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Views: 100 Author: 深圳市博森威电气有限公司 Publish Time: 2026-08-05 Origin: BSUMWELL
This article covers the following content
This article is aimed at photovoltaic EPC contractors, system integrators, and project procurement personnel, helping you quickly grasp the key points of selecting photovoltaic combiner box enclosures in 20 minutes:
1️⃣ Photovoltaic casing vs. ordinary junction box - Why can't they be used interchangeably?
2️⃣ IP65 protection rating - Why is it a standard feature in most photovoltaic projects?
3️⃣ How to choose between PC/SMC/stainless steel - Advantages, disadvantages, and applicable scenarios of the three main materials
4️⃣ UV protection and heat dissipation - Two "invisible killers" of photovoltaic casings
5️⃣ 1500V system requirements - Creepage distance, UL94 V-0 flame retardancy and arc resistance
6️⃣ Quick Selection Cheat Sheet - Lock in the solution in 3 seconds based on project type
This article was written by the technical team of BSUMWELL. We are a source manufacturer specializing in photovoltaic combiner box shells. Our products cover materials such as UV-resistant PC, SMC fiberglass, and 304/316 stainless steel. All series come standard with IP65 protection rating and we support OEM customization.
If you've ever done procurement for a solar power project, you've probably encountered this situation: the supplier says, "This waterproof junction box is IP65, it can be used with solar power," and the price is indeed cheap. You buy it, install it, and two years later the outer casing cracks, the seal fails, and water gets inside the box.
It's not that the waterproof junction box is bad; it's that it wasn't designed for photovoltaics in the first place.
Photovoltaic combiner box casings and ordinary waterproof junction boxes may look similar, but they are actually two completely different categories. Let's explain why.
Ordinary waterproof junction boxes are designed for "occasional rain" – installed on outdoor walls or under eaves, they may be rained on dozens of times a year, but are in a "resting" state most of the time.
The design scenario for photovoltaic combiner boxes is "24/7 outdoor exposure, continuous power supply, and year-round operation"—the essential differences between ordinary junction boxes and photovoltaic combiner box casings can be summarized in three points: Continuous fever, extreme temperature differences, and accumulated ultraviolet radiation. These three factors combined mean that ordinary waterproof junction boxes simply cannot withstand the impact.
Ordinary ABS shells yellow and become brittle in photovoltaic applications within 6-12 months (industry-standard data based on UL 746C long-term aging test standard). This has nothing to do with the quality of the material itself; it's simply a matter of "using it in the wrong place."
Our photovoltaic series casings are specially designed to address three major challenges: continuous heat generation, large temperature differences, and high-intensity UV radiation. The material formulation and structural design are completely different from ordinary junction boxes.
① Heat
The fuses and busbars inside the junction box continuously generate heat when operating at full load. Since the outer casing is sealed, the heat cannot dissipate, and the internal temperature can be 20-30°C higher than the ambient temperature.
In desert regions, where the ambient temperature is 45°C, the temperature inside the enclosure can reach 65-75°C. If the heat deflection temperature (HDT) of the outer shell material is not high enough, the enclosure will slowly deform and creep under long-term high temperatures, the sealing surface will no longer be smooth, and the protection level will naturally be compromised.
② Temperature difference
In desert regions, the temperature difference between day and night can reach over 40°C. The sealing strips and the enclosure structure are subjected to this "fatigue test" every day due to the thermal expansion during the day and the contraction at night.
With repeated deformation day after day and year after year, the sealing strip will lose its elasticity, and tiny cracks may appear in the box structure.
③ UV
After 25 years of direct outdoor exposure, the damage from ultraviolet rays to plastic is cumulative. UV radiation breaks down the molecular chains of polymer materials, making the material brittle, causing surface powdering, and eventually cracking.
Ordinary plastics cannot withstand the conditions of photovoltaic applications. UV resistance is not something that can be solved by simply "adding some additives"; the base resin itself must possess UV resistance capabilities.
Our photovoltaic series casings are made of UV-resistant PC or SMC fiberglass with a heat distortion temperature (HDT) ≥120°C (based on ISO 75 standard), ensuring structural stability under extreme temperature differences and continuous heat generation scenarios.
The most common protection rating for photovoltaic combiner box enclosures is IP65. This is sufficient to meet the outdoor environmental requirements of the vast majority of photovoltaic projects and is one of the most frequently appearing technical specifications in domestic photovoltaic project bidding documents.
