On the Catalina, Hunter and Beneteau owner boards the lithium refit thread always reaches the same argument, usually on page three: is a Class T fuse really necessary, or will the ANL that was already there do? The answer from the marine electricians who post there is unambiguous, and the reasoning is worth understanding rather than taking on faith, because it tells you where every other fuse on the boat should go too.
The number that changed: short-circuit current
A fuse has two ratings. The one everyone knows is the current it carries before it opens. The one that matters here is its interrupt rating, the largest current it can safely stop. Above that, the fuse element vaporises but the arc keeps conducting across the gap, and the fuse has failed to do the one thing it is for.
A lead-acid bank can push a few thousand amps into a dead short. A lithium iron phosphate bank, with its very low internal resistance, can push well over ten thousand; the figure quoted on the sailing boards for a modest house bank is above 13,000 amps. That is the whole argument.
| Fuse type | Interrupt rating | Verdict at the lithium bank |
|---|---|---|
| MEGA and AMG bolt-down | roughly 1,000 to 2,500 A | Not enough |
| ANL | a few thousand amps at 32 V | Not enough |
| MRBF terminal fuse | 10,000 A at 14 V, 5,000 A at 32 V | Enough for a single 12 V battery at its own terminal; not for the whole bank |
| Class T | 20,000 A at 125 V DC and higher, depending on maker | The one for the bank's main fuse |
The marine electricians on those boards report breakers welded shut by drop-in lithium. That is what an inadequate interrupt rating looks like in practice: a device that was supposed to open and instead became a conductor.
The rule, from Victron's own wiring guide, is that a lithium installation must include at least one fuse whose interrupt rating exceeds the bank's prospective short-circuit current. On any bank above a single small battery, that means Class T.
Where each fuse belongs
The forum questions "how many fuses does a lithium install need" and "after the Class T, does the inverter feed also need Class T" have a single answer once you see the layout as layers.
Layer 1: at each battery terminal, an MRBF. Where batteries are paralleled, each one needs its own fuse so a fault in one battery or its cable cannot be fed by the others. An MRBF bolts directly to the positive terminal, so there is no unfused cable at all, which satisfies the "within seven inches of the terminal" placement rule cleanly. Its 10,000 amp interrupt rating at 12 volts is adequate for one battery's contribution.
Layer 2: at the bank's main positive, one Class T. This is where the combined short-circuit current of the whole bank appears, and it is the fuse that must have the 20,000 amp interrupt rating. The Lynx Class-T Power In is the busbar module built for exactly this position: the bank's main cables land on it, the Class T fuse sits inside it, and everything downstream bolts on. Class T fuse elements in the 225 to 400 amp range come from Bussmann, Littelfuse or Blue Sea and are bought separately.
Layer 3: at each branch, a MEGA. The inverter, the solar controller, the DC panel feed and the alternator charger each get their own fuse sized to their cable. Downstream of the Class T these can be ordinary MEGA fuses, because the Class T upstream now limits what any fault can draw. The Lynx Distributor holds four of them with an LED per fuse, and bolts to the Power In.
So the answer to "does the inverter feed also need Class T" is no. One Class T at the bank, then MEGA at each branch. The answer to "how many fuses" is one per battery, one for the bank, one per branch.
Parallel through a busbar, not post to post
The other recurring question, whether to parallel batteries directly or through a busbar, is answered by the same layout. Each battery's fused positive runs to the busbar; each negative runs to the shunt side of the negative busbar. Cables of equal length from each battery to the bars keep the batteries sharing current evenly, which daisy-chaining post to post does not. The Lynx modules are the busbar; for a smaller system, a Victron busbar does the same job.
Monitoring belongs in the same chain
The Lynx Shunt VE.Can is a shunt in Lynx form that also carries the bank's main fuse, so on a system with a GX device it replaces the Class-T Power In and adds monitoring. On a boat without a GX, a SmartShunt on the negative side does the counting and reports to your phone. Either way, on lithium, voltage stops telling you state of charge and something has to count amp-hours.
What the Class T costs, against what it protects
The objection on the boards is price: a Class T fuse and holder cost several times an ANL. Set against a lithium bank, an inverter-charger and the boat, and against the electrician's observation that the alternative is a welded breaker during a fault, the arithmetic is not close. It is also the fuse the marine standards are moving toward for lithium, so a survey or an insurer's electrician will ask for it.
A refit list for a mid-size sailboat
- Batteries: two or three Victron 12.8 volt Smart lithium in parallel, each with an MRBF at its terminal
- Bank fuse and busbar: Lynx Class-T Power In with a Class T element
- Branches: Lynx Distributor with MEGA fuses for the inverter-charger, MPPT, DC panel and alternator charger
- Monitoring: Lynx Shunt or SmartShunt
Our cable and fuse sizing guide gives the cable gauge for each run, and our Cerbo GX wiring guide shows the Lynx chain on a monitored system. Send us the bank size and the loads, and we will lay out the fuses before you buy anything. Everything ships from Upland, California.