What is the difference between plenum and riser cable?

Plenum cable (CMP for copper, OFNP for fiber) has a fire-retardant, low-smoke jacket rated for air-handling spaces like above drop ceilings or under raised floors. Riser cable (CMR/OFNR) is rated to stop fire from spreading vertically between floors but does not meet plenum smoke requirements.

Can I use riser cable in a plenum space?

No. You cannot substitute riser (CMR/OFNR) cable in a plenum air-handling space; it violates the National Electrical Code. Higher-rated plenum cable can substitute for riser, but not the reverse.

Is LSZH the same as plenum?

No. LSZH (low smoke zero halogen) describes a jacket that produces minimal smoke and no halogens, common in Europe and enclosed spaces. It is not automatically a substitute for a required CMP or CMR rating unless the product is specifically listed for that application.

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Reference · Fire Ratings

Plenum vs riser cable.

The jacket rating on a cable is a fire-safety and code requirement, not a preference. This guide explains plenum (OFNP/CMP), riser (OFNR/CMR), and LSZH — and where each is required.

The ratings

Rating	Copper / Fiber code	Standard	Where required
Plenum	CMP / OFNP	NFPA-262 / UL-910	Air-handling spaces (above ceilings, raised floors)
Riser	CMR / OFNR	UL-1666	Vertical runs between floors
LSZH	Low smoke zero halogen	Regional / listing-dependent	Enclosed spaces; common in Europe

Plenum (CMP / OFNP)

A plenum is airspace used for air circulation by HVAC systems — the space above a drop ceiling or under a raised floor. Plenum-rated cable meets NFPA-262 / UL-910: its jacket produces very little smoke and resists flame spread, often using fluorinated ethylene propylene (a Teflon-family material). Because those spaces move air throughout a building, code requires the strictest smoke performance there.

Riser (CMR / OFNR)

Riser refers to the vertical shafts that carry cabling between floors. Riser-rated cable meets UL-1666, which tests its ability to stop fire from climbing floor to floor. It is not held to the same low-smoke requirement as plenum, so it can't go in a plenum space.

LSZH

LSZH (low smoke zero halogen) jackets minimize smoke and emit no halogens, which reduces toxicity and corrosion in a fire. LSZH is common in Europe and in enclosed or poorly ventilated areas — but it is not automatically a substitute for a required CMP or CMR listing unless the specific product is listed for that use.

Substitution rule (important)

Ratings substitute in one direction only: plenum (CMP/OFNP) → riser (CMR/OFNR) → general purpose (CM). You can always use a higher-rated cable in a lower-rated space, but never the reverse. Putting riser cable in a plenum space violates the National Electrical Code and creates a real fire hazard — so when in doubt, plenum is the safe spec.

Need plenum- or riser-rated assemblies?

Tri-Valley Optics builds fiber and copper assemblies in plenum (OFNP/CMP) and riser (OFNR/CMR) ratings to your spec — any length, any quantity, most orders shipped in about 2 business days with a lifetime warranty.

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