When a specification asks for the main classification of fuses, it usually means one specific thing: the standard letter code the fuse carries. IEC 60269 builds that code from a lower case letter for the breaking range and one or two upper case letters for the application. UL 248 takes a different route and assigns a class letter that fixes dimensions and interrupting rating as well as duty. Those codes are the classification that decides whether a fuse may sit on a motor branch or a semiconductor leg, and for anyone buying the insulation inside it, what the tube has to survive. This page covers the code systems themselves. For the wider picture of how fuses are grouped, see our fuse classification framework or the overview of how many types of fuses exist.
Why the letter code is the main classification
Voltage, shape and body material all describe a fuse, but none of them tells you whether it may be used to protect a motor branch or a rectifier. The standard code does, and it is the one printed on the label next to the current rating. IEC 60269 forms its codes from two parts. UL 248 assigns a class letter, then locks the physical dimensions, the rejection feature and the interrupting rating to that letter.
One warning before the tables. A class letter does not, on its own, tell you the speed of the fuse. UL classes define dimensions and ratings; whether a fuse is time delay or fast acting is a separate designation that has to be checked on the same label.
IEC 60269 utilisation categories
The first letter states the breaking range. Lower case g means the fuse clears every overcurrent from its minimum melting current up to its breaking capacity, so it covers overload and short circuit on its own. Lower case a means partial range, which clears short circuits only and needs separate overload protection. The upper case letter or letters state the duty.
| Category | Breaking range | Protects | Notes |
|---|---|---|---|
| gG | Full range | Cables and general circuits | Former general purpose types gL, gF and gI were merged into it |
| gM | Full range | Motor circuits | Dual rated, widely used in the UK, Australia and South Africa |
| aM | Partial range | Motor circuits | Short circuit only, overload handled by a thermal relay |
| gR | Full range | Semiconductor devices | Optimised for low let-through energy (I squared t) |
| aR | Partial range | Semiconductor devices | Back-up protection for drives and rectifier bridges |
| gS | Full range | Semiconductor devices and cable | Combines gR clearing with gG cable protection |
| gPV | Full range | Photovoltaic strings | Covered by IEC 60269-6 and rated for DC |
| gTr | Full range | Distribution transformers | Rated in transformer kVA rather than in amps |
UL 248 classes
In North America the class letter fixes the physical dimensions and the rejection feature as well as the duty, which is why a Class CC fuse cannot be fitted into a Class J holder and why substitution across classes is not a valid cost saving. Classes C, CC, G, J, K, L, R and T are current limiting. Class H is not.
| Class | UL 248 part | Voltage | Current range | Rejection feature | Typical use |
|---|---|---|---|---|---|
| C | 248-2 | 600 V | 0 to 1200 A | None | Older cartridge installations |
| CC | 248-4 | 600 V | 0 to 30 A | Ferrule rejection pin | Control circuits, PLC I/O, small transformers |
| G | 248-5 | 480 to 600 V | 0 to 60 A | Length keyed | Control circuits |
| H | 248-6 | 250 to 600 V | 0 to 600 A | None | Legacy panelboards, not current limiting |
| J | 248-8 | 600 V | 0 to 600 A | None, dimension keyed | Motor circuits and feeders |
| K | 248-9 | 250 to 600 V | 0 to 600 A | None | Legacy, largely replaced by Class R |
| L | 248-10 | 600 V | 601 to 6000 A | None | Large feeders, service entrance |
| R (RK1 and RK5) | 248-12 | 250 to 600 V | 0 to 600 A | Slot and notch on the ferrule | Motor, feeder and general purpose |
| T | 248-15 | 300 to 600 V | 0 to 1200 A | None | Compact panelboards |
How to read a class code in a specification
- Start from the code the equipment listing requires, not from the code you hold in stock. The two families look similar on a datasheet and behave differently in a holder.
- Check whether the code is full range or partial range. Partial range categories cover short circuits only and need separate overload protection, which is the most common coordination mistake.
- Confirm the body size and the rejection feature that match the holder, especially when a panel is being repaired with a substitute class.
- Match the insulation to the code. Semiconductor categories demand low let-through energy and fast clearing, while motor categories have to tolerate inrush and repeated thermal cycling.
What the code fixes about the tube
A utilisation category is not just a label. gG and gM fuses are asked to ride through normal overloads and motor inrush, so the insulating tube is cycled thermally far more often than the current curve alone suggests. aR and gR fuses are asked to clear in milliseconds, so the arc energy arrives at the liner in a much shorter and harder pulse. In practice those two demands pull the tube in different directions: semiconductor categories suit dense, high arc-resistance constructions such as melamine and synthetic arc quenching liners, while general purpose and motor categories are more often built on vulcanized fiber and its composites.
The material side of that decision is covered in the fuse tube range, and the grade specifications are on the datasheets and downloads page.
Standards that define these codes
- IEC 60269-1 general requirements, with -2 for industrial and commercial, -3 for household, -4 for semiconductor protection, -6 for photovoltaic and -7 for batteries and energy storage
- UL 248, one part per class: 248-4 for Class CC, 248-8 for Class J, 248-10 for Class L, 248-12 for Class R and 248-15 for Class T
- CSA C22.2 No. 248 mirrors the UL classes for the Canadian market
Frequently asked questions
Q: What is the main classification of fuses, in one sentence?
A: The standard code carried on the label, which is the IEC 60269 utilisation category or the UL 248 class, because that code is what constrains where the fuse may legally and safely be used.
Q: Is gG equivalent to a UL class?
A: No. gG and a UL class such as Class J may cover similar general purpose duty, but they come from different standards and differ in dimensions, holders and interrupting ratings. Confirm which one the equipment listing calls for before you standardise.
Q: Can I use an aM fuse instead of a gG fuse?
A: Only if separate overload protection is present. aM is partial range, so it clears short circuits and leaves overload duty to a relay or a thermal device. Without that device the circuit is unprotected against overload.
Q: How do I choose the tube material for a given class?
A: Start from the breaking range and the body construction, then confirm wall thickness and liner chemistry. Our plant runs an ISO 9001 quality management system and SGS material testing is available to support your qualification. Send the class, the rating and the drawing to sale@fusestube.com and we will confirm the tube construction that matches.
