Fuse check for a LiFePO₄ bank

LiFePO₄ fuse size calculator

A battery's amp-hours don't pick its fuse. Start from the current the circuit really carries, keep the fuse under what your cable and equipment can take, then check the exact fuse and holder for DC voltage and fault current.

Checked Sep 11, 2026 · 12 V DC planning · your manual wins

Worked schematic

See which cable the battery fuse protects.

A 40 A continuous load gives a 50 A preliminary baseline with the calculator's 125% headroom. The fuse sits in the positive path near the source; the negative cable completes the circuit.

Interactive Crimp schematic. Pan and zoom to follow the circuit. This fixed example illustrates the calculation; your form inputs are separate.

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Red = positive · dark = return. Crossings are not junctions. Read the assumptions before adapting a circuit.

C1: the short unfused lead

The example draws 0.1 m from battery positive to the fuse holder. That lead is upstream of the fuse. Check the permitted placement and protect the lead against a short.

C2: the downstream load cable

The remaining 2.9 m is downstream of the proposed 50 A fuse; C3 is the 3 m negative return. The exact fuse and holder still need to be verified.

Illustrative 12 V circuit with 40 A discharge and no charging source. The 50 A shown is only a continuous-current baseline, not a selected or verified fuse. Surge behavior, equipment limits, cable derating, exact fuse/holder DC ratings, whole-bank fault current, BMS and placement need evidence. The 0.1 m lead is an example dimension, not a universal permitted distance.

Open this example in Crimp →Use your own inputs ↓

The calculator

Start with the circuit. Then check a fuse.

No fuse selected yet? Enter the current and cable limits you know to see a preliminary sizing baseline. Add the manufacturer's surge guidance for a candidate rating, then open the exact-part checks. Leave unverified inputs blank. Nothing you type here is saved.

You can begin without a fuse part number. A baseline is only a starting constraint; it is not a fuse to buy.

I already have a fuse to check

1. What the circuit carriesRead these off the load's and the chargers' manuals, not off the battery.

Maximum simultaneous chargeAEvery charge source that can run at once through this fuse. Enter 0 only if charge can't flow here.Maximum simultaneous dischargeAEvery sustained load that can run at once through this fuse. Enter 0 only if discharge can't flow here.Documented discharge surgeAThe peak current the load's manual gives.Surge durationsHow long that peak can last.

What we can calculate so far

Continuous-current baseline: Unknown
Known cable/equipment ceiling: Unknown

Enter both charge and discharge current, including a verified zero where a direction cannot flow. Cable ampacity must already account for conductor size, insulation, temperature, bundling and routing. A size in mm² or AWG alone does not give that limit.

Next, supply the surge duration and manufacturer-backed fuse guidance below. The baseline can rise to carry that surge; it must still fit every cable and equipment ceiling. Exact-part voltage, interrupt rating and placement are separate checks.

How Crimp calculates cable sizes

What the manuals and the cable allowThe fuse protects the cable, so the cable's real ampacity is a hard ceiling.

Guidance sourceName the documents you took the surge and fuse limits from.Minimum fuse that carries the surgeAFrom that guidance — not the peak value itself.Manufacturer maximum fuseAThe hard ceiling for the protected equipment.Manufacturer recommendationAOptional, if the manual names an exact rating.Derated cable ampacityAAfter derating for heat, bundling and routing.Other equipment ceilingAOptional lower limit elsewhere on this segment.

2. Check an exact fuse and holder

Use a candidate rating to look up possible parts, then enter their actual datasheet evidence here. A family name or a BMS cutoff cannot prove fault interruption. Unknown checks stay unknown, even when a rating can be calculated.

The exact fuse and holderA family name is context. The check needs the part you'll actually buy.

Fuse family being checkedA family is context, not proof of a specific device.Exact fuse part numberThe check can't pass on a family name alone.Exact fuse amp ratingAHas to match the calculated candidate.Maximum DC system voltageVThe maximum charging or transient value, not nominal.Exact fuse DC voltage ratingVFrom the exact part's datasheet.Exact holder part numberThe holder you'll pair with this fuse.Installed holder current limitAThe maker's limit after derating for ambient, enclosure, mounting and terminal condition.Exact holder DC voltage ratingVThe holder has to pass on its own.

Fuse-holder compatibility sourceName the source that confirms this holder takes the exact fuse family and rating.

Fault current at the fuseADocumented or calculated for the complete bank and the installed path.Exact fuse interrupt ratingAAt your DC voltage.

Fault-current basisName the manufacturer data and the calculation. Amp-hours and the BMS cutoff current are not fault-current evidence.

Your bank and where the fuse sitsCapacity is recorded for context only. We never use it to size the fuse.

Battery capacityAhFor context only; never used to size the fuse.Battery countTotal batteries in this bank.Parallel stringsStrings that can feed this fault.Fault-current scopeHas to cover the complete source the fuse sees.Proposed fuse locationName the exact source-side segment.Governing standard or manualThe rules for this boat, vehicle or installation.

I checked how close the fuse sits to the source, and any exception allowed, against the named standard and the component manuals.

Not verified yet

Not enough information to verify a safe fuse.

Fill in the current the circuit carries and the manuals' limits to get a candidate.

