Fuse Types Explained: Class J, Class RK, Class L & More
Industrial fuses are grouped into classes defined by UL and NEC standards. The class determines the fuse's physical dimensions, interrupting capacity, current-limiting ability and the type of fuse holder it fits. Choosing the correct class is not optional — using the wrong fuse class can compromise protection and violate electrical codes.
Class RK1 & RK5 — The Workhorses
Class RK fuses fit standard "rejection-style" fuse holders (the "R" means rejection — they have a pin or notch that prevents non-current-limiting fuses from being installed). They are the most common industrial fuses in North America.
- RK1 — current-limiting, dual-element (time-delay). Better peak let-through and I²t limiting than RK5. Examples: Bussmann LPN-RK / LPS-RK (Low-Peak), Mersen TRS-R / TRS.
- RK5 — current-limiting, time-delay. Adequate for most motor and transformer circuits. Less expensive than RK1. Examples: Bussmann FRN-R / FRS-R (Fusetron), Mersen TR / TRS.
Rule of thumb: use RK1 when you need superior current-limiting performance (downstream component protection, series-rated systems). Use RK5 for general motor branch circuits where cost matters.
Class J — Compact & Current-Limiting
Class J fuses are physically smaller than Class RK for the same amperage, which allows more compact panelboards and switchboards. They offer excellent current-limiting performance and have their own unique physical dimensions that prevent interchangeability with other classes. Examples: Bussmann LPJ / LPJ-SP (Low-Peak), Littelfuse JTD / JLLN / JLLS.
Class J is the preferred choice for new installations where space is tight and strong current-limiting is important.
Class L — High-Amperage Mains
Class L fuses serve main bus and feeder applications at 601–6,000 A. They are bolt-in (not clip-mounted) and are used in switchboards and large distribution panels. Examples: Bussmann KRP-C / KTU, Littelfuse KLPC, Mersen A4BQ / A4BY.
If your main disconnect or switchboard bus fuse is a large bolt-in unit rated above 600 A, it's almost certainly a Class L.
Class CC — Control Circuit & Lighting
Class CC fuses are small, compact units used for control circuits, lighting panelboards, PLCs and other low-amperage applications (typically 0.1–30 A at 600 V). Their small size allows very dense packaging. Examples: Bussmann LP-CC / FNQ-R, Littelfuse CCMR / KLKR.
Class T — Space-Saving Alternative
Class T fuses are the most compact current-limiting fuses available. They're used where panel space is extremely limited — typically in service-entrance equipment and compact switchboards. Examples: Bussmann JJN / JJS, Littelfuse JLLN / JLLS.
Class G & H — Legacy & One-Time
Class G fuses are small, compact, current-limiting fuses often found in older panelboards (up to 60 A, 480 V). Class H fuses are non-current-limiting, one-time (non-renewable) fuses. Class H is largely being replaced by Class RK in modern installations, but many older systems still use them.
Key Specs When Ordering
- Fuse class — determines the holder and performance level.
- Voltage rating — must meet or exceed system voltage (250 V or 600 V are the two main ratings).
- Amperage — match the existing fuse or the circuit design.
- Time-delay vs. fast-acting — time-delay (dual-element) fuses tolerate motor inrush; fast-acting fuses protect semiconductor devices and welders.
- Interrupting rating (AIR) — the maximum fault current the fuse can safely clear. 200 kA and 300 kA are common for industrial current-limiting fuses.