Class T vs ANL: 200A fuses, 3x the AIC

The number a fuse's amp rating doesn't tell you

A fuse's rated current (35 A, 200 A, 400 A) describes the continuous load it carries and the overload curve at which its element melts. A completely separate number — the interrupting rating, or AIC (amps interrupting capacity) — describes something else: how much fault current the fuse can safely clear during a dead short without the arc across its element continuing to conduct, or the fuse body rupturing. A fuse asked to interrupt more fault current than its AIC allows doesn't fail safe; it can fail as a sustained arc or a physical rupture, which is a fire risk at the exact moment the fuse was supposed to be preventing one.

This distinction matters more for lithium than for lead-acid because of internal resistance. A LiFePO4 cell's internal resistance is low, and it stays low across nearly the entire state-of-charge range — unlike a flooded lead-acid battery, whose internal resistance rises as it discharges and acts as a natural brake on fault current. A dead short at the terminals of a lithium bank, especially one built from several cells or batteries in parallel, can push fault current into a range an ANL fuse's 6,000 A interrupting rating was never tested against. This project's own rule text states the reasoning plainly: a Class T fuse is required for a lithium bank because "its short-circuit current exceeds the interrupting rating" of a lower-AIC fuse class (src/rules.json, rule fuse-class-wrong). The rule applies to every lithium bank in the catalog, not just large ones, because a bank's fault current is a function of cell chemistry and wiring impedance the parts catalog does not model precisely enough to draw a safe cutoff below which ANL would be acceptable.

Paralleling batteries makes the physics worse, not better. Wiring two identical lithium batteries in parallel roughly doubles the fault current available at a dead short, because each battery's low internal resistance now contributes current to the same fault path simultaneously — the BMS peak rating scales the same way in this project's own model (bms_peak_a × battery_parallel), which is exactly why the multiplied peak number is what feeds the surge check in the inverter/BMS guide on this site. A single small lithium battery already sits in fault-current territory an ANL wasn't rated for; four of them in parallel behind one busbar make the case for Class T stronger, not weaker, even though nothing about the wiring diagram looks more dangerous at a glance.

The formula for the fuse's amp rating (a separate question from AIC)

min_fuse_a = current_a × 1.25 (Calc::FUSE_FACTOR, ABYC E-11's 125% continuous-load rule), and selectFuse() picks the smallest catalog rating at or above that minimum, filtered to a required class when one applies, and capped at whatever the wire's ampacity allows above it. requiredFuseClass(chemistry) returns 'class-t' for 'lifepo4' and null for anything else — a categorical answer, not a magnitude-based one. Sizing the amp rating and choosing the class are two independent steps, and getting the first one right does nothing to satisfy the second.

A concrete example: sizing the battery-to-inverter fuse

Continuing the pairing from this site's inverter/BMS guide — an EG4 WallMount Indoor 280Ah LiFePO4 battery (48 V, lithium) feeding a Victron MultiPlus-II 48/8000/110-100 inverter, whose real DC-side continuous draw (Calc::inverterDc) works out to 149.4 A — the minimum fuse rating is 149.4 × 1.25 = 186.7 A. selectFuse() against this project's fuse catalog returns the smallest rating at or above that minimum:

Both fuses satisfy the 186.7 A minimum with the same 13.3 A of margin before hitting the next size up. The amp-rating math is identical either way — this is exactly why the amp rating alone is not a sufficient spec for a lithium battery fuse, and why requiredFuseClass() exists as a separate check layered on top of selectFuse() rather than folded into it.

A second example: a smaller circuit, same rule

The same logic applies at much lower current. A 48 V charge controller rated for 60 A continuous into the same lithium bank needs a minimum fuse of 60 × 1.25 = 75 A. selectFuse() picks the smallest catalog rating clearing that minimum: 80 A in either class. An 80 A ANL fuse (6,000 A AIC) and an 80 A Class T fuse (20,000 A AIC) both size correctly and both blow at the same overload — the ANL is still the wrong choice for this bank, for the same reason it was wrong at 200 A. The circuit's current has nothing to do with which class is required; the chemistry does. A small solar charge controller circuit on a lithium bank needs Class T exactly as much as a large inverter circuit on the same bank.

This is also why requiredFuseClass() takes only a chemistry string as its argument, not a current value — there's no magnitude threshold in this project's logic below which ANL becomes acceptable on a lithium bank, because the fault current problem doesn't scale down with the circuit's normal operating current the way the amp-rating problem does.

Where Class T isn't required

requiredFuseClass('lead-acid') and requiredFuseClass('agm') both return null in this project's rule set — no class restriction. A flooded or AGM lead-acid bank's rising internal resistance under discharge keeps fault current within a range ANL fuses have been rated for since long before lithium chassis batteries existed in this market, which is part of why ANL remains the default, lower-cost fuse for those chemistries. Class T fuses also cost more and are physically larger for the same amp rating, which is a real reason not to over-specify them on a circuit where the interrupting-rating margin was never in question. The rule in this project's catalog is specifically chemistry-gated for that reason — it isn't "always use the bigger fuse," it's "match the fuse class to what the battery chemistry can actually push through a fault."

The chemistry gate also means the same physical circuit can have two different correct answers depending only on which battery sits behind it. Swap a lead-acid bank for a lithium one on an otherwise identical inverter and wiring run, and the fuse rating computed by minFuse() doesn't change at all — the current draw is the same — but the fuse class does, from unrestricted to Class T only, purely because requiredFuseClass() takes the chemistry string as its one argument. A fuse selected correctly for the old lead-acid bank becomes the wrong part the moment the battery is swapped, even though nobody touched the wire, the inverter, or the breaker panel.

Reading a fuse's own markings

Both numbers this guide covers are stamped on the fuse body itself, not hidden in a datasheet somewhere — an ANL fuse reads its amp rating prominently and its AIC in smaller print near the manufacturer's name, and the same is true of a Class T fuse and its holder. The physical size difference is also visible without reading anything: a Class T fuse and its fuseholder are larger than an ANL of the same amp rating, because the ceramic body and the internal clearances needed to safely interrupt 20,000 A rather than 6,000 A take more room. That size difference is often the first clue, during a build, that a kit shipped with the wrong class — an ANL fuse holder simply doesn't accept a Class T fuse, and the mismatch is a fit problem before it's ever tested as an electrical one. This project's kit-level rule (kit-fuse-class) exists specifically to catch this before a kit ships, rather than relying on someone noticing the fuse doesn't seat correctly during installation.

Try it with your own battery and circuit

Enter a circuit's continuous current and the battery chemistry it protects. The calculator sizes the minimum fuse rating and shows what each fuse class in this project's catalog offers at that size.

What this means for wiring a lithium bank

Three practical consequences follow directly from the two-number structure above:

1. Sizing the amp rating and choosing the class are separate steps, in that order. Run the current through minFuse() first to get the rating, then apply requiredFuseClass() for the chemistry — an ANL fuse sized correctly at the right amp rating is still the wrong fuse if the bank behind it is lithium.
2. The circuit's normal operating current doesn't tell you which class is safe. A 75 A minimum on a small charge-controller circuit needs Class T on a lithium bank exactly as much as a 187 A minimum on a large inverter circuit — the chemistry gates the class, not the load.
3. Don't over-specify Class T on chemistries that don't need it. Lead-acid and AGM banks keep their fault current within an ANL fuse's interrupting rating by nature of how their internal resistance behaves under discharge — Class T's added cost and size buys nothing there.

Every fuse rating in both classes and its AIC is in this project's own catalog, and every kit in the kit audit is checked against fuse-class-wrong before it's shown as a recommended build.