The short answer. A boat lithium bank needs a main fuse that can safely break the very large current lithium pushes into a short. That ability is the fuse’s interrupt rating. Class T fuses have the highest of the common types, MRBF fuses come next, and ANL fuses are lower. Mount the main fuse right at the battery positive, and size it to the cable.
You’ll need
- A main fuse at the battery with a known interrupt rating (class T or MRBF, as your battery’s manual says), in a holder made for that fuse type
- A fused feed from the positive busbar: Egis ANL fuse block, up to 4/0, 35-750A
- A branch fuse block for small circuits: Egis RT 12-position fuse block with ground
- A positive busbar to gather the feeds: Victron Busbar 250A 4-point with cover
- A battery switch after the main fuse: part of the Boat lithium upgrade kit
- Cable sized with the Wire gauge calculator, then the fuse picked with the Fuse size calculator
Fuse your bank, step by step
- Get your battery maker’s short-circuit current, or the fuse type it names, from the battery manual. Everything else follows from this number.
- Pick a main fuse whose interrupt rating covers that current.
- Size the cable for the current and the run length.
- Pick the fuse’s amp size at or below the lowest of the cable, battery and system ratings.
- Mount it on the battery positive, as close to the terminal as it fits.
- After the switch and busbar, give every feed its own fuse.
- Leave shore power and the inverter’s AC output to a licensed electrician.
You can do this. It’s your boat, so you’ll take care over it. When you swap lead-acid for lithium, the main fuse is easy to overlook. The old one fit the old battery. A lithium battery can dump far more current into a dead short, and a fuse only protects you if it can actually stop that current. Victron’s wiring book calls lithium short-circuit currents “far higher than in comparable lead-acid systems” (S8).
Below: what interrupt rating means, how class T, MRBF and ANL compare on the fuse makers’ own sheets, and how to pick the amp size. One honest limit up front. The number that decides all of this, your battery’s short-circuit current, lives in your battery’s manual, not here.
What is a fuse interrupt rating (AIC)?
Every fuse has two current numbers. The amp rating is the current it carries before it blows. The interrupt rating, often written AIC or kA, is the largest fault current it can stop without failing in a dangerous way.
Victron’s book puts it plainly: if the interrupt rating is lower than the current a short can deliver, “the fuse may fail catastrophically” (S8). The fuse can arc, rupture or keep conducting, so the cable it was meant to protect is still unprotected.
The same chapter sets the rule for lithium. One fuse or more in the DC line needs an interrupt rating at least as high as the bank’s maximum possible short-circuit current. Victron calls it a mandatory safety requirement.
Why does a lithium bank need a higher interrupt rating?
Because the fault current is higher, the amp size alone tells you nothing. A fuse that was fine on a lead-acid house bank can be under-rated on lithium at exactly the same amps.
How much current your battery can push into a short is a number only its maker can give you. Look in the battery manual or spec sheet for “short-circuit current” or a required fuse type. Some makers simply name the fuse. Victron’s Lithium SuperPack NG manual, for example, calls for “an MRBF-type or T-type fuse with an interrupt rating (IR) of at least 10 kA” on the main battery terminal (S6). That rule is for that battery. Use what your own battery’s manual says.
Class T vs MRBF vs ANL: which main fuse?
Here’s what each fuse maker states on its own sheet. Interrupt ratings depend on voltage, so the 12V boat figure is the one to compare.
| Fuse type | Sizes the maker sells | DC voltage on the sheet | Interrupt rating on the sheet | Where it fits |
|---|---|---|---|---|
| Class T (Eaton Bussmann JJN) | 1-1200 A; DC rated from 15 A | 160 Vdc (15-600 A) | 20kA Vdc (15-600 A) | Main fuse at the battery, in a class T holder |
| MRBF (Egis 3603) | 30-300 A | 58 VDC max | 10,000 A at 14 VDC; 2,000 A at 58 VDC | Main fuse bolted straight to the battery post |
| ANL (Eaton Bussmann) | 35-750 A | 80Vdc or less | 2700A | Feeds from a busbar to a panel or inverter |
| MAXI and ATO blade (Littelfuse) | MAXI 20-80 A; ATO 1-40 A | 32 V DC | 1000 A @ 32 V DC | Branch circuits in a fuse block |
Class T. The highest interrupt rating in the table, and fast. Victron’s book calls the T fuse “well-suited for lithium battery systems” (S8). The catch is size: a class T fuse needs its own holder, so plan the space next to the battery.
