When a motor starter faults or a control transformer shorts, the fuse is the last line of defense between a minor component failure and a catastrophic arc flash. But walking into an electrical supply house and asking for a '30-amp fuse' will get you the wrong part nine times out of ten. Fuses are strictly categorized by physical dimensions, rejection features, and interrupting capacity under the UL 248 standard. This reference provides the definitive UL 248 fuse classes chart, mapping out the exact ampere ranges, voltage limits, and interrupting ratings (AIC) you need to specify the correct replacement or design a new panel.

The Master UL 248 Fuse Classes Chart

The table below covers the most common low-voltage industrial and commercial fuse classes governed by UL 248 and referenced in NEC Article 240. Source Standard: UL 248 (Low-Voltage Fuses) and NEC 240.60.

Fuse Class Voltage Rating Ampere Range Interrupting Rating (AIC) Physical Style & Rejection Feature Primary Application
Class RK1 250V / 600V 1/10A - 600A 200,000A (200kA) Blade with rejection slot (prevents Class H insertion) Motor protection, mains, feeder (Time-Delay)
Class RK5 250V / 600V 1/10A - 600A 200,000A (200kA) Blade with rejection slot (identical footprint to RK1) General purpose, lighting, heating (Time-Delay)
Class J 600V 1A - 600A 200,000A (200kA) Offset blade (physically rejects RK, H, and K fuses) Space-constrained panels, motor circuits, feeders
Class CC 600V 1/10A - 30A 200,000A (200kA) 10x38mm ferrule (rejects standard 10x38mm Midget fuses) Control circuits, PLC power supplies, small transformers
Class T 300V / 600V 1A - 1200A 200,000A (200kA) Compact windowless ferrule with unique bolt-hole spacing Fast-acting protection, high fault current mains
Class L 600V 601A - 6000A 200,000A (200kA) Large bolt-on square body with specific terminal spacing Main service entrance, heavy industrial bus ducts

How to Read This Table

When sizing a fuse, the Voltage Rating must equal or exceed the circuit voltage (a 600V fuse can protect a 480V circuit, but a 250V fuse will violently explode on a 480V fault). The Interrupting Rating (AIC) is the maximum fault current the fuse can safely clear without rupturing; per NEC 110.9, this must exceed the available fault current at the point of installation (often 42kA to 65kA in commercial panels, making the 200kA rating of modern classes essential). Finally, the Rejection Feature is a mechanical keyway. It ensures you cannot accidentally install a cheap, low-interrupting Class H fuse (10kA AIC) into a holder designed for a 200kA Class RK1 or J fuse.

Quick-Jump: Most Queried Fuse Classes

Benchmark Tip: If you are replacing a fuse and the label is burned off, look at the holder. A Class J holder has a distinct offset slot. A Class CC holder is a small cylindrical tube (10x38mm). A Class RK holder has a standard blade slot with a small pin or ridge that blocks fuses lacking the corresponding notch.
  • Class RK1 (The Motor Workhorse): Dual-element, time-delay. Designed to hold through motor starting inrush currents (which can be 6x the full load amps for several seconds) while still clearing sustained overloads. Common Part: Littelfuse FLNR series.
  • Class J (The Space Saver): Delivers the same 200kA protection as an RK1 but in a package roughly 30% smaller. Because it is inherently non-interchangeable with older, dangerous Class H fuses, modern panel builders heavily favor Class J. Common Part: Mersen AJT (Time-Delay) or Littelfuse JLS (Fast-Acting).
  • Class CC (The Control Guardian): Used almost exclusively on the secondary side of control transformers and DC power supplies. They have exceptional current-limiting capabilities, protecting sensitive solid-state relays and PLC I/O cards from arc-flash let-through energy. Common Part: Mersen ATDR series.
  • Class T (The Fast Clearer): Lacks the time-delay elements of RK1 or J. Used where rapid clearing of high-magnitude short circuits is required, and where inrush currents are minimal (e.g., resistive heating banks or solid-state drive inputs). Common Part: Littelfuse JJT series.