Below, we will explain the true meaning of IP65 in photovoltaic scenarios.
Protection level | Test conditions | Photovoltaic Scenarios Interpretation | Applicable Scenarios |
IP65 | Anti-low pressure water jet (6.3mm nozzle, water flow rate 12.5L/min) | Capable of handling general rain and everyday outdoor conditions | Standard configuration for most photovoltaic projects |
The actual capabilities of IP65 in photovoltaic scenarios:
Dust-proof: Completely dustproof, sand and dust can't get in—no problem for the Gobi Desert project.
Water proof: It can withstand heavy rain, but the container cannot be directly washed with a high-pressure water gun.
Not waterproof: It cannot be submerged in water, and the installation location must take drainage into account.
Simply put: IP65 means protection against heavy rain and sandstorms, but not against water immersion, and it cannot be sprayed with a high-pressure water gun.
Our photovoltaic series all come standard with IP65 protection rating for their casings. For most ground-mounted power plants, commercial and industrial rooftop projects, and agro-solar hybrid projects, IP65 is a proven and reliable standard. View the specifications of photovoltaic series casing products.
IP65 covers the outdoor environment requirements of the vast majority of photovoltaic projects:
Ground-mounted centralized power station (Gobi Desert) ✅ IP65 is sufficient. Rainfall is scarce in the Gobi Desert; the real challenges are high temperatures and sandstorms—IP65's dustproof rating (completely dustproof) and waterproof rating (rainproof) are perfectly adequate.
Commercial and industrial rooftops ✅ IP65 is standard. Even with an unprotected roof and direct, torrential rain, it won't get wet—IP65 is perfectly adequate.
Solar-agricultural integration (animal husbandry/planting) ✅ IP65 + ammonia corrosion resistant material. IP65 protection is sufficient; the key is in the selection of materials.
Residential/Small-scale solar power ✅ IP65 is the mainstream choice.
Limitations of IP65 – In what scenarios is a higher level required?
Coastal mudflats/water surface photovoltaics: If the installation location may be temporarily submerged in seawater or rainwater, IP65 is insufficient; IP66 or a higher rating should be considered.
Low-lying areas prone to water accumulation: After heavy rain, the enclosure may be submerged by floodwater, which IP65 rating cannot withstand.
For scenarios requiring higher protection levels, we can provide IP66/IP67 solutions based on customer needs. However, it should be noted that our standard product line mainly focuses on IP65, and whether higher levels can be supported should be confirmed upon consultation.
IP rating is the result of testing "at the time of manufacture", not the status "25 years later".
Even the best IP65 enclosure will experience a significant drop in waterproofing after a few years as the sealing strips age, harden, and lose elasticity. Sealing strips will gradually harden and crack under UV radiation and temperature cycling – a physical law that no material can avoid.
A truth that the industry generally avoids: the lifespan of sealing strips is usually only 5-10 years (based on industry aging test data of EPDM/silicone materials under UV and temperature cycling), while the design life of photovoltaic projects is 25 years.
This means that the sealing strips will most likely need to be replaced once or multiple times during the project's lifecycle.
Therefore, when selecting a model, you should not only look at the protection level, but also whether the protection level can be "restored" - can the sealing system be replaced separately?
Our photovoltaic series housings feature a replaceable sealing strip design. During maintenance, the entire enclosure does not need to be replaced; only the sealing strips need to be replaced to restore the protection level. Sealing strip replacement kits can be purchased separately.
Plastic or metal? SMC or PC? These are the most frequently asked questions in photovoltaic EPC projects.
Follow these four steps to evaluate each option and find the material that best suits your project:
Step 1: Is the project located on coastal mudflats or saline-alkali land?
✅ Yes → 316 stainless steel (Corrosion prevention is the top priority; 316 is more resistant to chloride ion corrosion than 304.)
❌ No → Proceed to Step 2
Step 2: Is the project a combination of agriculture and solar power (breeding/planting, with ammonia corrosion)?
✅ Yes → 316 stainless steelorSpecial coating SMC
❌ No → Proceed to Step 3
Step 3: Is the project a ground-mounted centralized power station (requiring hail resistance IK10)?
✅ Yes → SMC fiberglassorStainless steel
❌ No → Proceed to Step 4
Step 4: Is the project an industrial or commercial rooftop (requiring lightweighting)?
