Class J, RK1, RK5 and CC are not simply four labels for the same kind of fuse. Each class has its own dimensional, rejection, voltage, and performance rules, and even two fuses within the same UL class may behave differently.
That means a replacement decision cannot be based on amperage or fuse class alone. The correct process is to identify the exact existing fuse, confirm the holder and equipment, compare the candidate’s current OEM data, and verify the protection behavior before ordering.
The practical differences are:
• Class J: compact branch-circuit fuse class, typically up to 600A, with unique rejecting dimensions.
• Class RK1: Class R rejection-style fuse with stronger current-limiting performance than many RK5 products, available in time-delay and fast-acting families.
• Class RK5: Class R rejection-style fuse family commonly used for motors and transformers, often with dual-element time-delay performance.
• Class CC: compact 600V branch-circuit fuse class generally used up to 30A, with rejection features and several response types.
None of these classes should be treated as automatically interchangeable. Verify the exact fuse family, voltage, interrupting rating, time-current behavior, dimensions, holder, and application.
Comparison at a Glance
Class J
Typical current range: Up to 600A
Typical voltage: Up to 600V class products
Physical format: Compact, unique rejecting dimensions
Examples: Eaton LPJ, JKS; Mersen AJT, A4J
Common behavior: Time-delay and fast-acting families both exist
Replacement note: Unique dimensions usually require a Class J holder or compatible equipment
Class RK1
Typical current range: Up to 600A
Typical voltage: 250V and 600V product families
Physical format: Class R rejection-style dimensions
Examples: Eaton LPN-RK, LPS-RK, KTN-R, KTS-R
Common behavior: Time-delay and fast-acting families both exist
Replacement note: Same basic physical dimensions as RK5 in comparable ratings, but performance may differ significantly
Class RK5
Typical current range: Up to 600A
Typical voltage: 250V and 600V product families
Physical format: Class R rejection-style dimensions
Examples: Eaton FRN-R, FRS-R
Common behavior: Commonly dual-element, time-delay
Replacement note: Do not assume an RK1 and RK5 fuse are equivalent because they fit the same holder
Class CC
Typical current range: Generally up to 30A
Typical voltage: 600V branch-circuit class
Physical format: Small 13/32 x 1-1/2 inch rejection-style format
Examples: Eaton LP-CC, FNQ-R, KTK-R; Mersen ATDR, ATMR
Common behavior: Time-delay and fast-acting families both exist
Replacement note: Requires Class CC-compatible holder; not the same as a generic midget fuse
1. Why UL Fuse Class Matters
Fuse classes establish standardized physical and performance requirements.
Eaton's fuse-class guidance explains that classes such as RK1, RK5, J, and CC are separate low-voltage fuse classifications. Mersen likewise notes that Class J, R, and CC products use unique dimensions or rejection features intended to prevent improper substitution.
This is important because fuse replacement is not just an electrical-rating decision.
The class can determine:
- Physical dimensions
- Rejection features
- Voltage class
- Interrupting performance
- Holder compatibility
- Current-limiting requirements
- Available time-delay or fast-acting families
The class should therefore be identified before a replacement candidate is selected.
2. Class J Fuses
Class J is widely used in industrial branch-circuit applications where compact size and high current-limiting performance are desirable.
Eaton's current Class J portfolio includes:
LPJ Low-Peak: Dual-element, time-delay, current-limiting.
JKS Limitron: Fast-acting, with no intentional time delay.
Eaton publishes Class J products from 1A through 600A in the LPJ family, with 600Vac ratings and high interrupting ratings on applicable products.
Mersen's AJT Class J fuse is another current-limiting, time-delay Class J example, while A4J is a fast-acting Class J family.
The critical point is that "Class J" does not tell you the response type.
A time-delay LPJ and a fast-acting JKS may share the same class and ampere rating but are not functionally identical.
3. Class RK1 Fuses
Class RK1 is part of the Class R family.
Class R fuses incorporate rejection features intended to prevent lower-performance Class H-type fuses from being installed in rejection-style Class R holders.
Eaton's current RK1 families include:
LPN-RK: 250V Low-Peak, dual-element, time-delay.
LPS-RK: 600V Low-Peak, dual-element, time-delay.
KTN-R: 250V Limitron, fast-acting.
KTS-R: 600V Limitron, fast-acting.
Eaton's current literature shows that both time-delay and fast-acting products exist within Class RK1.
That means "RK1" is not a synonym for "time-delay."
