10×38 mm vs 14×51 mm PV Fuses: gPV Ratings, String Protection and Selection
PV string protection is a difficult fuse application for a reason that is easy to underestimate: the circuit can be at very high DC voltage while the available fault current is only modestly higher than the array’s normal operating current. A utility-fed AC short circuit may deliver many multiples of normal load current and gives the fuse a natural current zero every half-cycle. A photovoltaic source does neither.
That is why a PV fuse is not merely an ordinary fuse with a higher voltage number printed on the label. gPV fuses are designed and tested for photovoltaic duty, including the low-overcurrent conditions that can arise when healthy parallel strings feed a faulted string. IEC 60269-6 covers PV fuse links for strings and arrays up to 1500 Vdc, while UL 248-19 is the active North American PV fuse standard and was revised in 2024 to extend its scope to PV systems up to 2000 Vdc.
The familiar 10×38 mm and 14×51 mm cylindrical bodies are two common ways of packaging that protection, but the size alone does not tell you the voltage, ampere range or breaking capacity. Those are product-series properties. The correct selection starts with the module and system data, then works toward the body size and holder—not the other way around.
Why DC PV interruption is different from ordinary AC fusing
When a fuse element melts, an arc forms between the separating element ends. On AC, current naturally passes through zero every half-cycle. That zero crossing helps the arc extinguish because the source stops feeding it momentarily and the gap has an opportunity to deionize.
DC does not provide that assistance. The fuse must create enough arc voltage, cooling and arc length to force the current to zero while the source voltage remains present.
PV makes the job harder because the source is current-limited. A PV string cannot supply the very high multiples of rated current that a strong utility source can produce. The fuse must therefore be capable of operating reliably at comparatively low fault-current multiples while still withstanding the full DC recovery voltage. Eaton describes its PV fuse families as specifically designed to provide fast protection under the low-fault-current conditions associated with PV systems.
This is the main reason a general AC fuse should not be substituted for a gPV fuse on a PV string. Even a general-purpose DC-rated fuse is not automatically equivalent to a PV fuse; the application needs the PV utilization category or the applicable PV listing specified by the equipment design.
The same distinction applies on the inverter side. UL 248-19 explicitly states that PV fuses covered by the standard are intended for strings, arrays and associated wiring and are not intended to protect downstream inverter components such as capacitors. Semiconductor-protection fuses used inside or directly around power electronics are a different protection problem and may use aR, gR or other high-speed categories rather than gPV.
What 10×38 mm and 14×51 mm actually tell you
The dimensions describe the cylindrical body: diameter by length. They determine the holder family and strongly influence how much internal arc path and thermal volume are available, but they are not a universal voltage code.
A current Eaton example makes the point clearly:
Those figures immediately correct a common assumption that 14×51 mm is automatically a 1500 V body. In Eaton’s present cylindrical PV range, the 14×51 PV-A14F family is a 1000/1100 Vdc product. Eaton moves to longer cylindrical bodies—such as 14×65 mm, 10×85 mm and 22×65 mm—for many of its 1300/1500 Vdc applications.
Other manufacturers may offer different ratings, so the conclusion is not “14×51 can never be 1500 V.” The conclusion is that body size does not guarantee voltage. Read the voltage and current from the exact product datasheet.
The holder follows the same rule. A 10×38 fuse belongs in a compatible 10×38 PV holder; a 14×51 fuse belongs in a compatible 14×51 holder. A holder with a higher voltage rating does not raise the voltage rating of the fuse installed inside it, and a high-voltage fuse does not make an under-rated holder suitable for the system. The assembly is limited by its lowest-rated component.
In compact combiner-box design, the 10×38 format is attractive because it uses less panel space and is widely available for lower string currents. The 14×51 format provides more physical volume and generally supports a higher current range in comparable product families, but it costs more space. The decision should come from the required electrical rating and the manufacturer’s product family—not from a blanket rule that residential equals 10×38 and commercial equals 14×51.
gPV string protection and the real source of reverse current
The key fault that makes string fusing necessary is usually backfeed from parallel strings, not the faulted string’s own short-circuit current. When multiple strings are connected in parallel, a fault in one string can create a path for current from the other healthy strings into the faulted branch. The purpose of the string fuse is to interrupt that reverse-current contribution before it exceeds the conductor or module’s permitted series-fuse level.