✅ Yes → UV-resistant PC
❌ No → Standard PC/ABSorSMCEither is acceptable (depending on budget).
Our photovoltaic series casings cover three major material series: UV-resistant PC, SMC fiberglass, and 304/316 stainless steel, with all series equipped with IP65 protection rating as standard. Whether your project is in the Gobi Desert, on a rooftop, or by the sea, there are corresponding material options.
Advantage：
Lightweight: Due to roof load-bearing limitations, the PC casing is more than 60% lighter than stainless steel.
Good insulation: No additional grounding is required, saving labor and materials.
Cost controllable: The initial procurement cost is the lowest among the three materials.
Transparent window optional: A transparent window can be opened directly, allowing maintenance personnel to conduct inspections without opening the box.
Shortcoming：
Impact resistance is not as good as SMC
Long-term UV exposure requires a special formula; ordinary PC cannot withstand it.
The difference between UV-resistant PC and ordinary PC is obvious: Ordinary PC begins to yellow and powder on the surface after 3-5 years outdoors; UV-resistant PC can extend its lifespan to 10-15 years (based on industry-standard data from the ASTM G154 standard QUV test).
Material identification techniques: UV-resistant materials are typically slightly milky white, while regular materials are more transparent — this is a difference visible to the naked eye. When purchasing, you can ask the supplier, "Are you sure this is a UV-resistant material?"
Our PC series photovoltaic casings are made of UV-resistant polycarbonate with an IP65 protection rating. They have passed long-term weather resistance verification, ensuring that they will not yellow or become brittle for more than 15 years under direct outdoor sunlight.
SMC (sheet molding compound) fiberglass is the "standard answer" for ground-mounted centralized power plants.
Advantage：
High strength: Impact resistance far exceeds that of PC
Excellent weather resistance: 25 years maintenance-free, virtually unaffected by UV radiation.
Good flame retardant properties: It can reach UL94 V-0 and produces no dripping during combustion.
Good insulation: No additional grounding required.
Shortcoming：
Higher cost than PC
Limited color options (usually gray/white)
Heavier than PC, but lighter than stainless steel
Why is SMC the preferred choice for ground-mounted power plants?
Hailstorms are frequent in the Gobi and desert regions. Hailstones with a diameter of 25mm or more fall at speeds of 20+ m/s, which can directly penetrate ordinary PC. SMC fiberglass has an impact resistance rating of IK10, which can withstand the impact (based on the definition of IK10=20J impact energy in the IEC 62262 standard).
Our SMC fiberglass photovoltaic housing series boasts an IP65 protection rating and an IK10 impact resistance, capable of withstanding hailstones up to 25mm in diameter impacting at a speed of 23m/s. It is specifically designed for ground-mounted centralized power plants.
Near coastal mudflats, floating photovoltaic facilities, and chemical industrial parks, stainless steel is not an "option," but a "must."
304 stainless steel: Suitable for general coastal environments (more than 2 kilometers from the coastline).
316 stainless steel: Suitable for environments with heavy salt spray (within 2 kilometers of the coastline, floating photovoltaic projects, and island projects).
The core advantage of stainless steel is that it requires no maintenance throughout its entire lifecycle. — No painting required, no replacement needed, no worries about corrosion. Although the initial purchase cost is the highest, it may be the most cost-effective over a 25-year period.
An easily overlooked detail: Fasteners must also be made of stainless steel. In many projects, the boxes are made of 304 stainless steel, and the screws are galvanized—five years later, the boxes are intact, but the screws rust and break, and the equipment falls off the wall.
We offer two material options: 304 and 316 stainless steel. All models come standard with IP65 protection and can provide material certification (MTC) according to project requirements. For coastal mudflat and water-based photovoltaic projects, we recommend prioritizing the use of 316 stainless steel. Learn more about 304/316 stainless steel photovoltaic housings
Most competitor articles only discuss protection ratings and materials, but almost never address the two core challenges of photovoltaic applications: "UV resistance" and "heat dissipation." These two aspects are crucial in determining whether a photovoltaic casing can last for 25 years.
The damage caused by ultraviolet radiation to plastics is cumulative. UV radiation breaks down the molecular chains of polymer materials, causing the material to slowly degrade from the inside out—first the surface turns yellow, then it becomes brittle, then it powders, and finally it cracks.