Amp rating
Continuous current and manufacturer-backed surge guidance are required before an amp rating can be calculated.
DC voltage
The maximum system, exact fuse, and exact holder DC ratings are required.
Interrupt rating
Prospective fault current at the fuse location and the exact fuse interrupt rating are required.
Placement
Installation location and its governing standard or manual are required.

Still missing · 23

Maximum simultaneous charge current
Maximum simultaneous discharge current
Documented discharge surge current
Documented discharge surge duration
Manufacturer guidance or exact fuse-curve source
Minimum fuse rating that carries the documented surge
Manufacturer maximum fuse rating
Installed conductor ampacity after derating
Fuse family being evaluated
Exact fuse part number
Exact fuse amp rating
Exact holder part number
Installed holder current limit after derating
Fuse-holder compatibility source
Maximum DC system voltage
Exact fuse DC voltage rating
Exact holder DC voltage rating
Documented or calculated prospective fault current at the fuse location
Source data and calculation basis for prospective fault current
Exact fuse interrupt rating at the relevant DC voltage
Proposed fuse location
Governing standard or manual
Confirmation that source-side placement was checked

Battery capacity: not supplied. We never use it to size the fuse.

This is a planning check, not an installation approval. When the evidence conflicts or is incomplete you get “Not enough information to verify a safe fuse.”

Check the whole circuit in Crimp

How the check works

Amp rating and interrupt rating answer different questions.

A fuse can have the right number printed on it and still fail to clear a lithium-bank short safely. Every check below has to pass for the exact part you're holding.

Load

Carry the normal current

Crimp's planning baseline applies 125% headroom to the larger of the charge and discharge current you enter. Your governing standard, the equipment's manual and the fuse's time-current curve still decide the final rating.

Cable

Protect the weakest link

The fuse can't be rated above what your installed cable carries after derating for heat, bundling and routing — or above any lower maximum the equipment sets.

Device

Cover the voltage and the fault current

Both the fuse and its holder need a DC voltage rating that covers your system. The fuse's interrupt rating has to cover the fault current the whole installed bank can deliver at that spot.

Location

Protect at the source

Check where the fuse sits against your standard and the component manuals, including any exception they allow. The BMS is not the branch fuse.

Amp rating

Will it carry the load without cooking the cable?

Interrupt rating (AIC)

Can it open the fault without arcing or welding shut?

Class T often comes up for lithium banks because exact Class T parts carry high published interrupt ratings. It isn't a universal answer: read the voltage and interrupt ratings for the exact fuse at your DC voltage.

Your bank and where the fuse sits

Check the fault source you actually built.

Parallel batteries feed the same short. A per-battery figure can't prove the interrupt rating for a fuse that sees the whole bank.

Name what's on each side of the fuse
Write down the battery terminal, bus, cable and load on either side of the fuse you're proposing. Decide exactly what that fuse is there to protect.Stop here
Use whole-bank fault data
With more than one battery, get the fault current for the complete bank at the spot where the fuse sits. Don't multiply amp-hours into a made-up short-circuit number.Stop here
Verify the exact fuse and holder
Match part numbers, DC voltage, interrupt rating, terminal size, how many cables the stud takes and the torque. A family name isn't a specification.Stop here
Put the fuse at the source
Follow your standard and the manufacturer's instructions on how close to the battery it sits, enclosed spaces, exposure, and any exception they allow.
Protect parallel cables
Check whether each battery or string needs its own fuse as well as the bank's main one. Your layout and your standard decide.

Sources

Check against the current datasheets.

Ratings change with series, voltage and certification. Open the current datasheet for the exact fuse, holder, battery and equipment before you commit to the design.

Victron EnergyWiring Unlimited: DC wiringVictron's plain walk-through of what a DC fuse has to be rated for: current, voltage, speed, type and interrupt rating.Blue Sea SystemsSelect a fuse and fuse holder for your DC product installationHow much headroom to leave over a continuous load, and why the fuse is there to protect the cable.Blue Sea SystemsClass T FusesThe DC voltage and interrupt ratings Blue Sea publishes for its Class T fuses.EatonBussmann series Class T fusesEaton's specifications for the Class T fuse families.LittelfuseMEGA Fuse Series DatasheetVoltage, interrupt rating and time-current curves for MEGA fuses, so you can see how long one holds a surge.LittelfuseFuseology selection guideHow heat, the holder and the enclosure lower what a fuse can really carry.LittelfuseFuses for energy storage systemsFault current and protection when several sources and a battery bank share one bus.Victron EnergyLithium Battery Smart: InstallationVictron's installation rules tying system current, cable rating and fuse rating to each other.International Organization for StandardizationISO 23625:2025 — Lithium-ion batteriesWhat the current small-craft lithium-ion battery standard covers, if your surveyor asks.American Boat & Yacht CouncilStandards Update, Supplement 65ABYC's update on how lithium batteries are to be installed on boats.

Keep goingCalculate the wire before checking its fuseUse current, one-way length and installation conditions to find the cable and its derated ampacity.Keep goingEstablish the DC-DC charging currentCompare vehicle input demand, alternator headroom and battery/BMS charging limits.Keep goingPlan the whole LiFePO₄ conversionStart with the battery, keep what already fits, and check charging and protection as you get to each part.

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