Definition:
UL 1666 is a standardized flame propagation test defined by Underwriters Laboratories (UL) that evaluates the vertical flame spread characteristics of electrical cables installed in riser applications—meaning, in vertical shafts that connect multiple floors in a building but are not air handling spaces (like plenums).

To assess whether a cable installed vertically in a riser shaft can resist flame propagation and prevent a fire from spreading rapidly from one floor to another.

Step	Details
1. Sample Setup	A 7.3 m (24 ft) section of cable is mounted vertically inside a fire test shaft that mimics a building riser.
2. Flame Source	A diffusion burner delivers a 154 kW (525,000 BTU/hr) flame at the base of the cable for 30 minutes.
3. Air Flow	A controlled airflow simulates chimney effects common in risers—pulling flames upward.
4. Flame Spread Measurement	A vertical ruler tracks the highest point of flame propagation above the ignition source.
5. Pass Criteria	To pass the test, the flame must not exceed 3.66 m (12 ft) of vertical spread.

Cables that pass UL 1666 are eligible for installation in vertical riser shafts and receive the following suffixes:

Cable Type	Example Code
Communications Cable	CMR
Class 2 & 3 Control Cable	CL2R / CL3R
Fire Alarm Cable	FPLR
Coaxial Cable	CATVR
Fiber Optic Cable	OFNR (nonconductive) / OFCR (conductive)
Broadband Cable	BMR

• Only Flame Spread is Measured: Unlike UL 910 (Plenum Test), UL 1666 does not measure smoke generation.
• Riser cable jackets are typically made of PVC or FR-PE; they are flame retardant but not low-smoke.
• UL 1666 is required by NEC Article 800, 725, 760, 770, and 820 for riser installations in commercial buildings.
• Fire barriers such as firestopping must still be used to maintain floor-to-floor fire separation integrity.

QuoteExample Application:
A fire alarm riser cable (FPLR) is routed in a 15-story building's vertical conduit shaft. It must comply with UL 1666 to prevent fire from spreading upward through the cable run in the event of a fire in lower floors.

Failure Scenario (non-compliance):
If non-rated cable is used in the riser, a fire could propagate rapidly upward, turning a localized floor incident into a full building emergency.