Choose the Right Port Configuration for Your Splice Closure
Created on 10.10
How to Choose the Right Port Configuration (Inlet/Outlet) for Your Splice Closure
"How many ports do you need?" is one of the first questions our engineers ask on every splice closure inquiry. It sounds simple, but picking the wrong configuration means either over-paying for ports you'll never use, or — worse — leaving a port empty and leaking water into a closure you just buried. This guide walks through the decision so you can order the right closure on the first quote.
The Basics: What "Inlet/Outlet" Actually Means
A fiber optic splice closure connects two or more fiber cables together. Every cable that enters the closure needs a sealed port. In ODN engineering, ports are described as 1-in-1-out, 2-in-2-out, 1-in-4-out, 4-in-4-out, and so on.
The first number counts incoming feeder or distribution cables; the second counts outgoing drop or branch cables. The sum of ports does not need to equal the fiber count — a 48-core closure can have 2-in-2-out and still splice 96 fiber ends if the fibers are looped through.
Match the Port Count to Your Network Topology
Feeder splice (1-in-1-out)
A straight through-splice where one main cable continues to the next joint. Typical in long-distance aerial routes, ADSS, or trunk fiber between cabinets. One port in, one port out — choose the smallest dome closure (e.g. 12–48 core mini dome).
Distribution splice (1-in-many-out)
The most common FTTH closure: one feeder cable comes in from the exchange, multiple drop cables fan out to neighborhoods. Typical configurations:
- 1-in-2-out
- 1-in-4-outstandard neighborhood node, 128–256 homes
- 1-in-6-out / 1-in-8-out
Bidirectional / hub splice (2-in-2-out, 4-in-4-out)
Used where the closure is a true junction — two feeders from opposite directions, plus two branches onward. Common in ring topologies and in large central offices feeding multiple cabinets. Plan extra slack for future re-routing.
Don't Leave Ports Open — Blank Them Properly
Here's the mistake we see most often: a crew orders a 4-port closure but only uses 2 ports, and leaves the other two grommets out "for future expansion." That's the fastest way to import water, insects and rodents into your splicing tray.
Every unused port must be sealed with:
- Blank plug / dummy grommet
- Heat-shrink end cap
- If you truly expect future fibers within 12 months, leave a sealed, capped port — don't leave an open hole.
Cable OD Range: The Number Nobody Reads
Every closure spec sheet lists a cable outer diameter range per port — for example, 5–16 mm, or 7–22 mm. If your feeder cable is 14 mm and your drop cable is 7 mm, make sure the closure ships with grommets for both ends. Sending a closure with only 14 mm inserts to a crew that needs to splice a 7 mm drop is a 2-hour return trip.
When in doubt, order a mixed insert kit. It's a few dollars and saves a truck roll.
Quick Decision Cheat Sheet
Topology | Ports | Typical Closure |
Straight trunk splice | 1-in-1-out | Mini dome 12–48F |
Rural subdivision | 1-in-2-out | Dome 48–96F |
Standard FTTH node | 1-in-4-out | Dome 96–144F |
MDU / dense block | 1-in-6/8-out | Horizontal 144–288F |
Ring / junction node | 2-in-2-out / 4-in-4-out |
Key Takeaways
- Port count follows topology: 1-in-1-out trunk, 1-in-4-out standard FTTH node, 4-in-4-out ring junctions.
- Blank every unused port with a proper dummy grommet — never leave it open.
- Match grommet inserts to both feeder and drop cable OD; order a mixed insert kit if in doubt.
- Plan 10–20% headroom on port count for future growth, not 100%.
Not sure which port configuration fits your network? Send us a quick sketch or your cable route diagram — our engineers will recommend the exact closure and port configuration, with stock availability from our LATAM warehouses. Reach out at [email protected].