I'm looking at running some OM3 fiber (outdoor, armored), 6 strand between two residential buildings (think from a house to a garage for example). It's about 150ft total.

I do not want to run copper (ground loop concerns, grounding in general, lightning/surge protection).

The fiber cable has a minimum bend radius (unloaded) of about 11.5cm (4.5 in).

I had been planning direct bury the cable (it's rated for it), then in to conduit as it exits the ground and in to a conduit body to enter/exit the buildings. But seeing the required minimum radius I don't think I'll be anywhere near the proper bend radius with any of the LBs I've looked at.

(I am considering using conduit for the full run as well, but that's here nor there right now).

For now the main thing I'm concerned with is how best to get the cable in/out of the building while maintaining the required minimum bend radius?

  • Fiber sounds like low voltage, you are not limited to electrical rated/listed conduit. Soft/bendable plumbing pipes will work or hard plumbing fixtures that have a more softer bend.
    – crip659
    Commented Oct 13, 2022 at 20:51
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    Compare the cost (time & money) of digging that trench to the cost of conduit. For the small added money of laying conduit in an existing trench (you'll have to dig, one way or the other), you'll save a lot of time & money in the future should you ever need to replace the fiber, pull another line or, well, anything.
    – FreeMan
    Commented Oct 14, 2022 at 11:45
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    @crip659 Low voltage? Isn't fiber definitionally no voltage at all? It's optical, not electrical. Commented Oct 14, 2022 at 14:55
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    @DarrelHoffman Low voltage as in not having code regulations to deal with.
    – crip659
    Commented Oct 14, 2022 at 15:09
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    NEC Section 800 applies. But it lets us do a lot of things you can't with line voltage. Then again, many ignorant folks do act like nothing applies. One issue with armored fiber is that it's not (if the armor is metal, as per usual) non-conductive (or "all dielectric") and the same applies to buried fiber with a locating wire (conductive.)
    – Ecnerwal
    Commented Oct 14, 2022 at 17:23

2 Answers 2

  1. For your stub from underground, use large-diameter sweeps instead of standard 90 degree bends. For ease of pulling think at least double your min. bend radius.

  2. When entering the building, run the sweep parallel to the wall rather than perpendicular. To enter the bottom of the sweep from the trench, make a big wide horizontal bend.

  3. At the top of your sweep where you intend to penetrate the wall, use an LB, but use a big one (we use 4" on jobs I've done, but we're pulling 24 strand minimum. You could probably get away with 1-1/2" or 2". I always figure conduit is cheaper than wire so don't be afraid to oversize. I believe the Verizon fiber coming into my house underground is in 1" PVC but they price it by the truckload.)

  4. When you get inside, run your fiber onto your backboard or into your pull box vertically, so that if you are looking at a cutaway cross section of your wall, you'll have a mainly vertical run from the ground, through your LB, and up the interior side, with the fiber running almost vertical.

You want the cable to be only slightly offset as it goes through the LB and into whatever indoor enclosure you will use. Think of a racing driver cutting two opposite corners with a single sweeping line.

  • Thank you for the detailed reply! I had to re-read a few times, but I get the gist of what you're saying here. This combined with the response from @vidarlo makes me feel a lot better about how to approach this.
    – Jonathan
    Commented Oct 14, 2022 at 18:51

Don't do OM3 today. Run single mode. The fiber is cheaper as it's more commonly deployed, it supports higher speeds and longer reaches, and the optics is almost at the same price.

In addition, remember that the bend radius given in data sheets is the bend radius where the cable will retain the rating. You probably have a short run, so a 3dB loss in the cable is probably not a disaster: your link budget allows for this.

In short: for such a deployment I'd be comfortable with going down to 1/4 of the bend radius given in datasheets, and accepting the increased loss.

As pointed out by ecnerwal, it's also possible to get reduced bend radius fiber. It's slightly more expensive, but allows for tighter installation runs.

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    OM3 was outdated .vs. singlemode 12 years ago - the "common knowledge" that the optics were far too expensive for using on short runs was already invalid then. It's even more true now. I haven't shopped recently, but I suspect the singlemode fiber is also still cheaper. So, spot on those points. Not a big fan of disrespecting the bend radius, but then I am a big fan of "spend the extra penny per meter for reduced bend radius fiber" so that's not even an issue.
    – Ecnerwal
    Commented Oct 14, 2022 at 0:59
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    @Ecnerwal I'm not a big fan of ignoring bend radius either - and would not do it in different settings, but for providing internet to a shed 150ft away? You have plenty link budget, and can allow for some extra loss...
    – vidarlo
    Commented Oct 14, 2022 at 7:05
  • I appreciate you mentioning this! When first planning the project I was going to run CAT6, then I started to become concerned with the things I noted in my original description. When I considered fiber, most of what I found was mentioning OM3, so I just assumed that was the current recommended fiber to run. I did some quick research just now and I do see I can get some much lower bend radius options with single mode, that combined with my being okay with a few db loss (if unavoidable) makes me feel better about this. Thank you again!
    – Jonathan
    Commented Oct 14, 2022 at 18:43

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