Technical Foundation
What Is a Class G Fuse?
A Class G fuse is a UL Class G current-limiting fuse defined by UL 248-5, purpose-built for branch circuit protection in equipment where compact size matters — HVAC condensing units, lighting panels and control cabinets. Unlike Class RK1/RK5 fuses, Class G fuses use a smaller, rejection-style body diameter that scales with amperage (1/4" for 1/2-15A, 13/32" for 20-30A, 13/32" for 35-60A but longer), physically preventing a higher-rated fuse from being installed in a lower-rated holder.
Class G fuses are rated 480VAC or less and offer a 100,000A interrupting rating, current-limiting to protect downstream components from short-circuit let-through energy. Because the body length changes with amperage bracket rather than staying fixed like Class RK fuses, fuse holders and blocks are matched to a specific amperage range — a 15A Class G fuse will not physically fit a holder sized for 30A.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Standard | UL 248-5, CSA C22.2 No. 248.5 | Confirms the fuse meets recognized current-limiting and interrupting-rating requirements for branch circuits |
| Voltage Rating | 300VAC or 480VAC | Must meet or exceed the circuit's nominal voltage to safely interrupt fault current |
| Interrupting Rating | 100,000 AIC | Determines the maximum fault current the fuse can safely clear without rupturing |
| Body Size by Amperage | 1/4" dia (1/2-15A), 13/32" dia (20-60A), length varies by bracket | Rejection feature prevents installing an over-amperage fuse in an undersized holder |
| Time-Current Class | Fast-acting, non-time-delay | Suited to protecting solid-state and electronic loads that don't need inrush tolerance |
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Applications
Where Class G Fuses Get Used
Class G fuses are specified wherever branch circuit protection must fit a compact enclosure without sacrificing current-limiting performance.
HVAC & Refrigeration Equipment
Branch circuit protection for condensing units, compressors and control boards where panel space is limited.
Lighting Panels & Ballast Circuits
Compact current-limiting protection for fluorescent and LED lighting branch circuits in commercial fixtures.
Control Panels & Machine Builders
Protecting individual control circuits and low-amperage branch loads inside industrial control cabinets.
Appliance & Equipment Manufacturing
OEM branch circuit protection specified for compact enclosures in packaged equipment.
Facilities & Electrical Maintenance
Replacement stock for Class G fuse holders already installed in panelboards and disconnects.
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FAQ
Class G Fuses Questions, Answered
What makes a Class G fuse different from a Class RK1 fuse?
Class G fuses use a smaller body diameter that scales with amperage, giving a rejection feature that prevents an over-amperage fuse from fitting an undersized holder. Class RK1 fuses use a larger, fixed-diameter body regardless of amperage. Both are current-limiting, but Class G is chosen specifically where panel space is tight.
Can I substitute a Class RK5 fuse for a Class G fuse?
No. The physical dimensions differ and Class G holders are sized to reject fuses outside their amperage bracket, so an RK5 fuse generally won't fit a Class G holder at all. Always match fuse class to the holder and the equipment's listing.
Why does the fuse body get longer at higher amperages?
UL 248-5 defines a body length for each amperage bracket (1/2-15A, 20-30A, 35-60A) specifically so that a fuse holder can only accept fuses within its rated bracket, preventing an installer from over-fusing a circuit.
What voltage and interrupting ratings do Class G fuses carry?
Most Class G fuses are rated 300VAC or 480VAC with a 100,000A interrupting rating, giving them enough current-limiting capability to protect downstream components even on circuits with high available fault current.
Are Class G fuses time-delay or fast-acting?
Standard Class G fuses are fast-acting, non-time-delay devices, making them well suited to protecting electronic and solid-state loads. Consult the equipment manufacturer's fuse table before substituting a different time-current characteristic.
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