How to size fuses and disconnects in PV combiner boxes

Correct fuse and disconnect sizing in PV combiner boxes protects modules, limits fault energy, and enables safe isolation. You will see clear rules, worked math, and field-ready checks that fit residential, C&I, and DC-coupled storage projects.

Distributed PV keeps growing, pushing more protection tasks to the distribution level. As noted by the IEA, distribution grids now host many small plants, so protection and isolation quality matter. Practical sizing supports uptime and safety without overspend.

Key concepts and standards to anchor your sizing

• Use short-circuit current Isc and open-circuit voltage Voc from the module datasheet. Multiply Isc and Voc with required factors for continuous duty and cold temperature.
• String fuses: apply PV-specific fuses (gPV per IEC 60269-6 or UL 248-19). Choose interrupt ratings and DC voltage ratings that match your array.
• Disconnects/isolators: use DC-rated switches with suitable utilization category (e.g., IEC 60947-3 DC-PV2) or relevant UL listings for PV service, with load-break capability at the array voltage.

Module technology keeps improving. Higher power modules often carry higher string currents, shifting typical fuse ratings from 15 A toward 20–25 A in many arrays. Module efficiency trends from IRENA show mainstream gains (e.g., common P-type to N-type advances), which indirectly increases current in some designs, so re-check legacy fuse assumptions during retrofits.

For solar-plus-storage designers, the reference on real-world performance from Anern: Ultimate Reference – Solar & Storage Performance compiles practical ranges for mainstream PV strings (Isc commonly in the 10–14 A band and Voc per module near 40–50 V for many products). Those ranges align with the sizing workflows below and help validate inputs early in design.

How to size PV combiner box fuses

Core rules you can use

• Array maximum current per string: Istring,max = 1.25 × Isc (continuous factor).
• Overcurrent protective device (OCPD) minimum rating: not less than 125% of Istring,max. That yields a simple rule of thumb: Fuse size ≥ 1.56 × Isc of the string.
• Fuse must be PV-rated (gPV) with DC voltage rating ≥ array maximum open-circuit voltage at the lowest site temperature (Voc,cold).
• Respect the module’s maximum series fuse rating. The selected string fuse must be ≤ the module series fuse rating stated by the manufacturer.
• Reverse-current protection: When strings are paralleled (typically ≥ 3 strings), string fuses protect a faulted string against backfeed from the others.

Cold weather raises Voc. Compute Voc,cold per string: Voc,cold = Voc,STC × . Use the absolute value of the (negative) temperature coefficient of Voc. Then multiply by modules in series.

Device ratings to verify

• Voltage: Fuse voltage rating ≥ Voc,cold of the series string (or array section it protects).
• Interrupt rating: ≥ the maximum available fault current from paralleled strings and upstream sources.
• Thermal derating: Check ambient and enclosure temperature. Fuseholders and fuses derate at higher temps; consult datasheets.

Why the extra 25% factors? PV is a continuous source. Applying 125% for continuous duty and 125% for OCPD ensures conductors and devices run with margin under irradiance peaks. This approach aligns with code practices widely referenced in PV design guides (non-legal advice).

How to size disconnects and isolators in PV combiners

Switch selection checklist

• Current rating: ≥ 1.25 × array maximum current for the circuit it isolates (string, sub-combiner, or main output).
• Voltage rating: ≥ Voc,cold of the circuit.
• Utilization category: DC-PV2 (IEC 60947-3) or appropriate UL listing for PV DC switching. Confirm load-break capability at your voltage and current.
• Poles: Use enough series poles to achieve the DC voltage rating (common for 1000 V or 1500 V arrays).
• Environmental: Match enclosure and switch components to ambient, altitude, and enclosure heat rise.

For foundational background, see U.S. DOE Solar Energy resources on PV basics and protection. Also, IEA on System Integration of Renewables highlights the operational impacts of higher variable renewable shares, reinforcing the need for robust isolation in distributed PV. These themes are consistent with the IEA’s note that distribution systems are evolving to host generation at scale.

Worked example: fuses and disconnect in a 14-string combiner

Assume these module nameplate values and site conditions. The ranges reflect typical mainstream products noted in the Anern performance reference and current efficiency trends reported by IRENA.

• Modules per string: 12
• Module Isc: 13 A
• Module Voc (STC): 49 V
• Temp coefficient of Voc: −0.28%/°C (use absolute value 0.0028/°C)
• Minimum site temperature: −10 °C
• Strings in parallel: 14

Compute Voc,cold per module: 49 V × = 49 × = 49 × 1.098 = 53.8 V. String Voc,cold = 53.8 × 12 = 646 V. Select fuses and disconnects rated ≥ 1000 V DC in this case (or 600/800 V if supported by your exact calculation—here 1000 V keeps clear margin).

