Box Size Calculation-Belden Cable

[chaos5505]

I have a job that requires Belden cable to be run to several devices like cameras and other security sytem components. We use Belden cable because all of the devices are 12-24 volts AC.

What is the requirement for sizing the boxes in the conduit runs out to this equipment? Mostly, 1 inch conduit is used. To me a 4 square box would be sufficient for a one or two pair 18 gauge Belden cable. But.........

Article 314.28 A ( Minimum size Pull and Junction boxes and conduit bodies)states that if the conductor is #4 or larger, the following box calculation will be used and so forth.

Ive been arguing this fact for months with other electricians and designers that say if the Belden cable diameter is equal to #4, you have to use this article.

I say BS. Sometimes you have to use some common sense. Belden cable is so flexable, it will go in a 4 square box easy considering there is enough sq inches in the box for that application.

Currently under their rule, the length of the box on a straight pull would have to be 8 times the size of the largest raceway or 8 x 8 x 4 box. This is actually what they are doing on our projects.

There is little to no heating effect by induction in the conduits. Its 24 volts AC and our limit on conduit fill is 25%.

What would be the correct size box to use for these very low voltage conduit runs? Is there anything in the code that would cover this? Article 314 A is the only rule I can find.

 

[steve66]

Re: Box Size Calculation-Belden Cable

A cable with a diameter equal to #4 is not the same as #4 wire. You struck the nail on the head when you said it is more flexible. I'm not sure if 314.28 applies to low voltage. I sure someone else will have an opinion on this.

Be aware though, that camera's require a high bandwidth to transmit video. If the cable is Cat 5e ethernet, the cable has a minimum bend radius of 1". If the cable is bent sharper than this, it won't be able to transmit the data fast enough. The result may be a poor picture or worse.

If even faster data is required, the bend radius may have to be larger. Make sure you find out what the requirements of the equipment is.

Steve

 

[hbiss]

Re: Box Size Calculation-Belden Cable

Problem is your thinking on this is wrong. Article 314 has nothing to do with LV wiring. There is no derating either. Look at Article 725, specifically 725.3(A) " Number and Size of Conductors in Raceway". It refers you to 300.17 which says nothing that you already didn't know. You need then to look at the manufacturers recommendation, not the NEC.

Belden used to have a slide calculator to determine the maximum number of cables allowed in a particular size conduit. You entered the product number and it would tell you how many you can put in 1/2" on up. I have an old one around here someplace. Dunno, check out their web site to see if there is any info there. If your limit is 25% though you should be more than fine.

There is nothing anywhere on box fill. You can use any size you want or none at all. For 24 volt power bend radius doesn't matter but for coax and CAT5 stuffing wiring into a box is a no no! Use a large enough box so that the cable tail can be looped and won't be kinked on the back of the connector when you put the plate and connector on the box. That should be your criteria to determine box size. This is the reason I never use boxes in stud wall construction.

 

[pierre]

Re: Box Size Calculation-Belden Cable

Hbliss

Your post is very good, I think you hit the spot!

Pierre

 

[tom baker]

Re: Box Size Calculation-Belden Cable

Hbliss, yes I agree boxes on stud walls for LV should be avoided. BICSI says on cat 3 etc, to use a mud ring and coil up 18" in the wall. 18" allows for retermination, and laying the jack on the floor to punch it down,not your knee...
Also belden PVC cable is hard to pull has a lot of friction, needs a lot of soap.