Derating and Ambient Temperature Modifications

The ampere ratings in the master chart assume an ambient temperature of 40°C (104°F) inside the enclosure. If your panel is located in a hot mechanical room, near a boiler, or in direct sunlight, the fuse will open prematurely at its rated current because the thermal mass of the element is already pre-heated.

You must apply a derating factor to the base value. Here is the standard modification matrix for dual-element time-delay fuses (RK1, RK5, J Time-Delay):

Enclosure Ambient Temp Derating Multiplier Example: 100A Fuse Max Continuous Load
40°C (104°F) or below 1.00 100.0A
50°C (122°F) 0.95 95.0A
60°C (140°F) 0.90 90.0A
70°C (158°F) 0.85 85.0A

Worked Example: You are protecting a 480V, 3-phase motor with a Full Load Amp (FLA) rating of 88A. The motor is in a rooftop enclosure that reaches 60°C in the summer. If you select a standard 100A Class RK1 fuse, the derated capacity is 90A (100 x 0.90). This leaves almost no margin for the 88A continuous load, leading to nuisance tripping. You must step up to a 110A fuse (110 x 0.90 = 99A capacity) to safely carry the 88A load in that thermal environment. Always verify the specific manufacturer's derating curves, as Mersen and Littelfuse publish exact TCC (Time-Current Characteristic) graphs for their specific element alloys.

Decision Tree: Which Fuse Class Do You Actually Need?

Use this if-then path to terminate your selection process with a concrete part specification.

Circuit Condition / Requirement Concrete Pick (Class & Type) Reference Part Number
Motor circuit with high starting inrush (across-the-line starting) Class RK1 or Class J (Time-Delay) Littelfuse FLNR (RK1) or Mersen AJT (J)
Panel space is severely restricted; upgrading an old panel Class J (Fast-Acting or Time-Delay) Littelfuse JLS (Fast) or AJT (Delay)
Control transformer secondary (120VAC/24VDC control power) Class CC (Time-Delay) Mersen ATDR or Littelfuse CCMR
Solid-state drives, VFD inputs, or capacitive banks (no inrush tolerance needed) Class T or Class J (Fast-Acting) Littelfuse JJT (T) or JLS (J)
Main service disconnect or bus duct tap exceeding 600A Class L (Time-Delay or Fast) Mersen A4BQ or Littelfuse L

What This Chart Cannot Tell You

While this chart dictates physical fit, voltage safety, and maximum fault clearing, it omits three critical engineering parameters required for complex system design:

  1. Let-Through Energy (I²t): The chart tells you the fuse will clear a 200kA fault, but it doesn't tell you how much thermal and magnetic stress passes through to the downstream contactor before the arc extinguishes. For protecting sensitive semiconductor rectifiers, you must consult the manufacturer's I²t melting and clearing curves.
  2. Selective Coordination: If a 20A branch circuit faults, you need the 20A fuse to blow before the 200A feeder fuse upstream. The chart cannot guarantee this. You must plot the Time-Current Curves (TCC) of both fuses on log-log paper (or use software like ETAP) to ensure a minimum 10:1 ratio or verify published coordination tables.
  3. DC vs. AC Ratings: A fuse rated for 600VAC may only be rated for 250VDC or 500VDC. DC arcs do not have a natural zero-crossing to help extinguish the plasma. If you are sizing fuses for a solar combiner box or a battery bank, you must explicitly check the manufacturer's DC voltage rating, which is often lower than the AC rating printed on the ferrule.
Safety & Code Caveat: Never adapt a fuse holder to accept a different class of fuse (e.g., drilling out a rejection pin to force a Class H into a Class RK holder). This violates NEC 110.3(B) and defeats the 200kA interrupting rating, creating a severe arc-flash hazard. Always replace like-for-like, or upgrade the entire holder block.