This article, part of the Fuse Fundamentals series, has been prepared for the EPCI Knowledge Blog with cooperation and permission from Littelfuse. You can download the PDF version of the Fuse Fundamentals from Littelfuse’s website here. This article provides overview of fuse UL classes and CSA standards.
Key Takeaways
- The article provides an overview of fuse classes, including UL and CSA standards.
- Key labeling requirements include manufacturer name, current rating, and voltage ratings.
- Current-limiting fuses include classes R, L, J, CC, CD, T, G, and K.
- Plug fuses do not possess UL letter class designations and are regulated for specific applications.
- To select the right fuse class, consider application requirements and compliance with UL and CSA standards.
UL and CSA Fuse Classes
Fuse labels must include the:
- UL or the CSA fuse class
- manufacturer’s name or trademark
- current rating
- ac or the dc voltage rating or both
- ac or the dc interrupting rating or both
Fuses that meet the requirements for current-limiting fuses are required to be labeled “current limiting.”
“Time Delay”, “D”, “TD” or equivalent may also be included on the label when the fuse complies with the time-delay requirements of its class.
Current-Limiting Fuse Classes
The classes for current-limiting fuses include:
- Class R
- Class RK1
- Class RK5
- Class L
- Class J
- Class CC/CD
- Class T
- Class G
- Class K
- Class K5
There are no classes for special-purpose fuses and fuses for supplementary overcurrent protection.
Plug fuses do not have a UL letter class designation.
| CLASS R | |
| STANDARDS: | UL Standard 248-12 CSA Standard C22.2 No.248.12-11 |
| VOLTAGE RATINGS: | 250 V and 600 V, ac; 125 V and 300 V dc |
| CURRENT RATINGS: | 0 A–600 A |
| INTERRUPTING RATING: | 200 000 A rms symmetrical |
| TWO CLASSES: | RK1 and RK5 |
Time delay is optional for Class R fuses. Time delay fuses are required to hold 500% current rating for a minimum of ten seconds. Same dimensions as UL Class H fuses, terminals modified to provide rejection feature. Fits UL Class R fuse holders which reject non Class R fuses. Physically interchangeable with UL Class H, NEMA Class H, and UL Classes K1 & K5 when equipment has Class H fuse holders.
| CLASS RK1 |
| High degree of current limitation.Provides IEC Type 2 (no damage) protection for motor starters and control components.Time delay optional, LLSRK_ID series provides visual indication of blown fuse. |
| CLASS RK5 |
| Moderate degree of current limitation, adequate for most applications.Time Delay otional.FLNR_ID, FLSR_ID and IDSR series provides visual indication of blown fuse. |
| CLASS L | |
| STANDARDS: | UL Standard 248-10 CSA Standard C22.2 No.248.10-11 |
| VOLTAGE RATINGS: | 600 V, ac and/or dc |
| CURRENT RATINGS: | 100 A–6 000 A |
| INTERRUPTING RATING: | Ac: 200 000 A rms symmetrical Dc: 50 000, 100 000, or 200 000 A |
| KLPC & KLLU: LDC: | 10 seconds at 500% current rating 4 seconds at 500% current rating |
Not interchangeable with any other UL fuse class. Time delay: Class L fuses may be marked “Time-Delay” although UL does not investigate time-delay characteristics of Class L fuses.
| CLASS J | |
| STANDARDS: | UL Standard 248-8 CSA Standard C22.2 No.248.8-11 |
| VOLTAGE RATINGS: | 600 V, ac |
| CURRENT RATINGS: | 0 A–600 A |
| INTERRUPTING RATING: | 200 000 A rms symmetrical |
Not interchangeable with any other UL fuse class. Time delay optional: Minimum of 10 seconds at 500% current rating.
| CLASS CC AND CLASS CD | |
| STANDARDS: | UL Standard 248-4, 248-18 CSA Standard C22.2 No.248.4-00; C22.2 No.248.18:22 |
| VOLTAGE RATINGS: | 600 V, ac |
| CURRENT RATINGS: | UL Class CC: 0 A–30 A UL Class CD: 35 A–60 A |
| INTERRUPTING RATING: | 200 000 A rms symmetrical |
Time delay optional: Minimum of 12 seconds at 200% current rating.
| CLASS K | |
| STANDARDS: | UL Standard 248-9 CSA Standard C22.2 No.248.9-00 |
| VOLTAGE RATINGS: | 250 V and 600 V dc, ac |
| CURRENT RATINGS: | 0 A-–600 A |
| INTERRUPTING RATING: | Three permitted: 50 000, 100 000, and 200 000 A rms symmetrical |
Time delay is optional for Class K fuses. Time Delay fuses are required to hold 500% current rating for a minimum of ten seconds. Same dimensions and physically interchangeable with UL Class H fuse holders. Class K fuses are not permitted to be labeled Current Limiting because there is no rejection feature as required by NEC Article 240-60(B).
