up any room since they are part of the mold. I don't see how the clamps can be removed to calculate the box volume since there is no way to remove them.

 

[infinity]

How is it any different from a metal box? My reaction was the same as yours but then I realized if you have to count them in a metal then you should count them in the plastic. The difference that I see is one clamp in a metal box takes up way more space then the clamps in a plastic box-- at least it seems that way. I have not known an inspector that enforces it.

Dennis, with a metal box the clamps actually take up room in the box when they are installed. With a molded plastic box the clamps don't take up any room since they are part of the mold. I don't see how the clamps can be removed to calculate the box volume since there is no way to remove them.

Article 314 is pretty specific in stating that if the clamp is left in the metal box then it gets counted. The stamped cu in size or wire fill quantity in 314.16(A) doesn't reflect the clamp. Like Curt stated the plastic box the clamp is part of the box which IMO is different.

 

[jumper]

Like Curt stated the plastic box the clamp is part of the box which IMO is different.

UL, manufacturer, and CMP says otherwise...........:angel:

Not that I am totally agreeing, but how can you refute their statements?

My current job does not use plastic boxes, so I do not have a dog in this fight other then discussing code.

 

[Dennis Alwon]

Dennis, with a metal box the clamps actually take up room in the box when they are installed. With a molded plastic box the clamps don't take up any room since they are part of the mold. I don't see how the clamps can be removed to calculate the box volume since there is no way to remove them.

Well I disagree as the volume must be based on the box as a rectangle. The clamps protrude into the box and according to the art. in 314 we must count it (I spelled it right this time- Thanks Trevor) Personally I think it is such a small difference in measurement that it is universally ignored.

 

[infinity]

Well I disagree as the volume must be based on the box as a rectangle. The clamps protrude into the box and according to the art. in 314 we must count it (I spelled it right this time- Thanks Trevor) Personally I think it is such a small difference in measurement that it is universally ignored.

Where does it say it must be based on a rectangle, is that part of the listing? 314.16(A)(2) requires nonmetallic boxes to be marked with their volume by the manufacturer. Does this mean the manufacturer cannot figure out the volume with those built in clamps like shown in post #11?

One other thought, boxes like the one in post #11 have no KO's which means that one of the integral clamps must be used, shouldn't the manufacturer include that in their calculation?