A 400 amp-hour lithium bank is the size most van builds land on: enough for an induction hob and a 3,000 watt inverter. It is also the size at which the fusing questions on the van boards stop being about amps and start being about something most builders have never heard of, the interrupt rating. Get that wrong and the fuse does not open in a short; it welds shut and becomes a conductor.
The rule
Victron's wiring guide states it in one sentence: a lithium installation must include at least one fuse with an interrupt rating at or above the prospective short-circuit current of the bank.
Lithium cells have very low internal resistance. Four 100 amp-hour batteries in parallel, or one 400 amp-hour bank, can push well over ten thousand amps into a dead short for the instant before something gives. A lead-acid bank of the same size cannot. The fuses that were fine on AGM are not fine here, and the battery's own BMS short-circuit protection is a backstop, not the fuse.
The interrupt ratings
| Fuse type | Interrupt rating | Verdict for a 400 Ah lithium bank |
|---|---|---|
| ANL | 6,000 A at 32 V | No. Common on van builds and the wrong choice at the battery |
| MEGA | Low thousands of amps | Branch circuits only, behind the main fuse |
| MRBF | 10,000 A at 14 V | Acceptable on a single small 12 V lithium battery; marginal at 400 Ah |
| Class T | 20,000 A at 125 V DC; Bussmann JJN rated 200,000 A | Yes. The main battery fuse |
The current rating is a separate question and an easier one: it is set by the cable, not the load, and it sits above the continuous draw with margin.
Where each fuse goes
Main fuse, at the battery, Class T. As close to the battery positive as the hardware allows, before anything else. Its job is the catastrophic short: a dropped spanner, a chafed inverter cable. The cleanest way to mount it is the Lynx Class-T Power In, which holds one Class T per battery input and bolts straight to the rest of the Lynx system, so there is no separate holder and no exposed stud.
Branch fuses, MEGA, in the distributor. Behind the main fuse the Lynx Distributor takes a MEGA fuse per consumer: one for the inverter-charger, one for the DC-DC charger, one for the solar controller, one for the 12 volt distribution panel. Each is sized to its cable. A six-way MEGA holder with busbar does the same job in a smaller build, or a plain busbar where the branches are fused elsewhere.
Inverter fuse. Follow the inverter manual. A 12 volt 3,000 watt MultiPlus-II draws around 250 amps continuously and its manual specifies the DC fuse; on a Lynx system that fuse is the MEGA in the distributor slot feeding it.
Cable for the 3,000 watt inverter
At 12 volts a 3,000 watt inverter is roughly 250 amps at full output. That wants 2/0 AWG (70 mm²) cable, and the total run, positive plus negative, kept under about 5 meters. Longer than that and you go up a size or, better, move the inverter closer to the bank. Our cable and fuse sizing chart has the full table by length.
The layout, in one list
- Battery positive → Class T in the Lynx Class-T Power In
- → Lynx Distributor with a MEGA per branch
- → Inverter-charger, DC-DC charger, MPPT, 12 volt panel, each on its own fused branch
- Battery negative → SmartShunt → negative busbar, with nothing else on the battery side of the shunt
- Every fuse rated to its cable; the Class T rated to the bank's short-circuit current
What the van boards get wrong
An ANL at the battery because it was the biggest fuse at the auto parts store. A single MRBF on the terminal of a four-battery bank. A fuse chosen by the inverter's wattage rather than the cable's rating. And, most often, no fuse at all between the battery and a busbar because "the inverter has its own". The inverter fuse protects the inverter cable; nothing protects the run from the battery to the busbar except the fuse at the battery.
Our sailboat article on Class T, the Lynx Class-T Power In and MRBF covers the same rule from the marine side. Send us the bank size, the inverter and the cable runs and we will list the fuses. Everything ships from Upland, California.