| CLASS K5 |
| Same prescribed degree of current limitation as RK5 fuses when tested at 50 000 or 100 000 A rms symmetrical. |
| CLASS T | |
| STANDARDS: | UL Standard 248-15 CSA Standard C22.2 No.248.15-18 |
| VOLTAGE RATINGS: | 300 V and 600 V ac, 125 V and 300 V dc |
| CURRENT RATINGS: | 0 A–1200 A |
| INTERRUPTING RATING: | 200 000 A rms symmetrical |
Fast-Acting fuses.High degree of current limitation. Very small fuses; space-saving and non-interchangeable with any other UL fuse class.
| CLASS G | |
| STANDARDS: | UL Standard 248-5 CSA Standard C22.2 No.248.5-00 |
| VOLTAGE RATINGS: | 480 V ac and 600 V ac |
| CURRENT RATINGS: | 0 A–60 A |
| INTERRUPTING RATING: | 100 000 A rms symmetrical |
Not interchangeable with any other UL fuse class. Time delay optional: Minimum of 12 seconds at 200% current rating.
| FUSES FOR SUPPLEMENTARY OVERCURRENT PROTECTION | |
| STANDARDS: | UL Standard 248-14; CSA Standard C22.2 No.248.14-00.Three Classifications covered: |
| NOTE: Fuses may be rated for ac and/or dc when suitable for such use. | |
| (1) Micro fuses Voltage ratings: Current ratings: Interrupting rating: | UL, 125 V; CSA, 0 V–250 V UL, 0 A–10 A; CSA, 0 A–60 A 50 A rms symmetrical (CSA classifies these as Supplemental Fuses) |
| (2) Miniature fuses Voltage ratings: Current ratings: Interrupting rating: | UL, 125 V or 250 V; CSA, 0 V–600 V UL, 0 A–30 A; CSA, 0 A–60 A 10 000 A rms symmetrical (CSA classifies these as Supplemental Fuses) |
| (3) Miscellaneous Cartridge fuses Voltage ratings: Current ratings: Interrupting rating: | UL, 125 V–1 000 V; CSA, 0 V–100 V UL, 0 A–30 A; CSA 0 A–60 A 10 000, 50 000, or 100 000 A rms symmetrical |
Time delay (optional); Minimum delay at 200% fuse rating: 5 seconds for fuses rated 3 A or less 12 seconds for fuses rated more than 3 A
| SEPECIAL PURPOSE FUSES | |
| VOLTAGE RATINGS: | up to 1 000 V ac and/or dc |
| CURRENT RATINGS: | up to 6 000 A |
| INTERRUPTING RATING: | up to 200 000 A |
There are no UL Standards covering this category of fuses.These fuses have special characteristics designed to protect special types of electrical or electronic equipment. Fuses may be UL Recognized for use as a component in UL Listed equipment. UL Recognized fuses are tested for characteristics such as published interrupting capacity.They are also covered by UL re-examination service.
Many of these fuses are extremely current limiting.When considering application of these fuses, or if you have special requirements, contact Littelfuse Technical Support Group for assistance.
| PLUG FUSES | |
| STANDARDS: | UL Standard 248-11 CSA Standard C22.2 No.248.11-11 |
| VOLTAGE RATINGS: | 125 V ac only |
| CURRENT RATINGS: | 0 A–30 A |
| INTERURUPTING RATINGS: | 10 000 A rms symmetrical.Interrupting rating need not be marked on fuse. |
Two types: Edison-base and Type S Edison-base: Base is the same with an E26 light-bulb base.All amp ratings interchangeable.NEC permits Edison-base plug fuses to be used only as replacements for existing fuses, and only when there is no evidence of tampering or overfusing. Type S: Not interchangeable with Edison-base fuses unless non-removable Type S fuse adapter is installed in Edison-base fuse socket.To prevent overfusing, adapters have three current ratings: 10 A–15 A, 16 A–20 A, and 21 A–30 A. Time delay: Fuses may be time delay, if so, they are required to hold 200% of rating for 12 seconds minimum.
