Fuse Class Chart

The UL low-voltage fuse classes decoded — voltage and ampere ranges, interrupting ratings, the rejection features that keep the wrong fuse out of the wrong holder, time-delay availability, and where each class belongs. Ratings shown are the UL class listings that Bussmann, Littelfuse, and Mersen publish in common; a handful of manufacturers self-certify higher interrupting ratings on specific series, and the fuse label governs.

Low-voltage fuse classes

UL 248
The nine classes you meet in distribution and control gear, smallest interrupting rating first. IR = interrupting rating, kA RMS symmetrical. Class K9 (discontinued) omitted; Class H renewable fuses are the ancestral 10 kA hardware most rejection schemes exist to keep out.
ClassVoltageAmpsIR (kA)Time delayRejection featureWhere it belongs
H250 / 600 V0–600 A10Not UL-labeled time-delayNoneLegacy light-duty circuits, resistive loads — not for high fault current or new installs
K5250 / 600 V0–600 A50Cannot carry the UL time-delay labelNone — same dimensions as H, fully interchangeableGeneral-purpose feeders and branch circuits where fault current stays under 50 kA
RK5250 / 600 V0–600 A200Yes — dual-element (holds 500% for 10 s)R slot — grooved ferrule ring / knife-blade slot; fits H clips, but H/K can’t enter R holdersThe workhorse motor-circuit fuse: mains, feeders, MCC buckets, combination starters
RK1250 / 600 V0–600 A200Yes (dual-element) and fast-acting versionsSame R rejection as RK5Everything RK5 does with tighter current limitation — arc-flash energy reduction, upgrades
J600 V0–600 A200Yes (time-delay) and fast-acting versionsUnique dimensions — physically interchanges with no other classSpace-limited panels, motor circuits, Type 2 coordination with IEC starters, panelboard mains
T300 V (to 1200 A) / 600 V (to 800 A)1–1200 A200No — fast-acting onlyUnique compact dimensions and dedicated blocks — about a third the size of Class RMeter stacks, panelboard and load-center mains, VFD line protection
CC600 V0–30 A200Yes (time-delay) and fast-acting versionsRejection pin on one end cap — the holder rejects look-alike 13/32" midget fusesControl transformers, control circuits and PLC I/O, small-HP motor branch circuits
G600 V (½–20 A) / 480 V (25–60 A)½–60 A100Yes for 7–60 A (½–6 A fast-acting)Size-rejecting — case length changes by ampere bandLighting and appliance panelboards, HVAC equipment branch circuits
L600 V601–6000 A200Time-delay versions sold; UL defines no time-delay requirement for the classBolt-in blade construction; dimensions scale with ratingService-entrance mains, switchboards, large feeders and machinery disconnects
Class T is fast-acting only — no time-delay version exists despite occasional claims otherwise. Class T's 1200 A ceiling is the 300 V line; the 600 V line stops at 800 A. Some RK1, J, and L series carry manufacturer-certified 300 kA ratings above the 200 kA class listing.

How the classes fit together

Read the chart as three families. The legacy family — H and K — shares dimensions and interchanges freely, which is exactly its hazard: a 10 kA Class H fuse drops into gear seeing 40 kA of available fault current. The rejection family — R, J, T, CC, G — exists to prevent that: each pairs a 100–200 kA current-limiting element with a physical feature that locks lesser fuses out. And Class L stands alone above 600 A, bolted rather than clipped. On new gear the choice usually reduces to R vs J by what the switch accepts — see the fuse provisions row on the safety switch ratings chart — with the time-delay versions doing motor duty per the motor circuit protection chart.

Common questions

What is the difference between RK1 and RK5 fuses?

Same dimensions, same R rejection slot, same 200 kA interrupting rating — the difference is speed under fault. RK1 elements are more current-limiting: they clear a fault with less let-through energy, which means less arc-flash incident energy and better protection for downstream gear. RK5 dual-element fuses ride motor inrush a little more forgivingly and cost less. The two interchange physically, so upgrading RK5 to RK1 is a common arc-flash remediation.

What does the R in Class R actually do?

It is a rejection feature: a grooved ring in the ferrule (up to 60 A) or a slot in the knife blade (70–600 A). An R-rated fuse holder accepts only Class R fuses, so nobody can replace a 200 kA RK1 with a 10 kA Class H fuse of the same physical size. The asymmetry is deliberate — R fuses fit old H clips for retrofits, but H fuses can never enter an R holder.

When should I use Class J instead of Class R?

When space or coordination demands it. Class J bodies are dramatically smaller than Class R at the same rating, have no compatibility with anything else (their dimensions are their rejection feature), and are the usual choice for Type 2 coordinated IEC starter protection and compact panelboards. If the gear was built with J clips, only J fuses will ever fit — a feature, not a bug.

Are Class CC fuses interchangeable with midget fuses?

One-way only, and that is the point. A Class CC fuse is a 13/32" × 1-1/2" body with a rejection pin on one end cap and a 200 kA interrupting rating. Ordinary midget fuses share the body size but may interrupt as little as 10 kA and carry no UL class listing at all — a CC holder rejects them. Never spec plain midget fuses where fault current matters.

Which fuse class for a motor circuit?

Dual-element time-delay Class RK5 or RK1 up to 600 A is the standard answer — sized at 175% of table FLC per 430.52, they ride inrush and provide backup overload protection. Class J time-delay works identically where the gear takes J. Above 600 A it is Class L by definition. For small control-cabinet motors, time-delay Class CC (or CC-rated motor fuses) covers the branch circuit in minimal panel space.

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