The short answer
- Polycarbonate: the impact-resistant one. Easy to cut, available clear, but attacked by alkalis, ammonia and many solvents, and it needs a UV-stabilized grade outdoors.
- Fiberglass (glass-fiber-reinforced polyester, FRP): the stiff one, about five times stiffer than polycarbonate, with a higher temperature range and broad chemical resistance. Harder on tools.
- For outdoor use, ask for the UL 746C f1 rating of the material. For fire behaviour, ask for the UL 94 class and the thickness it applies to.
- Both pass Wi-Fi and cellular signals and neither shields EMI, unless coated.
Two different plastics
Polycarbonate is a thermoplastic with very high impact strength, available in opaque, translucent and clear grades. Fiberglass enclosures are a composite: polyester resin reinforced with glass fibers, typically molded from sheet or bulk molding compound (SMC or BMC). The glass makes FRP stiff; polycarbonate is tough and easy to machine. Both are molded, so a custom non-metallic enclosure starts from a standard molded size with the cutouts machined to your layout. For how these two compare with stainless, steel and aluminum, see enclosure materials compared.
Properties side by side
| Property | Polycarbonate | Fiberglass (polyester SMC) |
|---|---|---|
| Density | 1.20 g/cm³ | 1.65–1.90 g/cm³ |
| Tensile modulus | 2.4 GPa | 11–12.7 GPa |
| Thermal conductivity | 0.2 W/m·K | 0.30–0.36 W/m·K |
| Thermal expansion | 65 × 10⁻⁶ /K | not compared (sources inconsistent) |
| Relative thermal index (UL 746B) | 115–125 °C | 130 °C (one grade) |
| Operating range, enclosure makers | −50 to +120 °C (one maker); −40 to +80 °C long term (another) | typically −50 to +150 °C (one maker) |
| UL 94 flame class | HB to V-0, depending on grade and thickness | V-0 at 1.5–3.0 mm; 5VA at 3.0 mm for one grade |
| Light transmission | about 88% at 3 mm (clear grades) | opaque |
Values come from resin and compound datasheets and enclosure makers' technical sheets listed below; they vary by grade, so use the figures for the enclosure you are buying. The relative thermal index is a long-term aging rating, not a maximum operating temperature.
Impact, stiffness and cold
Polycarbonate's strength is toughness. A UV-stabilized grade's datasheet gives a puncture energy of 60 J at 23 °C and 65 J at −30 °C, so it keeps absorbing blows in the cold. Its notched impact strength, though, falls from 70 kJ/m² at 23 °C to 15 kJ/m² at −30 °C, so notches, scratches and sharp corners matter more in the cold. Enclosure makers' published impact ratings show the same pattern; one polycarbonate line is rated IK10 at +35 °C and IK09 at −25 °C under IEC 62262.
Fiberglass trades some of that toughness for stiffness. At 11–12.7 GPa it is about five times stiffer than polycarbonate, so large doors and walls flex less under load. Impact ratings for FRP enclosures are product-specific; one polyester line we found is rated IK08. Ask for the IK code of the actual enclosure.
For outdoor equipment, UL 746C (1995 edition) includes a cold impact test in which specimens are cooled to −35 °C for 3 hours and struck while still cold. Check the edition your certifier applies.
Sunlight, UV and the f1 rating
UL 746C rates polymeric materials for outdoor use. A material with an f1 rating is suitable for outdoor use with respect to ultraviolet light, water exposure and immersion; f2 means it was tested only partially or passed only to a restricted extent. In the 1995 edition of UL 746C, outdoor UV conditioning is 720 hours of carbon-arc or 1,000 hours of xenon-arc exposure, after which the flame class must not drop and key properties must keep at least 70% of their original value.
- Polycarbonate needs UV stabilization outdoors. One enclosure maker notes that clear polycarbonate exposed to UV will, over some years, turn yellowish and lose impact resistance; pigmented and UV-stabilized grades hold up better.
- Fiberglass surfaces degrade slowly under UV until glass fibers appear at the surface ("fiber bloom"). Makers disagree on how much it matters: one says bloom does not affect structural integrity or the NEMA rating, a composites fabricator says it can cause mechanical damage. UV-inhibited resin formulations slow it.
