Tight buffered and loose tube are the two fundamental fibre optic cable constructions. Every fibre backbone cable — whether multimode or single mode, internal or external, four fibre or forty-eight — is built on one of these two approaches, and the choice between them determines how the cable handles environmental stress, how it is terminated, and which installation scenarios it is suited for. Specifying the wrong construction for the application does not always cause an immediate problem, but it creates risks that become apparent over time — difficult terminations, moisture ingress, or mechanical damage that a correctly specified cable would have avoided.
This guide explains the difference between tight buffered and loose tube fibre, when each is the right choice, and how DTECH’s OM4 and OS2 cable ranges cover both constructions for internal and internal/external installation.
Tight buffered fibre
In a tight buffered cable, each optical fibre has a protective buffer material applied directly and tightly around it, increasing the overall fibre diameter from 250µm — the bare coated fibre — to 900µm. This buffer is a dual-layer construction: a soft inner layer that cushions the fibre against mechanical stress, and a harder outer layer that provides protection during handling and installation. The buffer is bonded directly to the fibre coating, so the fibre and buffer move as a single unit.
The 900µm diameter is significant because it is the standard size for fibre pigtails and patch leads. A tight buffered fibre can be terminated directly — a connector can be applied directly to the buffered fibre without any fanout or breakout hardware, in the same way a pigtail is terminated. This makes tight buffered cable the practical choice for any installation where the cable is terminated at connectors inside a building, whether at a patch panel, wall outlet, or equipment room enclosure.
The tight buffer also adds mechanical robustness. The fibre is protected against bending forces, crushing, and the general physical abuse of being pulled through containment and around corners. Tight buffered cables handle more like copper cable than loose tube — they are more forgiving of handling during installation and do not require the same degree of care in routing and bend radius management that loose tube demands.
The limitation of tight buffered construction is environmental. The direct bond between buffer and fibre means that if the cable expands or contracts significantly with temperature changes, the fibre is placed under strain. Over large temperature ranges or in installations with high moisture exposure, this can affect attenuation. Tight buffered cable is therefore the correct construction for controlled internal environments — inside buildings, in equipment rooms, in riser shafts — where temperature and moisture are managed.
Loose tube fibre
In a loose tube cable, the bare 250µm coated fibres — sometimes multiple fibres — are placed inside a protective tube that is larger than the fibres themselves. The fibres float freely inside the tube, which means they are not under any strain from the cable’s mechanical behaviour. When the cable expands or contracts with temperature, bends, or is placed under tensile load, the tube accommodates the movement without transmitting stress to the fibres. This makes loose tube fibre significantly more tolerant of environmental and mechanical stress than tight buffered.
Loose tube cables are typically gel-filled — the space inside the tube around the fibres is filled with a thixotropic gel that acts as a moisture barrier. If the outer jacket is damaged and water enters the cable, the gel prevents it from migrating along the tube length and reaching the fibres. This moisture protection is what makes loose tube the standard construction for external, underground, and long-distance installations where moisture ingress is a real risk over the cable’s lifetime.
The practical tradeoff is termination. The 250µm bare fibres inside a loose tube cable cannot be directly terminated — they need to be either spliced to pigtails (which are factory-terminated to 900µm and can then be connectorised), or broken out using a fanout kit that applies individual protective sleeves to each bare fibre before termination. The gel also needs to be cleaned from the fibre ends before splicing or termination, which adds time to the installation process. For long backbone runs where the cable is spliced into splice trays at each end rather than directly terminated at connectors, this is not a significant burden. For installations with many termination points or where direct connector attachment is required, tight buffered is more practical.
Loose tube cable also typically holds more fibres in a given outer diameter than tight buffered, because the 250µm bare fibres pack more densely than 900µm buffered fibres. This makes loose tube the preferred construction for high-fibre-count backbone cables where cable diameter and weight matter.
Which construction for which application
| Application | Correct construction | Why |
|---|---|---|
| Internal backbone — equipment room to floor distributor | Tight buffered | Direct termination, mechanical protection, controlled environment |
| Riser cables — vertical between floors | Tight buffered | Internal environment, direct termination at each floor |
| Campus links — between buildings above ground in duct | Loose tube internal/external | Environmental protection for external section, splice at each end |
| Underground in conduit | Loose tube internal/external | Moisture protection, temperature tolerance for external environment |
| Mixed internal/external route — through external wall | Loose tube internal/external or tight buffered internal/external | Both rated for transition routes — loose tube better where external section is long or exposed |
| High fibre count backbone | Loose tube | More fibres per cable diameter, more efficient for large counts |
DTECH OM4 and OS2 in both constructions
DTECH supplies both OM4 multimode and OS2 single mode fibre in tight buffered and loose tube constructions, covering internal and internal/external installation requirements across both fibre types.
