Solar Fuse Size Calculator

How to use this calculator

Pick the circuit type first — the sizing factor depends on it:

1. PV string / module — fuse rated at 1.56 × Isc per NEC 690.9(B)
2. Battery → inverter / load — fuse rated at 1.25 × continuous current
3. Inverter DC input — fuse rated at 1.25 × max continuous input current
4. Charge controller output — fuse rated at 1.25 × rated continuous output

Enter your current value, then round up to the next standard fuse size. The calculator does this rounding for you using the NEC 240.6 / IEC 60269 standard ratings list (1, 2, 3, 5, 6, 8, 10, 12, 15, 16, 20, 25, 30, 32, 35, 40, 45, 50, 60, 63, 70, 80, 90, 100 A and up).

Why 1.56× for PV strings — the math behind NEC 690.9(B)

Two separate de-rating factors stack on the DC side of a solar array:

• 125% for continuous duty. NEC defines a continuous load as one that runs for three hours or more. A PV array on a sunny day clearly qualifies. All continuous-duty circuits require an overcurrent device rated at 125% of the load.
• 125% for PV irradiance enhancement. Cloud-edge effects and high-altitude irradiance can push a module’s effective Isc 20–25% above the STC nameplate value. NEC bakes another 1.25 multiplier into the PV equation to cover this real-world headroom.

Multiply: 1.25 × 1.25 = 1.5625, which NEC writes as 156%. A module rated 10.5 A Isc requires a fuse of at least 10.5 × 1.56 = 16.4 A → round up to 20 A (next standard NEC size).

Where each fuse goes — circuit-by-circuit

A typical DIY off-grid or hybrid solar system has three or four distinct fused circuits:

1. PV combiner (one fuse per string) — only required if you have 3+ strings in parallel per the string sizing of your array. For 1–2 strings, the wire ampacity protects the cable on its own.
2. PV array → charge controller — protect the cable run, usually 1.56× Isc total combined array current.
3. Charge controller → battery — 1.25× the controller’s rated output current.
4. Battery → inverter — the big one. A Class T fuse sized at 1.25× the inverter’s continuous DC input current, mounted within 18 inches of the battery positive terminal per NEC 706.30(C).

Each fuse must be DC-rated for at least the open-circuit voltage of that part of the system. PV-side fuses must handle up to 1000 V DC (residential) or 1500 V DC (commercial). Battery-side fuses on a 48 V system need at least 80 V DC rating with adequate interrupt capacity (Class T is the typical choice — 20,000 A IR is standard).

Standard fuse sizes (NEC 240.6)

When the calculation produces a fractional answer like 16.4 A, you round up to the next listed standard size. The standard NEC fuse ratings are:

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250 A

Smaller fuses below 15 A are also standardized (1, 2, 3, 5, 6, 8, 10, 12) for module-level applications and microinverter trunk cables. Above 250 A, sizes step up to 300, 350, 400, 500 A and beyond for utility-scale combiners.

You never round down. Rounding down means the fuse can nuisance-trip under normal operating current — it’s both unsafe (premature failure) and a NEC violation.

Picking the fuse type — gPV vs Class T vs ANL

Three families of DC fuse dominate residential and small-commercial solar:

• gPV (IEC 60269-6) — designed specifically for PV string protection. 1000 V DC or 1500 V DC rating, low pre-arcing energy, fast clearing under fault conditions. Standard for combiner-box use. Brands: Mersen Helio Protect, Bussmann PV.
• Class T (UL 248-15) — extremely high interrupting capacity (20 kA at 160 V DC), fast-acting, typical for battery-to-inverter circuits. The standard choice for 12/24/48 V battery banks where fault current is enormous.
• ANL / MIDI / MEGA — slower-blow, lower interrupt rating. Acceptable for DC loads under 32 V where prospective short-circuit current is moderate. Common in RV and marine. Not appropriate for large lithium battery banks — use Class T instead.

The calculator gives you the amperage; you still pick the fuse type based on voltage and prospective fault current.

Worked examples

Example 1 — PV string fuse. Module Isc = 11.2 A, 2 modules in series, 3 strings in parallel.

Required fuse ≥ 11.2 × 1.56 = 17.5 A → standard 20 A gPV at 1000 V DC, one per string in the combiner.

Example 2 — Battery to inverter. 48 V battery bank, 5000 W inverter, 92% efficiency.

Continuous DC current = 5000 / (48 × 0.92) = 113 A Fuse ≥ 113 × 1.25 = 141 A → standard 150 A Class T fuse near the battery terminal.

Example 3 — Charge controller output. 60 A MPPT controller charging a 24 V battery bank.

Fuse ≥ 60 × 1.25 = 75 A → standard 80 A Class T or DC breaker between the controller and the battery.

Voltage drop and fuse choice are separate constraints

Don’t confuse a fuse with wire sizing. The fuse protects the wire and equipment from sustained over-current and short-circuit faults. Voltage drop sizing protects system efficiency by minimizing resistive losses.

Standard practice on the DIY off-grid side:

1. Size the wire for ≤3% voltage drop at full load.
2. Size the fuse at 1.25× (or 1.56× on the PV side) of continuous current.
3. Confirm the fuse rating does not exceed the wire’s ampacity — if it does, upsize the wire.

For grounded systems also check your equipment grounding via the solar grounding calculator — the equipment ground conductor must be sized to safely clear the largest expected fault, which is usually the fuse’s interrupt rating.

When to call an electrician

DIY-friendly: small RV and off-grid systems under 48 V DC, single-string roof installs, prefab plug-and-play kits.

Licensed electrician territory: anything tying to the utility grid, anything in conduit penetrating a roof or wall, 1000+ V DC strings, lithium battery banks above 5 kWh, and any installation in a jurisdiction that requires permits or inspections (most US states). The calculator gives the right fuse size, but final installation must comply with the local AHJ — and on the grid side, that means a licensed electrician’s stamp.

Limitations and disclaimer

This calculator implements the NEC 690.9 / 706.30 sizing rules for North American installations. International readers — IEC 60364-7-712 (Europe), AS/NZS 5033 (Australia/NZ), and CSA C22.1 Section 64 (Canada) use the same 1.56 factor for PV string sizing but may use different standard fuse ratings (e.g. 16 A is a standard IEC size but not a standard UL size). Round up to the nearest standard size available in your market.

Always confirm fuse selection against the equipment manufacturer’s maximum overcurrent device rating — printed on the module sticker, the inverter spec sheet, and the charge controller manual. The lower of (NEC calc, manufacturer spec) wins.