NOTE: Plug fuses may be used where there is not more than 125 V between conductors or more than 150 V from any conductor to ground.This permits their use in 120/240 V grounded, single-phase circuits.
| CLASS H | |
| STANDARDS: | UL Standard 248-6, 248-7 CSA Standard C22.2 No.248.6-00; C22.2 No.248.7-00.Also known as NEMA Class H, and sometimes referred to as “NEC” or “Code” fuses |
| VOLTAGE RATINGS: | 250 V and 600 V, ac |
| CURRENT RATINGS: | 0 A–600 A |
| INTERRUPTING RATING: | 10 000 A rms symmetrical |
Two types: one-time and renewable Physically interchangeable with UL Classes K1 & K5; Fits UL Class H fuse holders which will also accept K1, K5, RK5, and RK1 fuses. Manufacturers are upgrading Class H one-time fuses to Class K5 per UL Standard 248-9D, See Class K fuses.
| ONE-TIME FUSES (NON-RENEWABLE) |
| Time delay: Optional Time-delay fuses must hold 500% current rating for a minimum of ten seconds. |
| RENEWABLE FUSES |
| Only Class H fuses may be renewable.While time delay is optional, no renewable fuses meet requirements for time delay. Some renewable fuses have a moderate amount of time delay, referred to as “time lag” to differentiate from true time delay. |
Codes and Standards
National Electrical Code (NEC)
- NEC ARTICLE 110 Requirements for Electrical Installations
- NEC ARTICLE 240 Overcurrent Protection
- NEC ARTICLE 430 Motors, Motor Circuits, and Controllers
International Electrical Code (IEC)
- IEC 60269-1 Low-voltage fuses: Part 1: General requirements
- IEC 60269-2 Low-voltage fuses: Part 2: Supplementary requirements for fuses for use by authorized persons (fuses mainly for industrial application): Examples of standardized systems of fuses A to I
- IEC 60269-3 Low-voltage fuses: Part 3: Supplementary requirements for fuses for use by unskilled persons (fuses mainly for household and similar applications): Examples of standardized systems of fuses A to F
- IEC 60269-4 Low-voltage fuses: Part 4: Supplementary requirements for fuse-links for the protection of semiconductor devices
- IEC 60269-5 Low-voltage fuses: Part 5: Guidance for the application of low-voltage fuses
Standards
UL
- UL 248-1 Low-Voltage Fuses – Part 1: General Requirements
- UL 248-4 Standard for Low-Voltage Fuses – Part 4: Class CC Fuses
- UL 248-5 Standard for Low-Voltage Fuses – Part 5: Class G Fuses
- UL 248-8 Low-Voltage Fuses – Part 8: Class J Fuses
- UL 248-9 Standard for Low-Voltage Fuses – Part 9: Class K Fuses
- UL 248-10 Low-Voltage Fuses – Part 10: Class L Fuses
- UL 248-12 Low-Voltage Fuses – Part 12: Class R Fuses
- UL 248-13 Low-Voltage Fuses – Part 13: Semiconductor Fuses
- UL 248-14 Standard for Low-Voltage Fuses – Part 14: Supplemental Fuses
- UL 248-15 Low-Voltage Fuses – Part 15: Class T Fuses
- UL 248-18 Low-Voltage Fuses – Part 18: Class CD Fuses
- UL 248-19 Standard for Low-Voltage Fuses – Part 19: Photovoltaic Fuses
IEEE
- IEEE 3004.3-2020 IEEE 3004.3 Recommended Practice for the Application of Low-Voltage Fuses in Industrial and Commercial Power Systems
Fuse Classes and Standards – FAQ
UL and CSA fuse classes define standardized categories of fuses, each with specific voltage, current, and interrupting ratings. They include classes R, L, J, CC/CD, T, G, K (K5), H, and supplementary/special-purpose types, ensuring compatibility and safety in electrical installations.
Current-limiting fuses include Class R (RK1, RK5), L, J, CC/CD, T, and G, designed to limit peak current during short-circuit events and enhance equipment protection.
“Time Delay” means the fuse can withstand temporary overloads, such as motor startup currents, without blowing immediately. This feature is optional and marked when relevant for the specific fuse class.
No, plug fuses do not have a lettered UL class designation. They include Edison-base and Type S, and are regulated for replacement applications only.
Fuses in North America adhere to UL 248 series standards, CSA equivalents, and follow codes like NEC Articles 110, 240, and 430. Internationally, IEC 60269 series sets requirements for fuse applications and characteristics.
How to Select the Right Fuse Class
- Identify Application Requirements
Review your circuit’s voltage, current, and interrupting rating needs. Consider if current-limiting or time-delay features are required for the load or equipment.
- Check Standards Compliance
Make sure your fuse meets UL and CSA standards for your region (e.g., UL 248, CSA C22.2) and verify if IEC requirements apply for international installations.
- Select Appropriate Fuse Class
Choose the fuse class based on compatibility with your equipment and holders: R, L, J, CC/CD, T, G, K/K5, H, or supplementary/special-purpose types.
- Verify Labeling & Time Delay Needs
Look for correct labeling: manufacturer, voltage ratings, current ratings, and class. If time delay is needed (for motors, inductive loads), select a fuse marked “Time Delay,” “D,” or “TD.”
- Consult Fuse Manufacturer Resources
Use technical documentation or manufacturer support (e.g., Littelfuse Fuse Fundamentals) to confirm proper selection and usage for your specific scenario.
Further read from the Fuse Fundamentals:
