Class T vs ANL: battery fault current

This is the single most consequential number in a DIY or field-assembled battery build, and the one most often left unchecked.

Two different ratings

• Ampere rating — the current it carries continuously. A 200 A fuse.
• Interrupting rating (AIC) — the fault current it can safely break.

People shop on the first and ignore the second. NEC 110.9 is about the second: equipment intended to interrupt current at fault levels must have an interrupting rating sufficient for the current available at its terminals.

What a lithium pack can deliver

Lead-acid has meaningful internal resistance. LiFePO4 has very little, which is why it delivers its rated power so flatly — and why it dumps so much into a short.

A 16S bank of 280 Ah cells sits around 4–5 mΩ including busbars. At 51 V that is roughly 10–12 kA available. An ANL asked to interrupt that does not simply blow — it can vaporise, arc across its housing, or weld closed.

The multi-string problem, and the real fix

Add parallel strings and resistance falls further. Two strings roughly doubles available fault current; three roughly triples it. Past about 20 kA, even a Class T on the pack output is not enough.

The answer is how large banks are actually built: a fuse in each string's positive leg. For a bolted fault at the pack terminals each fuse carries only its own string's contribution — total divided by the number of strings. Three strings at 30 kA total means each device interrupts about 10 kA, comfortably inside a Class T.

This is not arithmetic sleight of hand. It is why per-string fusing is standard practice, and it is often the only way a large bank satisfies 110.9 at all.

Practical rules

1. Class T at the battery, as close to the positive terminal as practicable. 2. Per-string fuses on any multi-string bank. 3. Read the listing, not the catalogue page. If no AIC is published, you cannot evaluate 110.9 — which is itself the answer. 4. The BMS is not the overcurrent device. It is protection, not a device rated to make and break fault current. 5. State the available fault current on the drawing. 706.7(D) wants maximum short-circuit current on the ESS nameplate anyway.

Work out your own number

Fault current depends on your cells, your string count and your interconnects — not on a rule of thumb. The free 48 V battery lab computes it from the pack you configure and checks it against the device you select, per 110.9.

See also: 48V LiFePO4 and the NEC and NEC 706 energy storage.

> Design aid — a licensed electrician or PE must review, connect and inspect before energizing.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.