Size a DC fuse or breaker
Computes rated current, the required DC voltage rating and the breaking capacity needed. It checks two things routinely missed in self-build projects: whether the device is rated for direct current at all, and whether it can still interrupt the battery's short-circuit current.
- Breaking capacity is below the expected fault current of 2,560 A. The device cannot clear the fault — choose one with a higher interrupting rating.
BREAKING_CAPACITY_INSUFFICIENT
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
Safety-relevant calculation. This is an estimate based on the stated assumptions. The final design must be checked by a qualified electrician against the rules that apply where you are.
Data as of: 2026-07-15 · The technical review of this safety-relevant calculator is still outstanding.
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Continuous current | I_cont,max | 100 A | exact |
| Required rating | 1.25 * I_design | 125 A | exact |
| Selected commercial size | next standard size >= I_req | 125 A | exact |
| Required voltage rating | U_nom >= V_oc,max,cold | 14.6 V | exact |
| Prospective short-circuit current | U_bat / R_int,total | 2,560 A | exact |
- Formula
I_OCPD ≥ 1.25 · I_design · U_nom ≥ V_oc,max,cold · I_cn ≥ U_bat / R_int- Valid for
- Battery circuit (continuous current × margin, standard series) and PV source circuit (NEC 690: 156 % Isc per string, parallel strings) — both paths fully operable. Device checks: DC rating (critical — AC breakers do not quench DC arcs), voltage rating against cold open-circuit voltage, breaking capacity against the prospective short-circuit current (U/R; 12.8 V / 5 mΩ → 2,560 A). Coordination with conductor ampacity and BMS continuous current as optional, named checks. NO SUBSTITUTE FOR PROFESSIONAL DESIGN.
- Not covered
- Selectivity of multi-stage protection, trip curves (slow/fast) in detail, arc energy calculation, DC residual-current protection, manufacturer-specific derating curves of protective devices.
- Data sources
- Official publications of the standards bodies and state authorities (NFPA, IEC, DKE/VDE, CEN) · retrieved 2026-07-15
Frequently asked questions
What fuse do I need for my battery circuit?
The fuse rating must be at least 1.25 times the maximum continuous current, rounded up to the next standard size. Upper bounds come from the cable's ampacity and the BMS's maximum continuous discharge current — a fuse above those limits no longer protects the conductor. In addition, the DC voltage rating must be at least the maximum cold open-circuit voltage of the system.
Can I use an ordinary AC circuit breaker in a DC circuit?
No. Alternating current crosses zero a hundred times per second; direct current never does — an AC breaker will not extinguish the arc in a DC circuit. A 230 V AC breaker in a 48 V DC battery circuit can burn continuously in a fault. The calculator therefore explicitly checks whether the intended device is rated for DC and raises a critical warning if it is not.
Why is an automotive blade fuse not enough for a LiFePO4 battery?
Because of breaking capacity: a 100 Ah LiFePO4 block with 5 mOhm internal resistance can theoretically deliver over 2500 A into a short circuit. A typical automotive blade fuse rated to break 1000 A does not interrupt that current — it vaporises. The calculator estimates the prospective short-circuit current as V_batt / R_internal and checks it against the device's breaking capacity; the design must be verified by a qualified electrician under local rules.
Why must I not use an AC breaker in a DC circuit?
Because direct current has no zero crossing: an AC breaker relies on the arc extinguishing itself at the zero crossing — with DC it keeps burning, and the breaker welds or burns out. DC-rated devices have arc chutes and a declared DC voltage rating. The calculator checks both and warns critically if an AC device is planned in a DC circuit.
Does the fuse protect my device?
No — it protects the CABLE. The selection ladder shows it: the fuse must sit above the design current (so it does not trip in operation) and below the cable ampacity (so the cable does not become a glow wire in a fault). Device protection is the device electronics’ job. Choosing the fuse by device instead of cable leaves you, in a short, with a burning cable and an intact fuse.
What is breaking capacity — and why is it critical with batteries?
Breaking capacity is the largest current the fuse can safely interrupt. A large LiFePO₄ bank with a few milliohms of internal resistance delivers several thousand amps in a short — an automotive fuse with 1 kA breaking capacity then welds instead of disconnecting. The calculator estimates the prospective short-circuit current from the internal resistance and checks it against the breaking capacity; for battery circuits, Class T or NH fuses are the usual answer.
Why must no AC breaker sit in a DC circuit?
Because it does not quench the arc. Alternating current has a hundred zero crossings per second in which the arc extinguishes by itself — direct current has none. A 230 V AC breaker in a 48 V battery circuit can burn continuously under fault. This is the most common serious error in DIY builds and therefore this page's sharpest warning: it cannot be dismissed.
How does the page check the PV source circuit?
Per NEC 690: 156 % of the string short-circuit current as the design current, more for parallel strings — both inputs are reachable through the interface since 5 August 2026; before that the PV path ran silently on 11 A and one string. For 11 A and one string this yields 17.2 A, hence a 20 A fuse.
What does the short-circuit comparison tell me?
Whether the fuse survives the fault: a 100 Ah LiFePO4 block at 5 mΩ delivers a prospective 2,500+ A — an automotive blade fuse rated 1,000 A does not interrupt that, it vaporises. Breaking capacity is printed on every device (say “10 kA”); enter it and you get the comparison, remove the checkmark and you get the honest note that this check is missing.
Why coordinate with conductor and BMS?
A fuse protects the CONDUCTOR — so it must not exceed its corrected ampacity. And it should sit below the BMS continuous current, so that under overload the fuse blows rather than the electronics cutting out. Both limits are reachable as optional inputs; without them the page names the checks as open instead of filling them with invented values.
Why does the page warn specifically above 60 volts?
Because above roughly 60 V DC, arcs drawn when disconnecting under load can remain stable — the threshold above which connectors and switches must be explicitly DC arc-rated. A 48 V system easily exceeds it at cold open-circuit voltage; the calculator accepts the maximum system voltage from the Voc calculator.