Quick Answer: Quick Answer: OFNP is the plenum rating for optical fibre cable: Optical Fibre Nonconductive Plenum. It passes the NFPA 262 Steiner tunnel test, so it may be installed in an air-handling ceiling without a conduit. The P means plenum, the N means nonconductive, so the cable has no metal armour or strength member. OFCP is the conductive variant, OFNR is riser rated to UL 1666, and OFN is general purpose. Plenum fibre is required wherever the return air path runs through the ceiling void, in new fit-outs and retrofits alike.
Plenum rating is a fire and smoke classification, not an optical one. A plenum cable burns cleanly enough to satisfy the tunnel test and produces limited smoke, which matters because the space above a suspended ceiling is often the return air duct for the whole floor. The optical performance of the glass is unchanged; only the jacket and the construction around it differ.
Decoding the Letters
The code is read left to right. OF is optical fibre, the next letter is the flame rating, and the last letter says whether the cable contains metal. That is why OFNP and OFCP both carry a P but behave very differently in an air-handling space.
What N and C change
Nonconductive means no armour, no steel strength member and no metallic tape, so the cable cannot carry fault current or create a ground loop between cabinets. Conductive versions add armour for rodent and crush protection, and they must be bonded to earth at one end.
Why the distinction is enforced
An inspector is checking two things: the print on the jacket and the pathway. A conductive plenum cable in a metal raceway is acceptable, but the same cable lying open above a ceiling tile is a bonding question, and most specifiers avoid it by choosing OFNP.
Jacket Codes Side by Side
The table lists the codes that appear on indoor fibre jackets, with the test each one has to pass. Values are the ones printed in the standards, not supplier claims.
| Code | Rating | Test | Construction | Typical location |
|---|
| OFNP | Plenum | NFPA 262 | Nonconductive | Air-handling ceiling |
| OFCP | Plenum | NFPA 262 | Conductive armour | Plenum, rodent risk |
| OFNR | Riser | UL 1666 | Nonconductive | Vertical shaft between floors |
| OFCR | Riser | UL 1666 | Conductive armour | Riser with mechanical protection |
| OFN | General purpose | UL 1581 VW-1 | Nonconductive | Exposed run inside a room |
Where Plenum Rating Is Required
The trigger is the air path, not the building type. If the ceiling void is used as a return air plenum, every cable in it must be plenum rated, including the fibre. A plenum cable is cheaper to install than a conduit retrofit, which is why the rating exists at all.
Spaces that are not plenum
A closed joist space, a dedicated duct or a riser shaft is not an air-handling space. OFNR is the normal choice there, and the cost difference per metre is real on a long vertical run. The comparison in plenum vs riser cable applies to fibre jackets in exactly the same way.
Mixed pathways
A run that starts in a plenum ceiling and drops through a riser needs plenum rating only in the plenum section, but splicing at the boundary costs two terminations and a closure. Most projects run one indoor fiber cable type end to end to avoid that.
Installing Plenum Fibre
Plenum jackets are stiffer and less slippery than riser PVC, and the fluoropolymer used for the rating has a lower friction coefficient but a larger bend radius in practice. Pull tension for a 2 to 24 fibre distribution cable is typically 200 N, and the bend radius should stay above 10 times the cable diameter under load and 15 times once installed.
Practical Scenarios
- Office fit-out with a return air ceiling: specify OFNP for every fibre and copper run above the tiles, and keep the jacket print visible at both ends for the inspection.
- Riser between floors: OFNR is sufficient and cheaper, but firestop the penetration with an approved sealant rated for the same hour as the floor.
- Data centre with overhead trays: open tray in a plenum return needs OFNP too. Armoured OFCP only pays off where rodent damage or crush risk is real.
- Retrofit where the ceiling is already closed: verify the air path first. If the void is not a return plenum, riser cable in conduit is compliant and avoids the plenum price premium.
FAQ
What does OFNP stand for?
Optical Fibre Nonconductive Plenum. OF is the fibre family, N marks a nonconductive construction with no armour or steel member, and P marks the plenum flame rating tested to NFPA 262. The code appears printed on the jacket, usually with the fibre type and count.
Can I use OFNR in a plenum ceiling?
No. Riser cable passes UL 1666, which is a vertical flame test, not the Steiner tunnel smoke and flame spread test. An inspector who finds OFNR above a return air ceiling will require removal, whatever the cable’s optical spec says.
Does plenum fibre cost more?
Yes, typically 30 to 60 percent more per metre than the riser equivalent, because the jacket compound is a fluoropolymer rather than PVC. On a short horizontal run that difference is small compared with the labour of installing conduit, so the rating rarely decides the budget.