This means the number of parallel strings, the module’s maximum series fuse rating and the array architecture all matter. In some small arrays, the maximum possible reverse current is below the module and conductor limits, so individual string fuses may not be required. In larger parallel arrays, string overcurrent protection becomes necessary.
There is no general “two-diode rule” that requires a blocking diode in every pair of parallel strings. Modern PV design commonly relies on properly selected string overcurrent protection and equipment designed for reverse-current conditions. Blocking diodes can be used in particular architectures, but they are not a universal substitute for correct gPV fuse selection and they should not be introduced as a mandatory combiner-box rule without the equipment design calling for them.
Likewise, the fuse itself does not provide “reverse blocking” in the semiconductor sense. A fuse is bidirectional as a current path until it operates. Its job is to open when the fault current reaches the operating region defined by the fuse characteristic. If the system needs one-way current blocking, that is a separate circuit function.
At the combiner output or array level, the current is the aggregate of multiple strings, so the overcurrent device is sized for a different duty than the individual string fuses. Larger cylindrical bodies or bolted/bladed PV fuses are common at those higher-current points.
Sizing a PV string fuse
A practical PV string-fuse selection starts with the module datasheet. Two values are especially important: short-circuit current Isc and maximum series fuse rating (sometimes expressed as maximum overcurrent-protection rating). The final fuse must be large enough to carry the maximum expected operating current under the applicable code and temperature conditions, but it must not exceed the module manufacturer’s permitted series-fuse value.
Manufacturer application guides commonly use a starting calculation around 1.56 × Isc for North American PV source-circuit fusing. Littelfuse, for example, publishes In = 1.56 × Isc for a PV source circuit before temperature rerating is applied. Mersen publishes the same 1.56 × Isc relationship in its NEC-oriented PV sizing guidance.
That is a useful engineering starting point, but it should not be turned into a universal one-line rule divorced from the current electrical code, module listing and equipment instructions. The selected rating must still fall at or below the module’s maximum series fuse rating, and the conductor and holder must be suitable for the resulting current.
Temperature is then applied using the fuse manufacturer’s published rerating data. PV combiner boxes can operate at high ambient temperatures, and the fuse’s current-carrying capability changes with temperature and with the thermal environment created by the holder. Eaton publishes temperature-derating curves for its PV-A10F family, and Littelfuse’s sizing guide explicitly adds the manufacturer temperature derating to the current calculation.
The correct method is therefore not to apply a generic “0.75 NEC factor.” The thermal correction comes from the relevant code rules and the manufacturer’s rerating curve for the installed fuse/holder combination. Treating a fixed 0.75 multiplier as a universal PV-fuse rule can oversize or undersize the device depending on the actual product and installation.
System voltage must also be checked at the worst-case cold condition. PV open-circuit voltage rises as module temperature falls, so the fuse and holder voltage ratings must exceed the maximum calculated DC circuit voltage, not merely the nominal operating voltage shown on the inverter display.
Finally, check the minimum operating region of the selected fuse against the fault mechanism. gPV fuses are specifically designed to operate at low overcurrent multiples associated with reverse-current faults. Oversizing the fuse can move the expected backfeed current below the useful operating region and defeat the purpose of string protection even though the fuse has a very high maximum interrupting rating.
Choosing between 10×38 and 14×51
The body-size decision becomes straightforward once the electrical requirements are known.
Choose a 10×38 product when the required current and DC voltage fit a suitable 10×38 gPV series and the smaller holder is useful to the combiner or equipment layout. Eaton’s PV-A10F, for example, provides 1000 Vdc gPV protection from 1 to 20 A with a 50 kA interrupting rating and is matched to the CHPV modular holder.
Move to 14×51 when the exact product family provides the required current or other performance that the smaller family does not. Eaton’s PV-A14F covers 15 to 32 A, with 15 and 20 A parts rated 1100 Vdc and 25 and 32 A parts rated 1000 Vdc. The family uses the CHPV14 holder.
If the system is 1500 Vdc, do not assume that stepping from 10×38 to 14×51 is enough. In Eaton’s cylindrical range, 1500 Vdc drives the selection into longer formats such as 14×65, 10×85 or 22×65 depending on current and product family. This is a good example of why the body-size table should be read from the current manufacturer catalog rather than copied from a generic “PV fuse sizes” chart.