For either material outdoors, ask for an f1-rated material and allow for solar heat load, covered in outdoor electrical enclosures.
Chemical resistance
This is where the two differ most. The polycarbonate producer's chemical-resistance guide (six days at 23 °C) reports:
- Not resistant: alkalis even at 1% (sodium and potassium hydroxide), ammonia at 0.1%, amines and methanol. Alkalis catalyze hydrolysis, and ammonia and amines are "particularly aggressive in the presence of traces of water".
- Swells or dissolves: acetone, MEK, benzene, xylene, ethyl acetate and chlorinated solvents.
- Resistant: 10% acetic, hydrochloric, sulfuric and nitric acids, 10% citric acid, hexane, silicone and paraffin oils, and most industrial oils and greases (but not petrol or diesel).
Polycarbonate can also stress-crack where tensile stress and a chemical meet, for example plasticized PVC cable insulation pressed against a molded part. The producer notes that low-stress parts in suitable grades can withstand frequent cleaning with 2% caustic at 80 °C with short contact, so washdown is not ruled out, but caustic and ammonia-based cleaners are the main risk.
Fiberglass-reinforced polyester is used in petrochemical, water treatment and marine settings and in the presence of organic solvents, alcohols and moderately strong acids and alkalis. For strong alkalis such as concentrated caustic, enclosure makers' own resistance tables disagree, and one maker's document contradicts itself. If your plant uses a specific cleaner at a specific concentration, name it in your quote request and check it against the material data, or consider 316L stainless.
Flame ratings: UL 94 is a material property
UL 94 classifies how a plastic specimen burns in a small-flame test. In the vertical test the flame is applied twice for 10 seconds:
| Class | Afterflame, each specimen | Total, 10 applications | Flaming drips ignite cotton |
|---|---|---|---|
| V-0 | ≤ 10 s | ≤ 50 s | No |
| V-1 | ≤ 30 s | ≤ 250 s | No |
| V-2 | ≤ 30 s | ≤ 250 s | Yes |
HB is the horizontal test: the material burns slower than a set rate (40 mm/min for 3–13 mm specimens, 75 mm/min under 3 mm). The 5V classes use a flame about five times more severe than the V and HB tests, with no flaming drips allowed; 5VA plaques must not burn through, while 5VB plaques may form a hole.
Three points for buyers. First, the class depends on thickness: one flame-retardant polycarbonate grade is V-2 at 1.0 mm and V-0 at 3.0 mm, so a rating means little without its thickness. Second, UL notes that most UL 94 results do not apply to material thicker than 13.0 mm or larger than 1 m². Third, a V-0 material does not make a V-0 or listed enclosure: UL 94 rates the plastic, and UL's own material cards say the acceptability of the combination in an end product is determined separately. See UL 50E, NEMA 250 and listing.
Wireless signals, EMI and grounding
Both materials let radio signals through. Polycarbonate and polymer-fiberglass composites let a wireless device work inside the enclosure "without noticeable impact" on the signal, which is a common reason to choose them for radios, gateways and RFID readers. The other side of the same property: plastics offer negligible EMI shielding unless they are plated or coated with a conductive layer, so EMI-sensitive equipment belongs in metal. A non-metallic enclosure also provides no grounding path: NEC 314.3 allows nonmetallic boxes with metal raceways or metal-armored cables only where the metal parts are bonded together inside the box (check the edition your authority having jurisdiction enforces), as described in enclosure grounding and bonding.
Cutting and drilling
Polycarbonate machines easily with sharp tools. Cool with compressed air or a light water mist, and avoid petroleum cutting oils or aggressive coolants, which can cause stress cracking. One enclosure maker does not recommend hole punches for polycarbonate.
Fiberglass is abrasive. A dull cutting tool is the most common cause of failed holes; carbide-tipped or diamond-tipped hole saws at high speed and low feed reduce chipping, and high-speed steel dulls quickly. Cutting produces glass dust, so wear eye and respiratory protection.
Either way, every opening must be closed by a gland or component rated at least as high as the enclosure type, or the enclosure loses its rating: see cutouts, cable glands and HMI windows.