For internal backbone installations, DTECH’s tight buffered cables — OM4 tight buffered 8 fibre and OS2 tight buffered 8 fibre — are rated CPR Cca, LSZH jacketed, and suitable for internal installation in commercial buildings. The 900µm buffered fibres can be directly terminated onto pigtails or fusion-spliced without breakout hardware, making termination at patch panels and enclosures straightforward.
For campus links, underground runs, and any installation where the cable route passes externally, DTECH’s loose tube internal/external cables — OM4 loose tube 8 fibre and OS2 loose tube 8 fibre — provide the gel-filled moisture protection and environmental tolerance needed for external sections, with a UV-stabilised LSZH black jacket rated for both internal and external installation. These cables are rated CPR Eca and can run continuously from an equipment room through a building, out through an external wall, across an external route, and into a second building without a cable join at the transition point.
The CPR rating difference between the two constructions — Cca for tight buffered, Eca for loose tube — reflects their intended installation environments. Tight buffered cables are designed for permanent internal installation in occupied buildings where higher CPR performance is the standard requirement. Loose tube cables, while LSZH jacketed and fire-rated to Eca, are primarily specified for external and mixed routes where the environmental construction of the cable is the primary concern. Both ratings meet CPR requirements for their respective installation contexts. View the full DTECH fibre optic bulk cable range for all available specifications.
Frequently asked questions
Can loose tube cable be used inside a building?
Yes — where the cable is LSZH jacketed and CPR rated it can be installed internally. DTECH’s loose tube cables are LSZH jacketed and Eca rated, making them suitable for internal installation. The practical reason to choose tight buffered over loose tube for internal-only routes is termination simplicity — the 900µm buffered fibres of a tight buffered cable terminate more easily and do not require gel cleaning before splicing. For a purely internal run that terminates at patch panels at both ends, tight buffered is the easier choice. For a run that transitions between internal and external environments, loose tube internal/external is the correct specification.
Can tight buffered cable be used externally?
DTECH’s tight buffered cables are specified for internal installation. For external or internal/external routes, the loose tube internal/external variants are the correct specification — the gel-filled tube construction provides the moisture protection that tight buffered cables do not have, and the UV-stabilised jacket is rated for external exposure.
What is the difference between CPR Cca and Eca?
Both are CPR Euroclass ratings under EN 50575 for the reaction-to-fire performance of cables permanently installed in buildings. Cca is a higher rating than Eca — it requires more stringent testing including assessment of smoke production, flaming droplets, and acidity in addition to flame propagation. Eca requires only the basic vertical flame propagation test. For standard commercial buildings, Eca is an acceptable rating for permanently installed cables. Cca provides a higher level of fire performance and is specified in environments where fire safety requirements are more stringent, such as hospitals, high-rise buildings, and public spaces.
Do I need to use a fanout kit with loose tube cable?
If you are direct-terminating the loose tube cable at connectors, yes — the 250µm bare fibres need individual protective sleeves applied before termination, either through a fanout kit or by splicing to pigtails. If the cable is being fusion-spliced into a splice tray and terminated via pigtails at the panel end, the pigtail handles the connector and the splice handles the transition from bare fibre to connectorised fibre, so no fanout kit is needed. Pigtail splicing is the more common approach for loose tube installation in commercial structured cabling.
Why is 8 fibre the most common count for backbone installations?
Eight fibres provides enough capacity for most commercial building backbone requirements — typically one or two fibre pairs for current active links and spare fibres for future use or redundancy — while keeping cable diameter and cost manageable. Most 40G and 100G parallel optics transceivers use 8 fibres (four transmit, four receive). An 8-fibre backbone cable therefore provides one 40G or 100G link with no spare, or two duplex 10G links with four spare fibres for future use. For higher-density requirements, 12, 24, and 48 fibre counts are available.
Summary
Tight buffered fibre — with its 900µm directly terminated fibres — is the correct construction for internal backbone installations where direct termination at patch panels is required and the installation environment is controlled. Loose tube fibre — with gel-filled tubes and 250µm free-floating fibres — provides superior environmental and moisture protection for external, underground, and mixed internal/external routes, at the cost of more involved termination. DTECH supplies both OM4 and OS2 in tight buffered (CPR Cca, internal) and loose tube (CPR Eca, internal/external) constructions, covering the full range of commercial backbone installation requirements.
If you need help specifying the right fibre construction for your installation, get in touch with the DTECH team — we supply OM4 and OS2 fibre cable to installers and IT teams across the UK, Europe, and the Middle East.
