Solar Fuse Size Calculator

Size DC fuses for any PV string, battery bank, or inverter, using Isc and continuous-current NEC 690.9 rules.

Enter the module's rated Isc from its datasheet. This calculator fuses one string at a time — parallel strings only changes whether per-string fusing is required, not the fuse size.

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HOW IT WORKS

How does the solar fuse size calculator work?

The calculator applies the NEC 690.9 sizing rule that matches your circuit — 156% of Isc on the PV source side, 125% of continuous current everywhere else — then rounds up to the next standard fuse rating.

1

Pick the circuit type

PV string, battery→inverter, inverter DC input, or charge controller output — the sizing factor depends on which one.

2

Enter the current

Module Isc for a PV string, or the circuit's continuous current for everything else.

3

Get your fuse size

See the required minimum rating and the next standard fuse size to buy.

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

Two separate de-rating factors stack on the DC source side of a solar array. The first is the standard 125% continuous-duty factor: NEC defines a continuous load as one running three hours or more, and a PV array on a sunny day clearly qualifies, so every continuous-duty circuit needs an overcurrent device rated at 125% of the load. The second is a PV-specific 125% irradiance-enhancement factor, because cloud-edge effects and high-altitude irradiance can push a module's effective Isc 20–25% above its STC nameplate rating. Multiply the two together — 1.25 × 1.25 = 1.5625 — and NEC rounds that to the 156% figure in 690.9(B). Only the PV source side carries both factors; battery, inverter, and load-side circuits use the single 125% continuous-duty factor.

THE CALCULATIONPV string fuse ≥ Isc × 1.56. All other DC circuits: fuse ≥ continuous current × 1.25.

Round the result up to the next standard fuse rating — never down. Standard sizes: 1, 2, 3, 5, 6, 8, 10, 12, 15, 16, 20, 25, 30, 32, 35, 40, 45, 50, 60, 63, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250 A and up (NEC 240.6 / IEC 60269).

Worked example: a PV string fuse

EXAMPLE 1

Module Isc = 11.2 A, 2 modules in series, 3 strings in parallel

Three strings in parallel means per-string fusing is required under NEC 690.9(A), since a fault in one string could otherwise be back-fed by the other two.

  1. Required fuse: 11.2 A × 1.56 = 17.47 A.
  2. Round up: next standard size is 20 A.

Result: a 20 A gPV fuse rated at 1000 V DC or higher, one per string, in the combiner box.

Worked example: battery to inverter

EXAMPLE 2

48 V battery bank, 5,000 W continuous inverter, 92% efficiency

This is the circuit that carries the most fault current in a typical off-grid or hybrid system, which is why it gets a Class T fuse rather than a slower-blow type.

  1. Continuous DC current: 5,000 W ÷ (48 V × 0.92) ≈ 113 A.
  2. Required fuse: 113 A × 1.25 ≈ 141 A.
  3. Round up: next standard size is 150 A.

Result: a 150 A Class T fuse, mounted within 18 inches of the battery's positive terminal per NEC 706.30(C).

Where each fuse goes in a typical system

A typical DIY off-grid or hybrid solar system has three or four distinct fused DC circuits, each protecting a different cable run. The PV combiner fuses each string individually, but only matters once you have three or more strings in parallel — with one or two, cable ampacity alone usually protects the run. The PV array to charge controller run is fused at 1.56× the combined array current. The charge controller to battery run is fused at 1.25× the controller's rated continuous output. And battery to inverter — almost always the largest fuse in the system — is sized at 1.25× the inverter's continuous DC input current.

Every fuse also needs a DC voltage rating that covers the open-circuit voltage of its circuit: PV-side fuses typically need to handle up to 1000 V DC for residential arrays (1500 V DC for commercial), while battery-side fuses on a 48 V system need at least an 80 V DC rating with adequate interrupt capacity — Class T's 20,000 A interrupt rating is the typical choice.

Picking the fuse family: gPV vs. Class T vs. ANL

The amperage this calculator gives you is only half the spec — you still need to pick a fuse family suited to the circuit's voltage and prospective fault current. gPV fuses (IEC 60269-6) are purpose-built for PV strings at 1000–1500 V DC with fast clearing and low pre-arcing energy — the standard choice inside a combiner box. Class T fuses (UL 248-15) have very high interrupting capacity, around 20,000 A at 160 V DC, and are the standard for battery-to-inverter circuits, where a 12/24/48 V lithium or lead-acid bank can deliver enormous fault current. ANL, MIDI, and MEGA fuses are slower-blow with a lower interrupt rating — acceptable for DC loads under 32 V with moderate fault current, common in RV and marine systems, but not a substitute for Class T on a large lithium battery bank.

You never round down

When the calculation lands on a fractional value like 17.47 A, always round up to the next standard listed size. Rounding down means the fuse can nuisance-trip under completely normal operating current — that's both a reliability problem and a NEC violation, not a conservative safety margin. Always confirm your final selection against the equipment manufacturer's maximum overcurrent device rating too, printed on the module sticker, inverter spec sheet, or charge controller manual — the lower of the code calculation and the manufacturer spec wins.

Common ways to use this calculator

Sizing a combiner box

Get the right gPV fuse rating for each string before ordering parts.

Protecting a battery bank

Size the Class T fuse between your battery and inverter to the correct interrupt-rated amperage.

Checking an existing install

Verify a fuse someone else installed actually meets the 1.25×/1.56× minimum.

Planning an off-grid or RV system

Work through PV, charge-controller, and inverter circuits one at a time before you buy fuses.