String fuse sizing: Minimum fuse ≥ 1.56 × Isc = 1.56 × 13 A = 20.28 A. Choose a standard 25 A gPV fuse, provided the module’s maximum series fuse rating is ≥ 25 A. If the module series fuse rating were 20 A, select 20 A instead and re-check conductor ampacity.

Combiner output OCPD and disconnect: Array maximum current = 14 × 1.25 × Isc = 14 × 1.25 × 13 = 227.5 A. OCPD minimum ≥ 1.25 × 227.5 A = 284.4 A → select a 300 A PV-rated DC fuse or breaker. The main combiner disconnect should have a continuous current rating ≥ 300 A and a DC voltage rating ≥ 1000 V with suitable utilization category. Apply manufacturer derating for enclosure temperature.

Example summary table

Reverse-current check and module protection

Backfeed risk grows with more parallel strings. A simple screen: potential reverse current into a faulted string is approximately (N − 1) × Isc at high irradiance. The module’s maximum series fuse rating (e.g., 20 A or 25 A) indicates the safe reverse-current limit per string. PV string fuses clear faults and limit energy, protecting modules and conductors.

Coordination, derating, and practical tips

• Selective coordination: String fuses should operate for string faults without tripping the main output OCPD. Review time-current curves and I²t of gPV fuses.
• Temperature derating: Hot enclosures raise device temperature. Apply fuseholder and switch derating from datasheets.
• Voltage margins: Choose device voltage ratings with headroom above Voc,cold and any transient uplift.
• Polarity and pole count: Use enough poles in series to reach the DC rating. Maintain clear polarity labels.
• Mechanical: Torque lugs to spec, use touch-safe holders, and strain-relieve conductors to reduce thermal cycling stress.
• Documentation: Record fuse ratings, disconnect ratings, and calculation assumptions in the as-built pack.

Storage-coupled arrays: added notes

In DC-coupled ESS, the combiner still protects PV strings, while separate battery fusing and battery disconnects address ESS currents. Coordinate PV OCPD with the hybrid inverter’s DC input window and maximum input current. As module and storage efficiency improve, BOS losses matter more. The performance ranges summarized by Anern’s performance reference and module efficiency trends from IRENA both support careful device selection to avoid bottlenecks.

Verification and commissioning

• Labeling: Mark fuse sizes, disconnect ratings, and maximum voltage clearly at the combiner.
• Cold-day Voc test: Validate Voc at the lowest seasonal temperature where feasible.
• Thermal scan: After first full-sun operation, perform IR imaging to catch loose lugs or undersized parts.
• Maintenance: Inspect fuseholders and switchgear annually; re-torque per manufacturer instructions.

For broader context on PV deployment scale and impacts, see EIA market data and the IEA Solar Energy Perspectives. Both underline how scale and diversity affect system design and protection.

What this delivers

You can size PV combiner box fuses and disconnects with a transparent method: apply 1.56 × Isc for string fuses, compute Voc,cold for device voltage ratings, and rate disconnects with suitable PV categories and current margins. This reduces nuisance trips, improves safety, and helps approval speed.

Disclaimer

This content is technical education, not legal advice. Always follow local electrical codes, AHJ interpretations, and manufacturer documentation. Non-legal advice.

FAQ

Do I need string fuses if I have only two parallel strings?

Often no, because reverse current is limited. Once you have three or more parallel strings, string fuses are commonly required to protect against backfeed. Always check module series fuse ratings and local code.

Can I use AC-rated fuses or switches on the DC side?

No. Use PV-listed DC devices. DC arcs behave differently. Select gPV fuses (IEC 60269-6 or UL 248-19) and DC-rated disconnects with appropriate utilization categories.

How do I choose the fuse interrupt rating?

Sum the possible fault contributions from parallel strings and any upstream sources. Select a PV fuse with an interrupt rating equal to or greater than that value at the array voltage.

What if my module series fuse rating is 20 A, but my 1.56 × Isc math suggests 22 A?

Select a 20 A fuse and re-check conductor ampacity and performance. The module’s maximum series fuse rating is a hard limit for protection.

Does higher module efficiency change fuse sizing?

Not directly. Sizing still starts with Isc and Voc. But newer high-power modules may have higher Isc, which can push string fuse choices from 15 A to 20–25 A. Always use current module datasheets.