how do you calculate the sizes of a conduit for Fiber?

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dannyrzk

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Location
tulsa, OK
need to provide a conduit for fiber inside the building a coming from a near exterior box. I being searching, but I feel I need to ask the right questions before I'll do any design.
your thoughts are welcome!!
 

don_resqcapt19

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Location
Illinois
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retired electrician
First thing you need to do is find the minimum bending radius for the fiber, and then make sure that conduit bend have a radius larger than that. You can do the fill just like with wire, and if there is much length and a number of bends you should do a pulling calculation to make sure the pulling tension won't exceed that maximum permitted by the fiber specs.
 

JFletcher

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Location
Williamsburg, VA
Is there a spec? Who is the inspecting authority? When in doubt, 4" with long radius sweeps and never LBs


Yes there is. OP will find it in EIA/TIA specs/BICSI manuals. It is not code but recommended specs for all telecommunications cabling. Be glad they arent code, they are much more restrictive than the NEC (e.g., no more than 2 quarter bends in conduit and pull boxes every 100').

eta: have fun reading thru all the manuals:

1210151658.jpg

Those are the 2003 books, each 2x as big as the 08 NEC.
 
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ADub

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Midwest
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Estimator/Project Manager
Wow. I've piped miles of fiber runs and all I've ever used was 1.5" rmc as per our plant wiring standard. We (company guys) ran the pipe and pulled the fiber and outside contractors terminate. It is usually single mode fiber cable about the size of a finger and we've never had a problem with using regular one shot 90s and multiple offsets in the run. I'm guessing there's types of fibers out there that aren't as forgiving as the stuff we use.


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MattS87

Senior Member
Location
Yakima, WA
Wow. I've piped miles of fiber runs and all I've ever used was 1.5" rmc as per our plant wiring standard. We (company guys) ran the pipe and pulled the fiber and outside contractors terminate. It is usually single mode fiber cable about the size of a finger and we've never had a problem with using regular one shot 90s and multiple offsets in the run. I'm guessing there's types of fibers out there that aren't as forgiving as the stuff we use.


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I've done the same in a 1700' run using 2" and had zero problems. Nice to know there is a standard out there though.
 

don_resqcapt19

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Illinois
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retired electrician
For the bend as posted above. Usually all communications pipe is 4"
Fiber can pull surprisingly hard it seems.
My experience is that it pulls way easier than copper and around here, unless your are pulling for the phone company, it won't be in 4" conduit. It will be in conduit sized for the fill. I often pull 12 fiber cable in 1" conduit for industrial projects.
 

don_resqcapt19

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Illinois
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retired electrician
Yes there is. OP will find it in EIA/TIA specs/BICSI manuals. It is not code but recommended specs for all telecommunications cabling. Be glad they arent code, they are much more restrictive than the NEC (e.g., no more than 2 quarter bends in conduit and pull boxes every 100'). ...
That is just plain stupid....pulling the fiber in and out of j-boxes placed every 100' drastically increases the chance that the fiber will be damaged.
 

JFletcher

Senior Member
Location
Williamsburg, VA
That is just plain stupid....pulling the fiber in and out of j-boxes placed every 100' drastically increases the chance that the fiber will be damaged.

Not just for fiber but other cables as well. I remember one job we did early on where the EC sucked a string line then pulled a mule tape thru 170' of 3" conduit (that the EC installed) for our 100pr and maybe 15 cat5e/cat6 cables. 5 quarter bends, 4 after we removed the unnecessary fifth bend. Plenty of wire lube used. Conduit fill maybe 15%. Pull got to the 3rd quarter bend and stopped. No amount of massaging or brute force would get it thru. Mule tape broke. Used a 200' Greenlee metal fish tape, that too broke. ~130' of slicked up cables all came back out onto the floor. EC wound up disassembling conduit at/near 3rd quarter bend, pulling the cables out into the hallway, then back up the rest of the run, then reassembling the conduit.

A pull box mid run would have saved a LOT of man-hours there. Straight shot, sure, 1000', dont need any boxes.

I was happy at that point in my career just to have 4 or less quarter bends in the conduit. I think 358.26 was the first code I learned by heart.
 

ADub

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Location
Midwest
Occupation
Estimator/Project Manager
One of my steadfast rules when piping for fiber through plants is not to use the pull box to change direction but instead put it right before or right after the 90.


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J.P.

Senior Member
Location
United States
I remember making snide comments about the communications guys always wanting 4" pipe and putting one friggin wire in it.

I helped a local phone company guy pull his fiber in this summer. It was 200 ft. 90s up and into the buildings at both ends, pull box in the middle where it got fed.

That stuff pulled real tough. Broke the hook off my fish tape once. I actually ended up ruining my tape by having to grip it with linemans.

It was strenthcore/ strongcore fiber? Anyway it had a fiberglass rod in the middle of it, very stiff cable.

I have used some easy to handle stuff around on wellheads we wire up.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Not just for fiber but other cables as well. I remember one job we did early on where the EC sucked a string line then pulled a mule tape thru 170' of 3" conduit (that the EC installed) for our 100pr and maybe 15 cat5e/cat6 cables. 5 quarter bends, 4 after we removed the unnecessary fifth bend. Plenty of wire lube used. Conduit fill maybe 15%. Pull got to the 3rd quarter bend and stopped. No amount of massaging or brute force would get it thru. Mule tape broke. Used a 200' Greenlee metal fish tape, that too broke. ~130' of slicked up cables all came back out onto the floor. EC wound up disassembling conduit at/near 3rd quarter bend, pulling the cables out into the hallway, then back up the rest of the run, then reassembling the conduit.

A pull box mid run would have saved a LOT of man-hours there. Straight shot, sure, 1000', dont need any boxes.

I was happy at that point in my career just to have 4 or less quarter bends in the conduit. I think 358.26 was the first code I learned by heart.
My experience has been that the fiber pulls easier than other cables, either communication or electrical.

Some of the first fiber I installed had a solid fiberglass strength member that was about 3/16" in diameter. We pushed that fiber in 300-400' straight underground duct bank runs....I wish I would have had a few hundred extra feet that I could have kept for use as a fish tape:)

Even when we got to the buildings where we had to go around a couple of 90s, we still pushed the cable.

As far as pulling rules, a number of communications cables came with the following instructions...."maximum length between pull boxes, 100', no more than 270° of bend between pull points, deduct 30' from the 100' maximum for each 90° of bend".
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
I can just see three nineties joined together with no more than 10 feet of pipe and a pull box on each end. (One configuration that fits the formula.)
 
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