By Dustin Guttadauro, Product Line Manager - Telecom & Fiber, Infinite Electronics
Fiber optic cabling is an essential part of modern data center infrastructure, but selecting the right fiber cable involves more than choosing the fiber type, connector, or transmission performance. The cable jacket rating is also an important consideration because the cable may need to pass through different building spaces with different fire-safety requirements. For data center engineers, facility managers, network designers, and compliance teams, understanding plenum vs. riser cable classifications can help prevent installation problems and ensure that the selected cable is appropriate for the pathway where it will be installed.
In North American installations, common fiber fire classifications include OFNP (Optical Fiber Nonconductive Plenum) and OFNR (Optical Fiber Nonconductive Riser). International projects may also specify LSZH (Low Smoke Zero Halogen) cable based on local codes, project requirements, or customer specifications. The right selection depends on the installation environment, applicable building and electrical codes, fire-safety requirements, and project specifications.
Key Takeaways
- "Plenum vs. riser cable" refers primarily to different cable fire classifications and installation environments.
- OFNP is commonly associated with plenum-rated optical fiber cable.
- OFNR is commonly associated with riser-rated optical fiber cable.
- LSZH means Low Smoke Zero Halogen and describes material characteristics rather than being synonymous with OFNP or OFNR.
- A cable suitable for a riser should not automatically be assumed to be suitable for a plenum space.
- The applicable NEC edition, local codes, AHJ requirements, and project specifications should be verified before selecting cable.
- Cable price should be considered alongside compliance, installation requirements, availability, and lifecycle requirements.
- Both bulk fiber cable and pre-terminated fiber optic cable assemblies can have a role in data center deployments.
What Is a Fiber Cable Jacket Rating?
A fiber cable jacket rating describes the cable's suitability for particular installation environments, especially regarding its behavior during a fire. The jacket and overall cable construction can influence factors such as:
- Flame propagation
- Smoke generation
- Fire performance
- Installation location
- Regulatory compliance
- Building safety requirements
It is important to understand that the jacket rating does not describe the optical performance of the fiber itself. For example, two cables may contain the same type of single-mode or multimode fiber but have different jacket constructions and fire ratings. Therefore, when specifying fiber cabling, engineers generally need to evaluate two separate categories:
Optical characteristics: Fiber type, transmission distance, wavelength, bandwidth, insertion loss, and connector configuration.
Installation characteristics: Jacket material, fire rating, environmental rating, temperature range, and applicable code requirements.
Plenum vs. Riser Cable: What Is the Difference?
The basic difference between plenum and riser cable relates to where the cable can be installed within a building and the fire-performance requirements associated with those spaces. In North American terminology, optical fiber cables commonly use the designations OFNP and OFNR.
OFNP: Optical Fiber Nonconductive Plenum
OFNP cable is designed for use in plenum spaces and has a higher flame and smoke performance classification than riser-rated cable. A plenum space is generally an area used to transport environmental air as part of a building's HVAC system. Examples can include certain spaces above suspended ceilings or below raised floors, depending on the building's design and applicable code requirements.
Because air can circulate through these spaces, fire and smoke characteristics are particularly important. A plenum fiber cable may therefore be specified when the installation pathway is classified as a plenum space and the applicable code requires a plenum-rated optical fiber cable.
OFNR: Optical Fiber Nonconductive Riser
OFNR cable is designed for riser applications and is commonly used in vertical pathways between floors. Riser spaces can allow fire to travel vertically through a building, so cable installed in these pathways must meet applicable fire-performance requirements. OFNR generally has a lower classification than OFNP. This means that an OFNR cable should not automatically be substituted for an OFNP cable when a plenum-rated installation is required.
What Does LSZH Mean?
LSZH stands for Low Smoke Zero Halogen. LSZH cable jackets are designed to produce low levels of smoke and avoid halogenated materials in the cable jacket construction. This characteristic can be important in environments where smoke toxicity, corrosive gases, or international project specifications are major considerations. LSZH is particularly common in international telecommunications and data center projects, including installations where customer or facility specifications call for low-smoke, halogen-free cable constructions. However, LSZH is not simply another name for OFNP or OFNR. These terms describe different cable characteristics and classification systems. A cable's suitability must be determined according to the requirements of the specific installation and applicable codes.
NEC and Fiber Optic Cable Ratings
For installations in the United States, the National Electrical Code (NEC) provides requirements governing many aspects of premises wiring and cable installation. For optical fiber cables, Article 770 addresses optical fiber cables and raceways. NEC classifications include designations such as OFNP and OFNR. However, engineers should not select a cable rating based solely on the cable name. The installation environment, building construction, pathway, local amendments, and applicable sections of the adopted code should all be reviewed.
Code Reference
NEC reference: NFPA 70, National Electrical Code, Article 770 — Optical Fiber Cables and Raceways.
The current edition adopted by the project jurisdiction should be consulted because code editions and local requirements can differ. Where requirements are unclear, the project electrical engineer, code consultant, AHJ, or qualified installation professional should verify the appropriate cable classification before installation.
Why Plenum Fiber Cable Costs More
A common consideration when comparing plenum and riser cable is cost. Plenum-rated fiber can cost more than a comparable riser-rated cable because it requires a cable construction that meets the applicable higher fire-performance requirements. This can create a temptation to use a lower-rated cable where it appears physically suitable. That approach can create compliance problems. The objective should not be to select the cheapest cable. Instead, the cable should satisfy the installation requirements while meeting the project's performance, safety, and lifecycle objectives.
