Blue Sea Systems MRBF Terminal Fuse Block 30-300A

The Blue Sea Systems MRBF Terminal Fuse Block is the simplest way to get overcurrent protection right at the battery. It threads directly onto any 3/8" terminal stud — a battery post, a battery switch terminal, or a bus bar — putting fused protection within inches of the power source instead of several feet down the cable run.

Key features

• 300A maximum, 58V DC maximum
• Mounts on 3/8" terminal studs; isolated M8 output stud accepts 5/16" ring terminals
• Insulating cap prevents accidental shorts
• Ignition protected only when paired with a Blue Sea MRBF Terminal Fuse

This is the same block referenced by every Blue Sea MRBF Terminal Fuse (PN 5175 through 5190) as its required mount — the fuse and block are always sold separately, so plan on ordering both. Because the fuse sits right at the battery, ABYC's 7-inch main-circuit-protection rule is easy to satisfy without running unprotected cable to a separate fuse holder elsewhere in the boat.

Torque the terminal stud to 75 in-lb when installing, and note the isolated stud design lets you stack more than one ring terminal on the output side if a single fused circuit needs to feed more than one downstream connection.

When choosing which MRBF fuse to seat in this block, size it to the battery's maximum discharge current rather than the wire gauge alone — lithium-ion and TPPL battery banks in particular can deliver short-circuit currents well beyond what a simple wire-ampacity calculation would suggest. Check the battery manufacturer's recommended main fuse rating before ordering, and keep at least one spare fuse aboard since a blown MRBF fuse must be replaced, not reset.