When to call a licensed electrician

Small RV and off-grid systems under 48 V DC, single-string roof installs, and prefab plug-and-play kits are reasonable DIY territory. Anything tying into the utility grid, run through conduit penetrating a roof or wall, operating at 1000+ V DC, using a lithium battery bank above 5 kWh, or installed in a jurisdiction that requires permits or inspections — which is most U.S. states — is licensed-electrician territory. This calculator gives you the correct fuse size; final installation still has to satisfy your local Authority Having Jurisdiction, and any grid-tied work needs a licensed electrician's sign-off.

Privacy and appropriate use

Your inputs are processed directly in your browser and are not sent to a database. This tool applies NEC 690.9 and 706.30 sizing rules for North American installations — international readers should note that IEC 60364-7-712, AS/NZS 5033, and CSA C22.1 use the same 1.56× factor for PV strings but may stock a different standard fuse size list for your market. This is a planning-stage reference, not a substitute for a licensed electrician or your local code authority.

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FREQUENTLY ASKED QUESTIONS

Questions about the solar fuse size calculator.

Clear answers about the 1.56× and 1.25× multipliers, fuse families, and when to bring in an electrician.

Why multiply Isc by 1.56 for PV string fuses?

NEC 690.9(B) requires PV string overcurrent devices to be rated at least 156% of the module's short-circuit current. That 1.56 factor is two separate 125% de-ratings stacked together: 125% for continuous-duty loading (any circuit running 3+ hours, which a sunny-day array clearly does), and another 125% headroom for the way cloud-edge effects and high-altitude irradiance can push a module's real-world Isc above its STC nameplate value. Battery, inverter, and load-side circuits use a 1.25× factor instead — only the PV source side gets the extra irradiance de-rating.

Do I need a fuse on every string, or just on the combiner?

If two or more strings are paralleled, NEC 690.9(A) requires overcurrent protection on each string unless the module's short-circuit current is already below its own series fuse rating. With three or more strings in parallel, a fault in any one string can be back-fed by the others — that's the specific reason per-string fusing exists in a combiner box. With only one or two strings, the wire's own ampacity typically protects the cable without a dedicated string fuse.

What's the difference between a PV fuse and a regular fuse?

PV fuses (gPV, per IEC 60269-6 / UL 248-19) are rated for DC — typically 1000–1500 V DC — with a time-current characteristic that tolerates a string's steady operating current while still clearing short-circuits quickly. A standard AC household fuse is not rated to safely interrupt a sustained DC arc and must never be substituted.

Can I use a circuit breaker instead of a fuse?

Yes — a DC-rated circuit breaker sized the same way (1.56× Isc on the PV source side, 1.25× continuous current on battery, inverter, and charge-controller circuits) is acceptable under NEC 690.9. It must be rated for the circuit's DC voltage and prospective fault current. Most household AC breakers cannot safely interrupt DC fault current and are not interchangeable.

Do I need a fuse between the battery and the inverter?

Yes. A lithium or lead-acid battery bank can deliver thousands of amps into a short circuit. A Class T (or ANL, for smaller 12–24 V systems) fuse sized at 1.25× the inverter's continuous DC input current, with a DC interrupt rating that exceeds the battery's available short-circuit current, is standard practice — and NEC 706.30(C) requires it mounted within 18 inches of the battery's positive terminal.

gPV vs. Class T vs. ANL — how do I choose the fuse family?

gPV fuses are purpose-built for PV string protection at up to 1000–1500 V DC with low pre-arcing energy — the standard choice inside a combiner box. Class T fuses have very high interrupting capacity (around 20,000 A at 160 V DC) and fast clearing, making them the standard for battery-to-inverter circuits where fault current is enormous. ANL/MIDI/MEGA fuses are slower-blow with a lower interrupt rating — acceptable for DC loads under 32 V with moderate fault current (common in RV and marine setups), but not appropriate for large lithium battery banks. This calculator gives you the amperage; you still choose the family based on system voltage and prospective fault current.

How is fuse sizing different from wire sizing or voltage-drop sizing?

They protect against different things. The fuse protects the wire and equipment from sustained overcurrent and short-circuit faults, sized at 1.25–1.56× continuous current. Wire ampacity and voltage-drop sizing protect system efficiency and prevent overheating, typically targeting 3% or less voltage drop at full load. Standard practice: size the wire first for acceptable voltage drop, size the fuse at the appropriate multiplier, then confirm the fuse rating doesn't exceed the wire's ampacity — if it does, upsize the wire rather than undersizing the fuse.

Why do I always round up, never down?

A fuse sized below the calculated requirement will nuisance-trip under completely normal operating current, which is both a reliability problem and a NEC violation. Always round up to the next standard listed size — 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 per NEC 240.6 and IEC 60269. Note that international markets don't share an identical size list — 16 A, for example, is a standard IEC size but not a standard UL size, so confirm what's actually stocked in your market.

Is this a substitute for a licensed electrician?

For small RV and off-grid systems under 48 V DC, single-string roof installs, and prefab plug-and-play kits, this is a reasonable DIY reference. Anything tying into the utility grid, in conduit through a roof or wall, at 1000+ V DC, with a lithium battery bank above 5 kWh, or in a jurisdiction requiring permits, is licensed-electrician territory — this calculator gives the right fuse size, but final installation must comply with your local AHJ. Always confirm against the equipment manufacturer's maximum overcurrent device rating too; the lower of the NEC calculation and the manufacturer's spec wins.

Is my data stored?

No. All inputs and calculations run directly in your browser and are not sent to a server or saved in an account. General site usage may be measured through Google Analytics as described in our Privacy Policy.