The fuse protects the wire. Check three things: the continuous load current stays below what the fuse may carry at the temperature of its fuse box; the fuse rating is at or below the current the wire can carry where it is routed (after ambient and bundle derating); and the fuse's let-through energy (I²t) on a short circuit stays below what the smallest wire it protects can absorb (k²S²).
The three conditions
| Check | Condition | What you need |
|---|---|---|
| Continuous | Load current Ib ≤ the fuse's allowed current at its box temperature | The fuse maker's temperature derating table, the fuse box ambient |
| Wire protection | Fuse rating In ≤ wire capacity Iz along its whole route | The wire maker's rating for that insulation and ambient, with bundle derating |
| Short circuit | Fuse I²t ≤ k²S² of the smallest protected conductor | The fuse's I²t, the conductor's cross-section S and its insulation constant k |
Installation rules for buildings and machines write the same idea as Ib ≤ In ≤ Iz, with a second condition that the current which reliably opens the device stays within 1.45 × Iz (BS 7671 433.1.1, from IEC 60364-4-43). Blade fuses to ISO 8820 open between 0.75 s and 600 s at 135 % of rating (Littelfuse ATOF, 3–40 A) and must not open within 100 h at 110 %, so for them In ≤ Iz is the condition that binds.
Worked example
A 10 A continuous load on 1.5 mm² FLRY-B wire, the fuse in a box at 85 °C, protected by a 15 A ATOF blade fuse.
- Wire capacity. Two wire makers rate a single 1.5 mm² wire in free air at 40 °C ambient (105 °C conductor) at 27 A (Mueller, FLRY-B) and 28.1 A (Anixter). Take the lower: Iz = 27 A. In a bundle, apply the maker's grouping factor first — Iz falls.
- Fuse at temperature. Littelfuse's temperature table allows a 15 A ATOF to carry 11 A at 85 °C (the table includes a 20 % margin).
- Continuous check. 10 A ≤ 11 A: passes, but at 91 % of the allowance — above an 80 % warning line, so worth a second look.
- Wire protection. 15 A ≤ 27 A in free air: passes before bundle derating. Recheck with the bundle factor for the most crowded branch on the route.
- Short circuit. Compare the fuse's I²t with k²S² of the wire. For a 0.35 mm² PVC wire, k²S² is 1,620 A²s; a 30 A ATOF's typical melting I²t is 1,500 A²s, already 93 % before the clearing energy is added — that pairing is a warning.
What else to check
- Inrush. Cold lamps and capacitive inputs draw a pulse; the fuse's melting I²t must exceed the pulse I²t divided by the maker's pulse factor.
- Voltage rating. A 32 V fuse family is not for a 48 V system.
- Coordination. A downstream fuse should open before the upstream one.
- Terminal and contact ratings, which can be lower than the wire's.
In Zorowire
Zoro checks look at data only. The separate engineering checks in the Workspace run this method per wire and per fuse and show the source table, the assumptions and the margin of every result. Each one reads “Engineering check — verify against the manufacturer's data and the applicable standard”: a calculation to check, not an approval.
Questions
Does the fuse protect the device or the wire?
The wire. Size the fuse so the wire it feeds cannot overheat; the device may need its own protection.
Can I use the ISO 8820 test cable size to pick the wire?
No. Those sizes are the conditions the fuse was tested under; they are not a rule for protecting a wire in your harness.
Why does fuse box temperature matter?
Fuses carry less current when hot. Littelfuse's table, for example, allows a 10 A ATOF only 7 A at 85 °C.
Does Zorowire size fuses?
It checks them. The engineering checks compare each fuse with the wire it protects (continuous current, protection of the wire, short-circuit) and show the assumptions behind each result. They are labelled engineering checks to verify against the manufacturer's data and the applicable standard; the choice of fuse stays yours.
Sources
- Infinitybox, fuses protect the wire (article) · retrieved 2026-10-30
- professional-electrician.com on BS 7671 433.1.1 (Ib ≤ In ≤ Iz, I2 ≤ 1.45 Iz) · retrieved 2026-10-30
- Littelfuse ATOF 287 blade fuse datasheet (opening times, cold resistance, I²t, temperature table; distributor copy) · retrieved 2026-10-30
- Digi-Key, Littelfuse fuseology (pulse I²t and derating) · retrieved 2026-10-30
- Mueller, FLRY-B datasheet (ratings at 105 °C conductor, 40 °C ambient) · retrieved 2026-10-30
- Anixter, automotive wire ampacity table · retrieved 2026-10-30
- Zorowire electrical research and published test vectors — Zorowire test or research, 2026-10-30
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