Solar Fuse Size Calculator

How to use this calculator

Choose the circuit type first — the multiplier depends on it:

1. PV string / module — fuse rated at 1.56 × Isc (BS 7671 712.434.1 / IEC 60364-7-712)
2. Battery → inverter / load — fuse rated at 1.25 × continuous current (BS 7671 Section 433)
3. Inverter DC input — fuse rated at 1.25 × maximum input current
4. Charge controller output — fuse rated at 1.25 × rated continuous output

Enter the current value and the calculator picks the smallest standard IEC fuse size that exceeds the required rating. Standard IEC ratings are 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250 A and up — slightly different from the NEC list used in the US.

Why 1.56× — the BS 7671 / IEC reasoning

The 156% PV factor combines two distinct safety margins:

• 125% for continuous-duty loading. A PV array on a sunny day runs at full output for many hours. BS 7671 Section 433 requires the overcurrent device to be rated at least 125% of the steady-state design current.
• 125% for irradiance enhancement. Cloud-edge focusing and cold-temperature operation can push real-world Isc 20–25% above the STC nameplate. The second 1.25 factor covers this headroom.

Stack them: 1.25 × 1.25 = 1.5625. For a module rated 10.5 A Isc the minimum fuse rating is 10.5 × 1.56 = 16.4 A → round up to 20 A gPV (next IEC standard).

Where each fuse goes — typical UK domestic install

A grid-tied PV system with battery storage usually has four distinct fused circuits:

1. PV combiner string fuses — one gPV fuse per string when three or more strings parallel. For 1–2 strings on most UK domestic roofs, no per-string fuse is required because the module’s specified maximum reverse current isn’t exceeded.
2. Array DC isolator to inverter — a DC isolator combined with gPV fuses sized to the total array Isc × 1.56.
3. Battery DC isolator — a fused DC isolator between the battery and the hybrid inverter, sized at 1.25× the inverter’s continuous DC input current.
4. AC side — a Type B or Type C MCB in the consumer unit on the inverter’s AC output, sized per BS 7671 Section 433.

All DC fuses must be rated for the maximum DC voltage of that circuit, not just the nominal. A 600 V string can hit 800 V at -10 °C — your fuse needs a 1000 V DC rating to remain compliant.

Standard IEC fuse ratings

Round the calculator’s required figure up to the next standard size:

6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500 A

Smaller sizes (1, 2, 3, 4 A) exist for module-level protection and microinverter trunk cables. Never round down — a fuse smaller than the calculated minimum will nuisance-trip on a sunny day or under inverter inrush.

gPV vs gG vs aR — picking the right characteristic

Three IEC fuse classes appear in PV practice:

• gPV (IEC 60269-6) — designed specifically for PV strings. 1000/1500 V DC, low pre-arc energy, fast clearing on string faults. Standard choice for combiner boxes. Mersen, Bussmann, ETI all sell these in 10×38 mm and 14×51 mm cartridges.
• gG (IEC 60269-2) — general-purpose, AC and low-voltage DC. Acceptable on battery and load-side circuits where breaking capacity is adequate.
• aR (IEC 60269-4) — semiconductor-protection, very fast acting. Found inside some inverter DC inputs as factory protection. Not normally used as field-replaceable fusing.

For the battery → inverter circuit on lithium home batteries (Pylontech, BYD, Tesla, etc.), a Class T or NH gG-DC fuse with 20 kA interrupt rating is the standard professional choice.

Worked examples — UK domestic

Example 1 — 4 kWp roof, 2 strings of 8 panels. Module Isc = 11.2 A. Strings = 2, parallel = 2.

Per-string fuse ≥ 11.2 × 1.56 = 17.5 A → standard 20 A gPV. With only 2 strings, per-string fuses are technically not required by BS 7671 712.434 if the module reverse current rating allows — but installers often fit them anyway for clarity and easier fault isolation.

Example 2 — 5 kW hybrid inverter, 10 kWh LFP battery, 48 V.

Continuous DC current = 5000 / (48 × 0.94) = 111 A Fuse ≥ 111 × 1.25 = 139 A → standard 160 A Class T or NH gG-DC fuse mounted at the battery terminal.

Example 3 — 80 A MPPT controller charging 24 V LiFePO4.

Fuse ≥ 80 × 1.25 = 100 A → standard 100 A DC-rated MCB or Class T fuse between controller and battery.

Sizing the wire alongside the fuse

A fuse protects the cable — so the cable’s continuous current-carrying capacity must equal or exceed the fuse rating after de-rating for installation conditions. BS 7671 Appendix 4 tables give the ampacity for each cable size and method (clipped direct, in conduit, in insulation, in free air).

For a 20 A PV string fuse the cable must carry at least 20 A after de-rating. 4 mm² single-core PV cable (H1Z2Z2-K) in free air carries 50–55 A — comfortable. 2.5 mm² PV cable carries 30–37 A — also fine. Use the wire size calculator to verify both ampacity and voltage drop on long runs.

For the battery → inverter circuit, cable sizing dominates: a 160 A fuse demands at least 25 mm² battery cable in short runs, 35 mm² for runs over 1 m, and you should target a voltage drop below 2% to keep the inverter’s low-voltage cut-off margin healthy.

MCS compliance and commissioning

UK installations registered through the MCS scheme must pass commissioning paperwork that lists every DC overcurrent device by manufacturer part number, voltage rating, current rating, and interrupt rating. The certifier checks:

1. Fuse current rating equals or exceeds the 1.56 × Isc calculation.
2. Fuse voltage rating equals or exceeds the maximum array Voc at -10 °C (BS 7671 712.512.1.1).
3. Fuse breaking capacity equals or exceeds the prospective short-circuit current.
4. Manufacturer’s published maximum overcurrent rating on the module sticker is not exceeded.

If any of these four checks fail, the install does not get its MCS certificate and the customer cannot claim Smart Export Guarantee payments.

Limitations and disclaimer

This calculator implements the 156%/125% sizing rules adopted by BS 7671 Section 712 and IEC 60364-7-712. It returns the next standard IEC fuse rating — confirm the manufacturer’s maximum series fuse rating on the module datasheet, and the inverter’s specified DC input protection rating, before purchasing.

All DC fuses on a PV or battery system must be installed by, or under the supervision of, a competent person registered with a recognised UK installer scheme (MCS, NICEIC, NAPIT, ELECSA). Self-certification does not exempt the installation from Part P of the Building Regulations.