A fuse type chart categorizes overcurrent protection devices by physical form factor, interrupting rating (AIC), and time-current characteristics as defined by UL 248 and NEC Article 240. If you are sizing protection for a motor starter, a residential branch circuit, or a VFD, the correct fuse class dictates whether the device safely clears a fault or catastrophically explodes. Below is the master reference chart for North American fuse classifications, followed by the specific derating and application rules you need to select the right part.
The Master Fuse Type Chart (UL 248 / NEC 240)
How to read this table: The Fuse Class defines the physical dimensions and rejection features. Max Voltage is the maximum AC RMS operating voltage. Interrupting Rating (AIC) is the maximum fault current the fuse can safely clear without rupturing. Time Delay indicates if a slow-blow variant exists for inductive loads. Rejection Feature notes whether the fuse has physical dimensions that prevent it from being installed in a lower-rated holder. All data is sourced from the NFPA 70 (NEC) Article 240 and UL 248 standards.
| Fuse Class / Type | Max Voltage (AC) | Interrupting Rating (AIC) | Time Delay Option | Physical Rejection Feature |
|---|---|---|---|---|
| Class H (Renewable/Non-Renewable) | 250V / 600V | 10,000A | Yes (Non-Renewable) | None (Interchangeable with RK) |
| Class RK1 | 250V / 600V | 200,000A | No (Fast-Acting) | Yes (Notch in ferrule) |
| Class RK5 | 250V / 600V | 200,000A | Yes (e.g., Bussmann FRS-R) | Yes (Notch in ferrule) |
| Class J | 600V | 200,000A | Yes / No | Yes (Unique blade spacing) |
| Class T | 300V / 600V | 200,000A | Yes / No | Yes (Smallest footprint) |
| Class CC | 600V | 200,000A | Yes (e.g., Littelfuse FLQ) | Yes (10x38mm body) |
| Class G | 600V | 100,000A | No | Yes (Unique ferrule size) |
| Edison Base (Plug) | 125V | 10,000A | Yes (Time-Delay) | None (Interchangeable threads) |
| Type S (Plug + Adapter) | 125V | 10,000A | Yes (Time-Delay) | Yes (Threaded adapter T-15/T-20/T-30) |
Decoding the Columns: Application, Derating, and Limits
Reading the chart is only the first step. To apply these values safely on the bench or jobsite, you must understand how environmental factors and system fault currents interact with the fuse's rated specifications.
Which Column Applies to Your Installation?
The most critical column for safety is the Interrupting Rating (AIC). If your facility's available fault current at the panel is 42,000A (common in modern commercial services with large transformers), installing a Class H fuse (10kA AIC) is a severe code violation and a physical hazard. The fault current will exceed the fuse's clearing capacity, causing the cartridge to rupture and potentially ignite the panel. For services with >10kA fault current, you must specify Class RK1, RK5, J, or T fuses (200kA AIC). Littelfuse and Eaton application guides heavily emphasize verifying the available fault current before selecting a fuse class.
How Ambient Temperature Derating Modifies the Base Value
Unlike NEC Table 310.16 for wire ampacity, fuses do not have standardized "derating rows" printed in the codebook. However, fuse elements are thermally activated. A 100A Class RK5 fuse rated for a 25°C ambient environment will melt at a lower current if installed in a hot enclosure.
What the Table Cannot Tell You
A static reference chart cannot provide selective coordination or let-through energy (I²t). If you need to ensure that a downstream 20A branch fuse blows before the upstream 400A main fuse (preventing a total facility blackout), you must consult the manufacturer's time-current curves (log-log plots). Furthermore, protecting sensitive semiconductors like IGBTs in a VFD requires checking the specific I²t let-through value; a standard Class RK1 fuse may let through enough thermal energy to destroy a silicon junction before it fully clears, necessitating a specialized ultra-fast semiconductor fuse.
Quick-Jump Guide: Matching the Right Fuse to the Application
Bookmark this section for rapid component selection based on your specific circuit topology.
- Motor Control Circuits (HVAC, Pumps, Conveyors): Use Class RK5 or Class J (Time-Delay). Motors draw 600%+ of their full-load amperage (FLA) during startup (Locked Rotor Current). A fast-acting fuse will nuisance-trip on startup. The time-delay element absorbs the 10-second inrush without opening. Part example: Bussmann FRS-R-30 (30A RK5).
- Main Service Entrance & Feeder Protection: Use Class RK1 or Class T (Fast-Acting). These provide the highest current-limiting effect, chopping off peak fault currents in less than a half-cycle (8.3ms) to protect downstream busbars from magnetic bracing failure.
- Control Transformers & PLC Power Supplies: Use Class CC (Time-Delay). The compact 10x38mm footprint fits perfectly into standard 35mm DIN rail holders, and the time-delay handles the brief magnetizing inrush of small control transformers. Part example: Mersen ATDR10.
- Residential Branch Circuits (Legacy Panels): Use Type S Plug Fuses. Never use standard Edison base fuses in residential panels. Type S adapters physically prevent a homeowner from screwing a 30A fuse into a 15A lighting circuit socket.
Fatal Mismatches and the Rejection Feature Mandate
The most common point of failure in fuse selection is ignoring the physical rejection features designed into the UL 248 standard.
The Class H vs. RK Trap: Class H fuses share the exact same physical dimensions and blade spacing as Class RK1 and RK5 fuses. This means a 200kA Class RK5 fuse will physically fit into a cheap 10kA Class H fuse holder. However, the reverse is also true: a 10kA Class H fuse will fit into an RK-spec holder. If an inspector or engineer assumes the panel contains current-limiting RK fuses based on the holder, but a maintenance tech replaced them with cheap Class H fuses, the panel is no longer rated for high available fault currents. Always use holders with the physical rejection pins that block Class H ferrules.
The Edison Base Danger: The standard Edison screw base (E26) is identical across 15A, 20A, and 30A plug fuses. This allows a dangerous mismatch where a 30A fuse is installed on 14 AWG wire (rated for 15A), creating a fire hazard before the fuse clears. The NEC mandates the use of Type S fuses for new installations and replacements. Type S fuses require a threaded adapter (T-15 for 15A, T-20 for 20A, T-30 for 30A) that screws into the socket permanently. Once a T-15 adapter is installed, only a 15A Type S fuse can fit, physically eliminating the over-fusing hazard.
When replacing fuses, always read the class stamped directly on the ceramic body or the metal blade, not just the amperage. A 30A Class H fuse is not a substitute for a 30A Class RK5 fuse, even if they look identical on the shelf.