4. Class RK5 Fuses
Class RK5 is also a Class R rejection-style fuse category.
Eaton's Fusetron RK5 families include:
FRN-R: 250V family.
FRS-R: 600V family.
These are commonly used dual-element, time-delay fuses for applications with inrush current such as motors, transformers, welders, and similar industrial loads.
Eaton publishes RK5 Fusetron products with 200kA RMS symmetrical interrupting ratings for applicable AC ratings.
The important distinction is that RK5 generally provides less current-limiting performance than premium RK1 products.
A buyer should not assume that RK1 and RK5 are equivalent simply because both use the same basic Class R physical format.
5. Class CC Fuses
Class CC is a compact branch-circuit fuse class.
Eaton's current Class CC portfolio includes:
LP-CC Low-Peak: Time-delay.
FNQ-R: Time-delay, commonly associated with control-transformer and control-circuit applications.
KTK-R Limitron: Fast-acting.
Eaton describes Class CC as a 600V, 200kA interrupting-rating class in a very compact package.
Class CC also uses a rejection feature in appropriate fuse blocks to prevent insertion of lower-rated supplemental fuses.
Do not confuse a Class CC fuse with a generic 13/32 x 1-1/2 inch midget fuse just because the dimensions look similar.
6. RK1 vs RK5: Same Basic Size Does Not Mean Same Performance
This is one of the most common areas of confusion.
Eaton states that Class RK1 and RK5 fuses have the same basic dimensions as each other and as certain older Class H formats.
That physical relationship is useful for upgrade paths, but it does not mean every RK1 and RK5 product performs the same.
Eaton's own Fuseology guidance says that even two fuses within Class RK1 can have substantially different performance characteristics. For example, a Low-Peak dual-element time-delay fuse and a Limitron fast-acting fuse are both RK1 but may require very different sizing and application treatment.
The same logic applies when comparing RK1 with RK5.
Always verify the exact product family.
7. Class J vs Class RK1
Class J and RK1 products can both be high-performance current-limiting fuses, but they use different physical formats.
Class J
- Compact, unique Class J dimensions
- Typically requires Class J-compatible holder
- Available in time-delay and fast-acting families
Class RK1
- Class R rejection-style format
- Can fit compatible Class R holders
- Also available in time-delay and fast-acting families
Mersen describes Class J as particularly attractive where compact size is important, while RK1 is often used to upgrade existing installations that already use Class H, K, or RK5-size fuse equipment.
Do not change from RK1 to Class J without verifying the holder and equipment.
8. Class RK1 vs Class RK5
The most practical comparison is performance.
Eaton's current product families show:
RK1 Low-Peak: Higher current-limiting performance and, for applicable Low-Peak products, up to 300kA interrupting rating.
RK5 Fusetron: Dual-element, time-delay performance with 200kA interrupting rating on applicable AC products.
Both can be appropriate industrial fuses.
The better replacement is determined by the existing design and documented application, not by assuming one class is universally superior.
9. Class CC vs Class J
Both are branch-circuit fuse classes and both can be found in 600V industrial applications, but they are physically very different.
Class CC
- Compact 13/32 x 1-1/2 inch format
- Generally up to 30A
- Rejection-style holder
- Common in control circuits, small motors, and compact industrial equipment
Class J
- Larger current range
- Available through hundreds of amps
- Unique Class J dimensions
- Common in feeders, branch circuits, motor circuits, and larger industrial equipment
A Class CC fuse is not a smaller drop-in version of a Class J fuse.
10. Verify Voltage, Not Just Fuse Class
The class alone does not establish the exact voltage rating.
Within Class R families, 250V and 600V versions are common.
Examples include:
- LPN-RK: 250V RK1 family
- LPS-RK: 600V RK1 family
- FRN-R: 250V RK5 family
- FRS-R: 600V RK5 family
For Class J and Class CC, 600V products are common, but always verify the exact part.
For DC applications, verify the exact DC rating separately.
Do not infer DC capability from the AC class or voltage.
11. Verify Interrupting Rating
The replacement fuse must have an interrupting rating suitable for the system.
Current OEM literature shows examples such as:
- Eaton LPJ Class J Low-Peak products with up to 300kA interrupting rating at 600Vac.
- Eaton Fusetron RK5 products with 200kA interrupting rating at applicable AC voltages.
- Eaton Class CC products with 200kA interrupting ratings.
These are product-family examples, not universal values for every fuse carrying the same class designation.