MRBF. A compact fuse that bolts onto the battery terminal, so no extra cable sits between the post and the fuse. At 12V its stated interrupt rating is half the class T figure. Fine when your battery maker allows it, and some do, like the SuperPack NG manual above.
ANL. The bolt-down fuse with a clear window over the element, and a much lower interrupt rating. On a lithium boat it usually belongs downstream: the feed from the busbar to a fuse block or inverter, behind a class T or MRBF main fuse. Don’t use it as the only fuse on a lithium bank unless your battery maker says so.
Check the brand on the fuse you buy. Interrupt ratings differ between makers of the same fuse type.
Where does the main fuse go?
On the battery positive, as close to the terminal as you can get it. The SuperPack NG manual says to install all fuses “as close as possible to the positive battery terminal” (S6). Any cable between the post and the fuse has no protection at all.
The order from the battery is: main fuse, battery switch, positive busbar, then a fuse for each feed. Boat lithium battery wiring diagram: 2 simple setups shows that chain drawn out.
After the main fuse, every circuit gets its own. Victron’s book says each consumer connected to a battery needs an individual fuse in the positive cable, “no matter how big or small” (S8). Running more than one battery in parallel? Victron’s Lithium Battery Smart manual has you fuse each battery on its positive side (S7).
How do I size a lithium battery fuse?
The fuse protects the cable. The Lithium Battery Smart manual says the fuse “has to match the lowest current rating”: the cable, the battery or the system (S7). So:
- Find the battery’s maximum continuous discharge current in its manual.
- Size the cable for the current and the run length with the Wire gauge calculator.
- Pick a fuse at or below the lowest of those ratings, from the sizes the fuse maker actually sells.
Chargers and inverters come with their own numbers, and those win over a rule of thumb. Some examples from Victron’s manuals:
| Device | Fuse the manual gives | Source |
|---|---|---|
| SmartSolar MPPT 100/30 (battery side) | 35A minimum, 40A maximum | S10 |
| Blue Smart IP22 12/30 charger | 40A minimum, 70A maximum | S11 |
| Orion-Tr Smart isolated, 12V side | 60A | S12 |
| MultiPlus 12/2000/80 | 300A | S13 |
The IP22 manual is clear about why the charger fuse goes near the battery: it’s there “to protect the cabling” (S11). The MultiPlus has no internal DC fuse, per its manual, so the external one is the only protection on that cable (S13).
The Fuse size calculator holds every device fuse we could find in a maker manual and rounds up to a size the fuse makers list.
What is a Lynx Distributor (a fused busbar)?
A busbar and a set of fuse holders in one box. Victron’s Lynx Distributor has a positive busbar with four fuse-protected connections and a negative busbar, plus a light for each fuse (S9). It takes MEGA fuses, bought separately.
Two rules from its manual. On a 12V or 24V boat, use 36V-rated MEGA fuses. And the fuses on all circuits added together shouldn’t be more than the module’s current rating, which the manual lists as 1000A.
MEGA fuses are branch fuses here. The main fuse with the high interrupt rating still sits at the battery, ahead of the Lynx.
Building it from separate parts
Rather not use a Lynx? The same layout comes together from parts:
- Positive busbar: the Victron Busbar 250A 4-point with cover has 5/16 inch studs and a 250A rating (S16).
- Fused feed: the Egis ANL fuse block, up to 4/0, 35-750A takes ANL fuses from 35 to 750A, is rated to 80 Vdc, and Egis lists it as IP66, IP67 and IP6K9K (S14).
- Branch circuits: the Egis RT 12-position fuse block with ground holds ATO, ATC or ATF blade fuses, 30 amps per circuit, for 0-36 V DC systems (S15).
Busbars and fuse blocks: laying out a boat DC panel goes through the layout in more detail.
Does this guide fit your boat?
This applies if: – Your boat has a 12V lithium house bank, or you’re about to fit one. – You’re picking or checking the main fuse and the feeds downstream of it.
This doesn’t apply if: – Your battery maker names a specific fuse or holder. Follow the battery manual over this guide. – You run 24V or 48V. Interrupt ratings change with voltage, so recheck every fuse on its sheet. – The job is on the AC side (shore power, inverter output). That part needs a licensed marine electrician.
