For equipment destined for the North American market, fuses must be UL Listed under UL 248 and installed per NFPA 70 (the National Electrical Code). The US fuse system revolves around classes — each with defined dimensions, performance, and interchangeability rules.

Bussmann offers the broadest UL class fuse lineup in the industry: LPJ, JKS, JJS, FRN-R, FRS-R, LP-CC, JJN, and Class T series. This guide explains each class, when to use it, and how to select specific models.

Why Classes Matter

Under UL 248, a fuse class defines:

• Physical dimensions — fuses of the same class and rating are interchangeable between manufacturers
• Time-current characteristics — the speed at which the fuse opens
• Current-limiting ability — whether the fuse limits peak let-through current
• Rejection features — whether the fuse prevents inserting a lower-rated fuse into a higher-rated holder

The class system exists so that a maintenance technician can replace a blown fuse with a correctly-rated, physically-compatible unit from any approved manufacturer.

Class J — The Modern Standard

Class J is the most widely specified UL fuse class for new construction. Its compact dimensions deliver high interrupting capacity (200kA RMS symmetrical) and excellent current limiting.

Bussmann Class J Models

LPJ (Low Peak J) is Bussmann's flagship Class J fuse. The dual-element, time-delay design handles motor inrush while delivering exceptional current limiting.

Selection example — 30kW motor branch (480V, 40A FLC): - Inrush: 240A (6× FLC) - Selected: LPJ-50SP (50A, Class J, time-delay) - The time-delay curve rides through the 6× starting surge; interrupting capacity 200kA

Class RK5 and RK1 — Renewable and Non-Renewable

Class RK fuses come in two performance grades:

• Class RK5 — Standard time-delay, moderate current limiting
• Class RK1 — Enhanced current limiting, faster opening under fault

Bussmann RK5 Models

FRN-R is the classic general-purpose RK5 fuse found in millions of North American panels. FRS-R offers additional time-delay for high-inrush loads like transformers and large motors.

Bussmann RK1 Models

Class RK1 fuses provide the same physical envelope as RK5 but with superior current limiting — a straight upgrade where the panel allows.

Class CC — Branch Circuit Protection for Control

Class CC fuses are small, rejection-type fuses designed for control circuits, small motors, and branch circuits up to 30A.

LP-CC is the workhorse for control panels, PLC power supplies, and small motor branch circuits. Its rejection feature prevents inserting an oversized fuse — a key safety attribute.

Selection example — 3-phase 5HP motor branch (480V, 7.6A FLC): - Selected: LP-CC-15 (15A Class CC) - With rejection holder CHCC-3 (3-pole) - Interrupting capacity: 200kA

Class T — Ultra-Compact High-Current

Class T fuses pack very high current ratings into very small bodies, using a rejection-type ferrule. Excellent for space-constrained applications.

JJN is widely used in photovoltaic combiners, battery systems, and compact distribution where space is at a premium.

The Comparison Matrix

Time-Delay vs. Fast-Acting

The single most common selection error is choosing a fast-acting fuse for a motor circuit.

Bussmann time-delay fuses (LPJ, FRN-R, FRS-R, LP-CC) hold 5× rated current for 10+ seconds — enough for virtually all motor starting profiles.

Rejection Features

Rejection fuses have a special geometry that prevents inserting an oversized fuse into a smaller holder — a critical safety mechanism.

Do not defeat rejection features. They exist to prevent a maintenance technician from installing a 100A fuse in a 30A holder after exhausting the correct stock.

Selection Workflow

1. Determine system voltage (240V, 480V, 600V)
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2. Determine load type → time-delay or fast-acting
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3. Determine continuous current → apply 1.25× (NEC 430.52 for motors)
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4. Select fuse class based on panel design & space
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5. Verify interrupting capacity ≥ available fault current
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6. Match holder (rejection type) to fuse class
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7. Verify NEC compliance for the specific application

Worked Example: 100HP Motor Starter Panel

Parameters: - Voltage: 480V, 3-phase - Motor FLC: 124A - Available fault current: 42kA - NEC motor branch: 124A × 1.25 = 155A → next standard 175A

Selection: - Class J required (new construction, compact) - Selected: LPJ-175SP (175A, Class J, time-delay) - Holder: CH30J series (Class J, 3-pole) - Verify: Interrupting 200kA > 42kA available ✓

What each class tells you — and what to check before selecting

Class R includes RK1 and RK5; “R” refers to rejection features, not renewable construction. Eaton lists LPN-RK and LPS-RK as time-delay RK1 series and other fast-acting RK1 series. Do not assume that one listed prefix has the same time curve as all others.

Select from the installation, not the class name

First establish voltage, load and startup profile, conductor protection and available short-circuit current. Then compare the manufacturer's curve, interrupt rating and current-limiting data. In a panel, check the required short-circuit current rating and any documented tested combination. The fuseholder must accept the class, ampere frame and voltage duty.

Example: If a small control circuit requires a Class CC rejection fuse, a physically similar 10 × 38 mm supplemental fuse is not a substitute. If upgrading an RK5 feeder to RK1, examine motor starting, coordination and equipment instructions; a different curve may change nuisance operation and selectivity.

Brand catalogs may list ranges and features by series. Verify the specific part number before ordering, particularly where a product code appears in multiple markets or AC/DC variants.

Sources for verification

Eaton fuse classes  •  UL fuse and fuseholder services  •  UL short circuit rating guidance

Common Questions

Q: Can I replace a Class RK5 fuse with a Class RK1? A: Yes — RK1 and RK5 share the same dimensions and holder. RK1 is a performance upgrade (better current limiting) and is generally acceptable. Never go the other direction if the design specifies RK1.

Q: What's the difference between LPJ and JKS? A: Both are Class J. LPJ is time-delay (dual-element, for motors/transformers). JKS is fast-acting (for resistive loads and semiconductor-adjacent circuits).

Q: Are UL fuses usable in IEC panels? A: Physically no — Class J/RK/CC/T dimensions differ from NH/BS88. You must use the fuse class matching your holder system. Match fuse class to the panel standard.

Q: What does "200kA interrupting" mean? A: The fuse can safely interrupt a fault current of up to 200,000 amperes RMS symmetrical without rupturing or failing to clear. This is the maximum available fault current it's rated for.

Written by the asbeam.com technical team. asbeam supplies Bussmann Class J, RK1, RK5, CC, and T fuses with matching holders and rejection-compliant bases. Contact us for North American panel support.

Further reading: - NH Fuse Selection Guide - BS88 Fuse Selection Guide - Fuse Holders, Bases & Accessories Guide