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UL Fuse Classes Explained: CC, J, RK1, RK5, T and L
Quick answer: A UL fuse class defines standardized dimensions and performance requirements. Same-class fuses can often share holders, but speed, voltage, DC rating and let-through performance still differ. Different classes are generally not physically interchangeable. |
UL fuse classes allow engineers and maintenance teams to identify the protective role before choosing a brand. They also support rejection features that prevent installation of a lower-rated or wrong-class fuse. The class is the first filter; the product family and exact catalog number complete the selection.
Class-by-class comparison
Class | Typical rating envelope | Representative series | Key use / caution |
CC | Up to 30 A, commonly 600 Vac, 200 kA AC IR | Bussmann FNQ-R, LP-CC, KTK-R; Littelfuse KLDR, CCMR, KLKR; Mersen ATQR, ATDR, ATMR | Compact branch protection; distinguish control-transformer, motor time-delay and fast families |
J | Up to 600 A, commonly 600 Vac, 200 kA minimum class IR | Bussmann LPJ/JKS; Littelfuse JTD/JLS; Mersen AJT/A4J | Compact current-limiting main, feeder, motor and branch protection |
RK1 | Up to 600 A, 250 or 600 Vac families | Bussmann LPN-RK/LPS-RK; Littelfuse LLNRK/LLSRK; Mersen A2D-R/A6D-R | High current limitation and lower let-through than RK5 in many comparisons |
RK5 | Up to 600 A, 250 or 600 Vac families | Bussmann FRN-R/FRS-R; Littelfuse FLNR/FLSR; Mersen TRN-R/TRS-R | General-purpose dual-element time-delay protection |
T | 300 or 600 Vac; rating limits depend on voltage family | Bussmann JJN/JJS; Littelfuse JLLN/JLLS; Mersen A3T/A6T | Very compact, fast-acting, highly current-limiting; voltage families use distinct dimensions |
L | Typically 601-6000 A, 600 Vac | Bussmann KRP-C; Littelfuse KLPC/KLLU; Mersen Class L families | Bolted-blade main/service applications; check holder and equipment details |
Representative exact models
Model | Class / rating | What it illustrates |
FNQ-R-10 | Class CC, 10 A, time-delay | Control-transformer-oriented Bussmann family |
KTK-R-10 | Class CC, 10 A, fast-acting | Same class/current as FNQ-R-10 but different speed |
LPJ-30SP | Class J, 30 A, time-delay | Compact Class J branch/motor protection |
JKS-30 | Class J, 30 A, fast-acting | Same class/current as LPJ-30SP but different characteristic |
LPS-RK-60SP | Class RK1, 60 A, 600 V family | High current-limiting rejection fuse |
FRS-R-30 | Class RK5, 30 A, 600 V | Dual-element time-delay RK5 family |
JJS-100 | Class T, 100 A, 600 V | Compact fast-acting Class T; candidate cross A6T100/JLLS100 |
KRP-C-800SP | Class L, 800 A, 600 V family | High-current bolted-blade service/feeder protection |
RK1 vs RK5
RK1 and RK5 fuses use Class R rejection concepts and may fit the same Class R rejection holders at the corresponding voltage and amp frame, but they are not performance-identical. RK1 families generally provide a higher degree of current limitation and lower let-through energy. Replacing an RK1 with RK5 can reduce downstream component protection or affect an SCCR/coordination study. An upgrade from RK5 to an approved RK1 candidate may improve current limitation, but it still needs engineering confirmation.
Class T voltage families
Class T is compact and fast-acting, but the 300 V and 600 V families use different dimensions. Bussmann JJN is the 300 V family and JJS is the 600 V family; Mersen uses A3T and A6T; Littelfuse uses JLLN and JLLS. For example, JJS-100, A6T100 and JLLS100 are 600 V, 100 A family candidates. Do not select a 300 V model for a 600 V circuit.
Interchangeability rules
• Stay within the same UL class unless the equipment is redesigned with the correct holder and re-evaluated.
• Within a class, match voltage, current, speed, interrupting rating, DC rating and indication.
• Do not treat RK1 and RK5 as performance-equivalent merely because they may fit a Class R holder.
• Class CC and midget 10 x 38 mm products are not equivalent despite similar size.
• Class J and Class T have unique dimensions and require their corresponding holders or blocks.
• High-speed semiconductor fuses are often outside these general-purpose class comparisons and require I2t-based selection.
Frequently Asked Questions
Which UL fuse class is most common in industrial control panels?
Class CC is common for compact control and small branch circuits, while Class J is common for feeders, motor circuits and larger branch protection. The design determines the class.
Can a Class J fuse replace a Class RK5 fuse?
No direct physical replacement. The classes have different dimensions and holders, even when voltage and current ratings match.
Can RK1 replace RK5?
It may be an approved upgrade path within the correct Class R holder and rating, but coordination, equipment listing and exact performance must be checked.
Why are FNQ-R-10 and KTK-R-10 not interchangeable?
They share Class CC and 10 A ratings, but FNQ-R is time-delay for high-inrush control-transformer duty and KTK-R is fast-acting.
Bottom line
Use the UL class to establish physical and minimum performance compatibility, then select the exact family for speed and application. Product models make the distinction concrete: FNQ-R-10, KTK-R-10 and LP-CC-10 are all Class CC at 10 A, yet they solve different protection problems.
Send ASBEAM the installed fuse model, holder, system voltage and load type for a class-correct model shortlist across Bussmann, Littelfuse and Mersen.
Technical references
• Littelfuse - Fuse Fundamentals and UL/CSA class overview
• Eaton - Bussmann fuse data-sheet library
Mersen - A6T Class T official family
New industry Technology regarding to Bussmann fuse, ABB breakers, Amphenol connectors, HPS transformers, etc.
TAG: UL fuse classes Class CC fuse Class J fuse Class RK1 Class RK5 Class T fuse Class L fuse