Products used in this guide
| Product | Why it’s here |
|---|---|
| Egis ANL fuse block, up to 4/0, 35-750A | Fused feed from the busbar to a panel or inverter. |
| Egis RT 12-position fuse block with ground | One blade fuse per small circuit, with a ground block. |
| Victron Busbar 250A 4-point with cover | Gathers the positive feeds after the switch. |
Prices show live from the store. The battery switch comes in the Boat lithium upgrade kit.
Bundles for this job: Wiring and fusing kit (12-circuit panel), Boat lithium upgrade kit, small
Quick answers
Do I need a class T fuse for a lithium battery on a boat? You need a main fuse whose interrupt rating covers your battery’s short-circuit current. Class T has the highest rating of the common types, which is why many lithium makers ask for it. Some, like Victron for its SuperPack NG, also allow MRBF. Your battery manual decides.
Is an MRBF fuse good enough for lithium? Sometimes. Egis rates its MRBF fuses at 10,000 A interrupt at 14 VDC. If your battery maker’s minimum is at or below that, it qualifies. If the maker asks for class T, use class T.
Can I use an ANL fuse as the main fuse on a lithium bank? Eaton’s ANL sheet gives an interrupt rating of 2700A, much lower than class T or MRBF. Use ANL for feeds after the main fuse unless your battery maker says ANL is fine as the main.
What size main fuse do I need? No bigger than the lowest of three ratings: the cable, the battery’s maximum discharge current and the system current. That’s the rule in Victron’s Lithium Battery Smart manual. Size the cable first, then pick the fuse.
Does the fuse go before or after the battery switch? Before. The main fuse sits on the battery positive as close to the terminal as it fits, so it also protects the cable to the switch.
Sources
- Eaton Bussmann JJN Class T fuses, data sheet 1025: https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/bussmann-series-branch-circuit-fuses/class-t/bus-ele-ds-1025-jjn.pdf
- Egis Mobile Electric, MRBF fuses: https://www.egismobile.com/mrbf-fuses
- Eaton Bussmann ANL, data sheet 2024: https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/bussmann-series-supplemental-fuses/supplemental-fuses-limiters/bus-ele-ds-2024-anl.pdf
- Littelfuse MAXI 32V datasheet: https://www.littelfuse.com/assetdocs/littelfuse-datasheet-299-maxi32v?assetguid=68b8bbe8-3e51-4c7b-bbda-3c71667f75f3
- Littelfuse ATOF datasheet: https://www.littelfuse.com/assetdocs/littelfuse-datasheet-287-atof?assetguid=43dcdce8-8ca2-426f-8998-7e566f048d40
- Victron Lithium SuperPack NG manual: https://www.victronenergy.com/upload/documents/Lithium_SuperPack_NG/219778-Lithium_SuperPack_NG-pdf-en.pdf
- Victron Lithium Battery Smart manual: https://www.victronenergy.com/upload/documents/Lithium_Battery_Smart/15958-Manual_Lithium_Smart_Battery-pdf-en.pdf
- Victron Wiring Unlimited: https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf
- Victron Lynx Distributor M10 manual: https://www.victronenergy.com/upload/documents/Lynx_Distributor/24531-Lynx_Distributor_Manual-pdf-en.pdf
- Victron SmartSolar MPPT 100/30 and 100/50 manual: https://www.victronenergy.com/media/pg/Manual_SmartSolar_MPPT_100-30__100-50/en/installation.html
- Victron Blue Smart IP22 Charger manual: https://www.victronenergy.com/media/pg/Blue_Smart_IP22_Charger_230V_manual/en/installation—bsc—psc.html
- Victron Orion-Tr Smart isolated manual: https://www.victronenergy.com/media/pg/Orion-Tr_Smart_DC-DC_Charger_-_Isolated/en/installation.html
- Victron MultiPlus 2kVA 120V manual: https://www.victronenergy.com/media/pg/MultiPlus_2kVA_120V/en/installation.html
- Egis 3903 ANL fuse block: https://www.egismobile.com/shop/3903b-anl-fuse-block-for-up-to-4-0-cables-35-750-a-sealed-bulk-pack-8802
- Egis 8026 RT fuse block: https://www.egismobile.com/shop/8026b-rt-fuse-block-12-circuit-w-ground-bulk-4952
- Victron Busbar 250A datasheet: https://www.victronenergy.com/upload/documents/Datasheet-Busbar-250-A-4P-and-6P-with-cover-EN.pdf