Use the exact data sheet for the part being considered.
12. Verify Time-Delay vs Fast-Acting Performance
This is one of the most important checks.
Each of these classes can include different response families.
Examples:
Class J
- LPJ = time-delay
- JKS = fast-acting
Class RK1
- LPN-RK / LPS-RK = time-delay
- KTN-R / KTS-R = fast-acting
Class CC
- LP-CC = time-delay
- FNQ-R = time-delay
- KTK-R = fast-acting
A fuse can match the class, amperage, and voltage and still provide the wrong protection behavior.
13. Check the Fuse Holder
Before replacing a fuse, record the holder or block:
- Manufacturer
- Part number
- Fuse class
- Voltage
- Ampere range
- Poles
- Rejection feature
- Terminal style
- Mounting
A holder may be designed specifically for Class J, Class R, or Class CC.
Do not defeat a rejection feature.
If the holder is damaged or obsolete, treat holder replacement as a separate engineering and sourcing decision.
14. Do Not Use a Higher Class as a Shortcut
It may be tempting to assume that a higher interrupting rating or stronger current-limiting class is automatically acceptable.
That is not a safe general rule.
A replacement still needs to match:
- Voltage
- Ampere rating
- Holder
- Physical format
- Response characteristics
- Application
- Equipment listing
- Coordination requirements
A technical upgrade can be valid, but it should be documented.
15. What to Send JW for a Fuse-Class Replacement Request
Send:
- Original fuse manufacturer
- Complete part number
- Fuse class
- Ampere rating
- AC voltage
- DC voltage if applicable
- Interrupting rating if known
- Time-delay or fast-acting marking
- Fuse-holder or block number
- Equipment manufacturer and model
- Application type
- Clear label photos
- Quantity
- Required condition
- Ship-to location
- Required-by date
If the class is unclear, include the fuse dimensions and holder photos.
Related JW Fuse Resources
Buyers can review JW Electrical Supply's Fuses and Fuse Holders inventory while preparing a sourcing request.
JW's Industrial Fuse Cross-Reference workflow explains the broader verification process, while the Bussmann and Mersen cross-reference guides provide manufacturer-specific workflows once those pages are live.
For general condition terminology, JW's New vs. Reconditioned vs. Surplus vs. Used Circuit Breakers guide explains why condition and technical compatibility should be handled separately.
Common Fuse-Class Replacement Mistakes
Assuming same class means same performance: It does not. Eaton explicitly notes that two fuses in the same class can still have different functions and sizing requirements.
Assuming RK1 and RK5 are identical because they fit: Their basic dimensions can be similar, but their current-limiting and response performance may differ.
Using a midget fuse instead of a Class CC fuse: Similar dimensions do not prove branch-circuit equivalence.
Ignoring voltage family: FRN-R and FRS-R, or LPN-RK and LPS-RK, are different voltage families.
Ignoring response type: LPJ and JKS are both Class J but one is time-delay and the other fast-acting.
Ignoring the holder: The holder and rejection system are part of the approved application.
Frequently Asked Questions
Both are branch-circuit current-limiting fuse classes, but they use different physical formats and holders. Both classes also include different response families. Verify the exact product and application rather than treating one class as a direct substitute for the other.
Both are Class R rejection-style fuse classes with similar basic dimensions. RK1 products generally provide stronger current-limiting performance than RK5 products, but exact performance depends on the fuse family. Replacement still requires product-level verification.
Some RK1 products may be used as documented upgrades for certain RK5 applications because the physical Class R format is compatible, but do not assume every RK1/RK5 pair is interchangeable. Verify voltage, amperage, response type, holder, interrupting rating, and application.
Not automatically. Class CC is a branch-circuit-rated rejection fuse class. Many generic midget fuses are supplementary fuses with different listing and application limitations.
No. Eaton's LPJ Class J family is time-delay, while JKS is fast-acting.
There is no universal best class. The correct class is the one documented for the equipment and application, with the required voltage, interrupting rating, time-current behavior, physical format, and holder compatibility.
Need Help Comparing Industrial Fuse Classes?
Contact JW Electrical Supply with the original fuse part number, label photo, fuse-holder information, equipment model, quantity, and required-by date.
JW can help identify the correct product family and available sourcing options. Final substitution should remain supported by current OEM documentation and the application's electrical requirements.
For fuse sizing changes, coordination changes, fault-current questions, or protection redesign, involve an appropriately qualified electrical professional.