The UV resistance of different materials varies greatly:
Ordinary ABS: ABS begins to yellow and powder after 6-12 months of direct outdoor exposure (industry-standard data based on UL 746C long-term aging test standard). Using ordinary ABS for photovoltaic projects is practically "obsolete upon deployment."
UV-resistant PC/ABS: It can extend the aging time to 10-15 years (based on industry-standard data from the ASTM G154 standard QUV test). Suitable for industrial and commercial rooftop projects.
SMC fiberglass: 25+ years, virtually unaffected by UV radiation. Preferred choice for ground-mounted power plants.
An industry truth: Many low-priced combiner boxes use ordinary ABS composite materials for their casings. This isn't noticeable during project acceptance, but after 2-3 years of operation, the casing cracks and the protection level fails. The cost savings will ultimately be repaid at a much higher price.
Our photovoltaic series casings are strictly differentiated by material grade—both the UV-resistant PC series and the SMC fiberglass series have passed long-term weather resistance verification, and all series come standard with IP65 protection rating, ensuring structural integrity within a 25-year design life.
The 1500V system presents an additional challenge to the combiner box housing: heat.
The 1500V system has a dense array of fuses inside, and when operating at full load, the temperature inside the enclosure can be 20-30°C higher than the ambient temperature. In desert areas where the ambient temperature is 45°C, the temperature inside the enclosure can reach 65-75°C.
Heat Deformation Temperature (HDT): It is an indicator that measures a material's resistance to deformation under heat (based on ASTM D648 or ISO 75 standards). Simply put: will the box soften or deform at high temperatures?
The HDT of PC/ABS is typically 100-110°C.
SMC fiberglass can achieve a HDT of over 200°C.
High temperatures not only cause the outer casing to deform, but also accelerate the aging of the sealing strips. Heat plus UV radiation is a double whammy, further shortening the lifespan of the sealing strips under these conditions.
Our SMC fiberglass series housings have an HDT ≥ 200°C, maintaining structural stability even in high-temperature desert regions; the PC series has an HDT ≥ 110°C, suitable for regions with normal climates. All series come standard with an IP65 protection rating. If you are working on a project in a desert region, SMC is almost the only option.
Question: How is condensation formed?
Condensation is the most insidious "chronic disease" of photovoltaic combiner boxes. During the day, the sun's rays cause the air inside the box to heat up and expand; at night, the temperature drops sharply, the air contracts, and a negative pressure is created inside the box—drawing in external moisture through tiny gaps in the sealing strips. This cycle repeats itself, causing moisture to condense into water droplets on the inner walls of the box and the surfaces of components, slowly corroding the terminals, leading to short circuits, and ultimately burning out the equipment.
Solution: Waterproof and breathable valve
The function of a waterproof venting valve (Breathing Valve / Vent Plug) is: Breathable but waterproof, it balances the pressure difference between the inside and outside, allowing internal moisture to escape.
Waterproof and breathable valves should be suitable for areas with large temperature differences between day and night (deserts, plateaus, coastal areas). Standard configuration It is not an "optional" option.
Our photovoltaic series casings come standard with IP65 protection rating + waterproof and breathable valve, effectively preventing condensation buildup. For regions with large temperature differences between day and night, this is a key design feature for extending the lifespan of the junction box.
Photovoltaic systems are transitioning entirely from 1000V to 1500V. With the increased voltage, the requirements for the casing have also changed.
The first change brought about by the 1500V system is that the enclosure requires more internal space.
Creepage Distance - It is the shortest distance measured along the insulating surface. Electrical clearance - It is the shortest distance measured through the air. Both distances must be large enough to prevent high-voltage breakdown.
The creepage distance requirement for a 1500V system (calculated according to pollution level 2, material group IIIa, based on IEC 60664-1 standard) is approximately ≥20mm, which is much greater than the 12mm requirement for a 1000V system.
This means that the 1500V combiner box enclosure needs a deeper cavity and a larger internal space. If the enclosure size is insufficient and the creepage distance of the components is inadequate, the acceptance batch will not pass inspection.
Our 1500V series photovoltaic housing features a deep cavity design with ample internal dimensions to allow for creepage distances and electrical clearances. All models come standard with IP65 protection, meeting the safety requirements of 1500V systems.
UL94 V-0 is one of the highest levels of vertical burning test: the sample extinguishes itself within 10 seconds of ignition and there are no burning droplets (based on UL 94 standard).
