FMX Fuses Explained: The Go-To “Auto Link” Fuse for High-Current Circuits
When your design outgrows standard blade fuses (ATO/ATC, Mini, Micro), the next bottleneck is usually high-current branch protection: blowers, pumps, heated seats, trailer circuits, and other loads that live in the 20A-60A range.
That’s where the FMX Automotive Link Fuse comes in. OptiFuse’s FMX series is a JCASE-style, female-terminal cartridge fuse designed for higher-current automotive/DC circuits. They’re color-coded for quick amp identification and built with a top window for fast blown-fuse checks.
Baseline ratings and standard
OptiFuse FMX (JCASE) is specified with:
- Voltage rating: 32VDC
- Current rating: 20-60A
- Interrupting rating: 1000A @ 12VDC
- Testing standard: ISO 8820-4
Why choose the FMX/JCASE instead of standard blade fuses?
Think of the FMX/JCASE as the “sweet spot” when your circuit is too large for small blade formats, but you still want compact packaging and easy service.
FMX fuses are perfect for circuits that need high-current branch protection without the bulk of larger cartridge fuses.
To learn how to select the fuse, see our Fuse Selection Guide for complete context.
Where FMX fuses are commonly used
FMX/JCASE link fuses are a strong fit for:
- Automotive accessory branches
- Work trucks / fleet upfits: radios, inverters (branch feeds), tool power, utility bodies
- Trailer and auxiliary circuits in the 20A-60A band
- Compact DC distribution panels where space is limited but current is non-trivial
Sebastian Castañeda is a circuit protection specialist and technical writer with application-focused experience in technical support and custom protection design. He contributes practical, application-driven insights to the OptiFuse Blog.