Detached Garage 10G Link Through a Loaded 20 mm Conduit
A homeowner in a 1988 two-story, 210 m² house wants 10 Gb/s connectivity to a detached garage 38 m from the house for a NAS and two 4K cameras. The only existing path is a 20 mm PVC conduit that follows a 55 m route with three 90° sweeps and one 45°; it already carries one Cat5e and one RG6 cable. The homeowner will not approve trenching and asks for “the fastest fiber,” plus one spare fiber “in case.” Garage temperatures reach 41 °C in summer and 4 °C in winter. The main switch is in a utility closet and has two empty SFP+ slots; the ONT is on a separate service and is not part of this link. The installer proposes either a point-to-point OS2 link using 10GBASE-LR BiDi optics or a 1x8 passive splitter fed from the closet. The homeowner also wants the work completed without removing the existing cables.
1. Determine whether two 3.0 mm OS2 cables can be added to the existing conduit, and identify the practical pulling risks. Show the fill calculation and state what you would verify before ordering cable.
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Fill calculation: A 20 mm trade-size PVC conduit has an internal diameter of roughly 20.9 mm, so its area is about 343 mm². Cat5e at 5.6 mm OD occupies about 24.6 mm²; RG6 at 6.9 mm occupies about 37.4 mm². Existing fill is therefore about 62 mm², or 18%. Two 3.0 mm cables add 2 × 7.07 = 14.1 mm², bringing the total to about 76 mm², or 22%. That is well under the usual 40% fill target, so capacity is not the limiting factor.
Practical risks: The 55 m route has three 90° sweeps and a 45°, which can multiply pulling tension. A pre-terminated SC or LC connector with its boot and pulling grip may be too large to pass a 20 mm conduit, especially around tight sweeps. The installer should verify the actual conduit ID, sweep radius, debris or water in the conduit, condition of the existing cable jackets, and the cable’s published installation tension (often around 200 N for 3.0 mm cable). If pre-terminated ends will not pass, pull unterminated cable and fusion splice pigtails, or use field-installable connectors. Use a compatible lubricant, a two-person feed, and a tension monitor rather than simply pulling harder.
2. Compare the point-to-point and splitter options for this link, including a loss estimate at 1310 nm and the effect on the spare-fiber request.
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Point-to-point: 55 m of OS2 at 0.4 dB/km adds only about 0.02 dB. Two connectors at 0.3 dB each add 0.6 dB, so the link is roughly 0.6 dB, far below the approximately 6.2 dB budget of 10GBASE-LR. A BiDi optic uses one strand for both directions, so two installed fibers provide one working link and one dark spare - exactly what the homeowner asked for.
Splitter option: A 1x8 splitter adds about 10.5 dB, plus 0.6 dB of connectors, for roughly 11.1 dB. That is still within a PON budget, but it is unnecessary for a single 10G link and forces the use of PON optics rather than simple point-to-point SFP+ modules. It also consumes a feeder port and leaves seven unused outputs.
Judgment: Choose point-to-point OS2 with no splitter. It gives the lowest loss, the simplest troubleshooting path, and preserves the second fiber as a genuine spare. A splitter here adds loss and complexity without serving any present requirement; the “spare capacity” argument is weaker than the benefit of a dedicated dark fiber.
3. Specify fiber type, connector polish, and environmental protection, and explain why your choices fit the garage conditions.
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Use bend-insensitive OS2 9/125 µm fiber, since the route has multiple tight sweeps and the garage is a single-mode distance irrelevant to multimode economics. For cable construction, a 2.0–3.0 mm indoor/outdoor or riser cable is acceptable inside the conduit, but it should be rated for the moisture and temperature range; a UV-resistant outdoor-rated jacket is prudent at the garage end if any cable is exposed. The 41 °C summer high is within normal indoor/outdoor cable ratings, but confirm the published range rather than assuming it.
Connector polish must match the optics: LC/UPC is typical for SFP+ ports, and APC should not be mated to UPC because the 8° angled endface against a flat endface produces high loss and backreflection. Use LC/UPC at both SFP+ ends unless the chosen modules specify APC. Respect bend radius: about 10× OD during pulling and 5× OD installed, so roughly 30 mm and 15 mm for 3.0 mm cable. Leave a 1 m service loop at each end, seal the conduit against water entry, label both ends, and verify with a VFL, optical power meter or OTDR, and an inspection probe before closing out.