The holder is part of the decision as well. Verify its DC voltage, current rating, accepted fuse size, conductor range and applicable PV fuseholder listing. In North America, UL 4248-19 covers photovoltaic fuseholders. A fuse and holder that are individually well rated but not compatible as an assembly are not a valid design.
Installation, replacement and common errors
The most common replacement mistake is treating “same amps” as sufficient. A 20 A 10×38 gPV fuse and a 20 A general-purpose 10×38 fuse share dimensions and current rating but can have very different voltage, interrupting and low-overcurrent performance. The replacement must preserve the PV category/listing and the required DC voltage.
The second common mistake is confusing holder voltage with fuse voltage. A 1500 Vdc holder does not make a 1000 Vdc or 1100 Vdc fuse suitable for a 1500 V circuit. The same is true in reverse: a 1500 Vdc fuse still needs a holder and equipment system rated for that voltage.
The third is using the wrong body assumption. 14×51 is physically larger than 10×38, but that does not automatically mean 1500 Vdc. Check the exact series.
The fourth is ignoring temperature and holder heating. A fuse operating near its catalog current in a hot combiner can run substantially hotter than expected. Use the manufacturer’s temperature-rerating guidance and the holder’s power-dissipation limits.
The fifth is forgetting that PV fuses protect against parallel-string backfeed. Adding strings, changing module type or adding another current source can change the maximum reverse current available to a faulted string. A system modification should therefore trigger a new string-fuse check.
Official product references
For exact product appearance, use the manufacturer’s real product media rather than generated images:
- Eaton Bussmann PV-15A10F, 10×38 mm official product page: https://www.eaton.com/us/en-us/skuPage.PV-15A10F.html
- Eaton Bussmann PV-15A14F, 14×51 mm official product page: https://www.eaton.com/us/en-us/skuPage.PV-15A14F.html
- Eaton Bussmann photovoltaic fuse family: https://www.eaton.com/us/en-us/catalog/fuses-and-fuse-holders/bussmann-series-photovoltaic-fuses.html
No AI-generated product photo is used in this article.
Frequently asked questions
Is 14×51 always rated higher than 10×38?
Not automatically. It is physically larger, but voltage, current and breaking capacity are properties of the product series. In Eaton’s current PV range, PV-A10F is 1000 Vdc and 1–20 A, while PV-A14F is 1000/1100 Vdc and 15–32 A.
Is 14×51 a 1500 Vdc PV fuse size?
Not as a general rule. Some manufacturers may offer particular high-voltage products, but Eaton’s PV-A14F 14×51 family is 1000/1100 Vdc. Eaton uses longer cylindrical bodies such as 14×65, 10×85 and 22×65 for many 1500 Vdc applications.
Why can’t I replace a gPV fuse with an AC fuse of the same size?
Because PV fault duty is DC, high-voltage and comparatively low-current. gPV products are designed and tested for those conditions, including the low-overcurrent reverse-current faults found in parallel PV strings.
How do I calculate a starting fuse size from module Isc?
Manufacturer NEC-oriented application guides commonly use about 1.56 × Isc as the starting current calculation for a PV source-circuit fuse, followed by any required temperature rerating and standard-size selection. The result must not exceed the module’s maximum series fuse rating.
Do parallel PV strings always need blocking diodes?
No. Blocking diodes are application-specific. Modern PV systems commonly use gPV string overcurrent protection where reverse current from parallel strings can exceed the permitted conductor or module value.
References and FuseStock pages
- gPV fuses
- PV string fuses
- PV array fuses
- Solar DC fuses
- gPV class
- 10×38 mm PV block
- 14×51 mm PV holder
- Eaton Bussmann photovoltaic application guide: https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/bussmann-iec-high-speed-semi-conductors-fuses/bussmann-iec-photovoltaic-high-speed-fuses/eaton-bussmann-series-photovoltaic-fuses-catalogue-ca135004en.pdf
- UL 248-19: https://www.shopulstandards.com/ProductDetail.aspx?productId=UL248-19
- IEC 60269-6: https://webstore.iec.ch/en/publication/68843
- Littelfuse PV fuse sizing guide: https://www.littelfuse.com/assetdocs/photovoltaic-fuse-sizing-application-guide?assetguid=1f1616ee-aada-4a68-b4fc-8ba2363bcad2