A useful way to approach the decision is:
Required rating → Appropriate cable construction → Optical requirements → Environmental requirements → Total project cost
Rather than:
Lowest cable price → Installation
The cost difference between cable ratings should therefore be evaluated alongside the potential cost of redesign, cable replacement, inspection issues, project delays, and compliance problems.
Can OFNR Cable Be Used in a Plenum Space?
This is one of the most important questions when discussing plenum vs. riser cable. In general, an OFNR cable should not be assumed to meet the requirements for a plenum installation simply because it is an optical fiber cable. If the installation space requires a plenum-rated cable under the applicable code, the cable should have the required classification. Engineers should verify the actual cable listing and the requirements of the applicable code rather than substituting an OFNR cable based on appearance or fiber type.
Can OFNP Cable Be Used in a Riser?
A higher-rated cable may sometimes be permitted in an application where a lower classification is required, but the project team should verify the applicable code and installation requirements before making substitutions. In practice, using a cable with a suitable or higher classification can provide design flexibility, but it may increase material costs. The important point is to verify the specific cable listing and applicable requirements rather than assuming that all higher-rated cables are interchangeable in every application.
Why LSZH Is Common in International Data Centers
Data center projects increasingly span multiple regions, and cable specifications can differ between markets. An international project may specify LSZH because of requirements related to:
- Smoke reduction
- Halogen-free materials
- Occupant safety
- Equipment protection from corrosive combustion products
- Customer specifications
- Regional standards
- Facility design requirements
This can be particularly relevant for data centers built in markets where European, IEC-based, or customer-specific cable requirements influence the physical-layer design. However, the exact requirement should always be verified against the applicable regional standard and project specification.
How to Select the Right Fiber Cable?
A practical fiber cable selection process can be broken into several steps.
1. Identify the installation environment
Determine exactly where the cable will run.
Is it:
- Above a suspended ceiling?
- Beneath a raised floor?
- Inside a vertical riser?
- In a dedicated cable tray?
- Inside conduit?
- In an outdoor pathway?
- In a mixed-use pathway?
The answer can affect the required cable classification.
2. Confirm the applicable code
Determine which edition of the applicable electrical and building codes governs the project. Do not rely on an older specification from another facility without checking the current requirements.
3. Confirm the required fire rating
Once the pathway classification is established, determine whether OFNP, OFNR, LSZH, or another cable construction is required.
4. Select the fiber type
Choose single-mode or multimode fiber based on the transmission requirements of the network. For multimode applications, the required OM classification should also be considered.
5. Select the fiber count
Determine the number of fibers required today and consider future expansion.
6. Select the cable construction
Choose the appropriate jacket, environmental rating, bend characteristics, and mechanical construction.
7. Verify the product listing
Check the manufacturer's specifications and certifications to ensure that the cable actually carries the required classification.
8. Document the installation
Record cable type, pathway, rating, fiber count, termination points, and routing information. Good documentation makes future upgrades and troubleshooting easier.
Bulk Fiber Cable vs. Fiber Optic Cable Assemblies
Data center projects may use both bulk fiber cable and pre-terminated fiber assemblies. Bulk fiber cable can be useful for custom-length installations where technicians terminate the cable on-site or where a structured cabling system requires field termination. Pre-terminated fiber optic cable assemblies can simplify deployment because connectors are installed during manufacturing and the assembly can be installed directly into the network infrastructure. L-com provides a range offiber bulk cable options as well asfiber optic cable assemblies for different data center and network connectivity requirements. The appropriate approach depends on installation method, required lengths, termination strategy, deployment schedule, and project specifications.
Building a Compliant and Scalable Data Center Fiber Infrastructure
Fiber cable selection is an important part of data center infrastructure planning. Optical performance is only one part of the specification. The physical environment, cable jacket construction, fire classification, local code requirements, and project standards must also be considered. Understanding plenum vs. riser cable helps engineers distinguish between cable classifications designed for different installation environments. OFNP and OFNR classifications address specific fire-performance requirements, while LSZH construction may be specified for international or specialized projects. The safest approach is to identify the pathway first, verify the applicable code and project requirements, and then select the appropriate cable construction.
How L-com Supports Data Center Fiber Infrastructure
L-com provides connectivity products for data center and network applications. For projects that involve pathway-specific cable requirements, L-com offers fiber bulk cable and fiber optic cable assemblies that can support a range of fiber counts, connector types, and installation requirements.
Frequently Asked Questions (FAQs)
What is the difference between plenum and riser fiber cable?
Plenum and riser fiber cables have different fire-performance classifications and are intended for different types of building pathways. OFNP is associated with plenum applications, while OFNR is associated with riser applications.
What is plenum fiber cable?
Plenum fiber cable is optical fiber cable with a plenum-rated construction, commonly designated OFNP in North American classification terminology. It is intended for installation in spaces where the applicable code requires a plenum-rated cable.
Is OFNR cable suitable for a plenum space?
OFNR should not automatically be considered suitable for a plenum space. If the applicable code requires a plenum-rated cable, the installed cable should have the required classification.
What does LSZH mean in fiber optic cable?
LSZH means Low Smoke Zero Halogen. It describes a cable construction designed to produce low smoke and avoid halogenated materials in the jacket. LSZH is not another name for OFNP or OFNR, and it is commonly specified for international projects.
Why does plenum fiber cable cost more than riser cable?
Plenum-rated fiber requires a cable construction that meets higher fire-performance requirements, so it can cost more than a comparable riser-rated cable. Using a lower-rated cable to save money can create compliance problems, so the cable should be selected to satisfy the installation requirements first.
How do you select the right fiber cable for a data center pathway?
Start by identifying the installation environment, then confirm the applicable code and the required fire rating. From there, select the fiber type, fiber count, and cable construction, verify the manufacturer's product listing, and document the installation.