Why must photovoltaic combiner boxes be made of V-0 material?
The arcing energy of a 1500V DC system is far higher than that of an AC system. Once an internal short circuit occurs, the resulting arc temperature can reach several thousand degrees Celsius. If the flame retardant rating of the casing material is insufficient, the arc will directly ignite the casing, and the fire will spread throughout the entire power station.
Procurement Reminder: Don't just look at the words "flame retardant"; make sure you ask whether it's V-0 or V-2. V-2 rating allows for the combustion of dripping materials—in a photovoltaic scenario, dripping materials could ignite equipment below, which is equally dangerous.
Our PC and SMC series photovoltaic housings are all UL94 V-0 flame retardant certified and all models come standard with IP65 protection. Please contact customer service for certification certificates. Contact customer service to obtain the UL94 V-0 certification certificate.
In addition to being flame retardant, the outer casing also needs to have good dielectric strength—that is, "breakdown resistance".
The energy of a high-voltage electric arc is sufficient to burn through a regular plastic casing. The casing material needs to have sufficient dielectric strength to withstand the impact of the arc, allowing time for the internal protective devices (fuses, circuit breakers) to activate.
The difference between SMC fiberglass and metal casings is obvious here:
SMC fiberglass: It is an excellent insulator by itself, requiring no additional grounding treatment, and can effectively block electric arcs.
Metal casing: It is conductive and must be reliably grounded; otherwise, the casing may become live, endangering the safety of maintenance personnel.
Our SMC fiberglass series enclosures require no additional grounding, as they are excellent insulators themselves and possess good arc resistance. All models come standard with IP65 protection.
How do I use this table?
1️⃣ Confirm your Project Type (Ground/Roof/Coastal/Agricultural-solar hybrid, etc.)
2️⃣ Find the corresponding table - Recommended materials
3️⃣ Check Special requirements - Are there any additional notes in this column?
4️⃣ For NEMA rating or special customization, please contact customer service to confirm the replacement plan.
Project Type | Recommended materials | Protection level | Recommended IK level | Special requirements |
Ground-based centralized (Gobi/desert) | SMC fiberglass | IP65 | IK10 | UV resistant, high HDT (≥200°C) |
Ground-based centralized (coastal) | 316 stainless steel | IP65 | IK08 | Salt spray resistant, all stainless steel fasteners |
Commercial and industrial rooftops | UV-resistant PC | IP65 | IK08 | Lightweight and UV resistant |
Agricultural-solar complementary | 316 stainless steel/SMC | IP65 | IK08 | Ammonia corrosion prevention |
Water surface photovoltaic | 316 stainless steel | IP65 | IK08 | Salt spray resistant, waterproof, and corrosion resistant coating |
Residential/Small | PC/ABS | IP65 | IK07 | Compact design, transparent window optional |
All models come standard with IP65 protection rating - The above selection recommendations are mainly based on differences in materials and impact resistance levels.
For higher protection levels (IP66/IP67) or project-specific selection support, please contact our technical team for professional advice. OEM customization is also available for special scenarios not covered in the table.
Pitfall 1: The inland project selected regular ABS.
They assumed "ABS is fine for indoor use," but it cracked after 2-3 years of UV exposure. In photovoltaic applications, both inland and coastal areas require UV-resistant treatment; the only difference lies in the level of salt spray corrosion protection.
Pitfall 2: Thinking that IP65 can prevent water immersion
IP65 is rainproof but not submersible. If the installation location is in a low-lying area that may be flooded, it is necessary to upgrade to a higher protection level or raise the installation location.
Pitfall 3: Ignoring the materials of fasteners and sealing components
Even if you choose the right casing, the screws are galvanized, and the seals are ordinary rubber—after five years, the casing may be intact, but the screws will rust and break, the seals will age, and the entire device will fail. Always confirm the material specifications of all components when purchasing.
Our products come with a complete set of stainless steel fasteners and replaceable sealing strips, and all models are equipped with an IP65 protection rating as standard, avoiding the problem of "the outer casing not breaking, but the accessories failing first." Please confirm the specifications and materials of all accessories when purchasing.
(Next article preview: Finished the technical selection of the photovoltaic combiner box enclosure? Next, to learn about installation details, 25-year cost comparison and OEM customization solutions, please read "Photovoltaic Project Combiner Box Enclosure Procurement and Installation Guide - A Must-Read for EPC Contractors".)
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