Which one to choose
| Application | Choose |
|---|---|
| Impact, vandalism, dropped tools | Polycarbonate |
| Clear cover or see-through door | Polycarbonate (UV-stabilized outdoors) |
| Large enclosure, heavy mounting panel | Fiberglass |
| High ambient temperature | Fiberglass, after checking the published range |
| Caustic, ammonia or solvent exposure | Fiberglass, checked against the specific chemical; or 316L stainless |
| Outdoor, long sun exposure | Either, with an f1-rated material |
| Wireless device inside | Either |
| EMI shielding needed | Neither: use a metal enclosure |
Custom builds. NEMA4 Enclosure builds custom polycarbonate and fiberglass (FRP) enclosures in standard molded sizes, cut to your layout, alongside 304 and 316L stainless, powder-coated carbon steel and aluminum. NEMA types are manufacturer ratings for the specified design; the enclosures are not UL listed. From one piece, quoted by email, delivered in 10 business days from your approval of the drawing.
Frequently asked questions
Is polycarbonate or fiberglass better outdoors?
Both work outdoors if the material is rated f1 under UL 746C. Polycarbonate needs a UV-stabilized grade and can yellow over years; fiberglass can show fiber bloom on the surface. Fiberglass has the higher temperature range.
Which is stronger, polycarbonate or fiberglass?
They are strong in different ways. Polycarbonate absorbs impact better; fiberglass is about five times stiffer, so it flexes less under load and on large panels.
Can polycarbonate enclosures be used in washdown areas?
Yes, with care over chemicals. Polycarbonate is attacked by alkalis, ammonia and many solvents, so check the cleaners and sanitizers used. For caustic washdown, fiberglass or stainless steel is usually the safer choice.
Does a UL 94 V-0 rating mean the enclosure is fire rated?
No. UL 94 classifies the plastic material at a stated thickness. It does not rate or list a finished enclosure, which is evaluated as a product under its own standard.
Do fiberglass enclosures block Wi-Fi?
No. Fiberglass and polycarbonate let radio signals through, which is why they are used for wireless equipment. Neither provides EMI shielding unless coated.
Sources
- UL Solutions, Combustion (fire) tests for plastics (UL 94 V, HB and 5V criteria)
- UL Code Authorities, Understanding UL 94 rating certifications and limitations (13.0 mm and 1 m² limits)
- UL Yellow Card for a polyester SMC (minimum thickness per class; acceptability of the combination determined by UL)
- UL Solutions, Understanding the UL 746 series (UL 746B aging, UL 746C f1 / f2)
- Bopla, outdoor capabilities (f2 definition)
- UL 746C (1995), Polymeric Materials: Use in Electrical Equipment Evaluations (UV conditioning, Table 26.1, cold impact 57.4)
- Covestro, Makrolon 2807 UV-stabilized polycarbonate datasheet
- Covestro, Makrolon 6555 flame-retardant polycarbonate datasheet (V-2 at 1.0 mm, V-0 at 3.0 mm)
- Covestro, "Makrolon: the chemical resistance" (resistance table, stress cracking, 2% caustic cleaning)
- Altraset AS8125 electrical SMC datasheet (specific gravity, modulus, UL 94 V-0 at 3.0 mm)
- Altraset AS8115 electrical SMC datasheet (V-0 at 1.5 mm, 5VA at 3.0 mm)
- LyondellBasell, Premi-Glas 1200H-30 SMC datasheet (density, thermal conductivity, RTI)
- LyondellBasell, Premi-Glas 2207-25 CR-SX SMC datasheet
- nVent HOFFMAN, Technical Information (polycarbonate and fiberglass temperature ranges and uses)
- Fibox, enclosure materials (UV yellowing of clear polycarbonate; no EMC shielding)
- Fibox ARCA polycarbonate declaration (IK10 at +35 °C, IK09 at −25 °C; −40 to +80 °C)
- Fibox Euronord P polyester enclosure (IK08)
- CompositesWorld, UV surface degradation of fiberglass
- Allied Moulded, Ultraguard bulletin (fiber bloom and structural integrity)
- Strongwell, UV weatherability of fiberglass products (fiber blooming)
- Stahlin, chemical resistance tables (fiberglass and polycarbonate)
- Hammond, fiberglass vs polycarbonate comparison (chemical resistance ratings)
- Control Global, "Enclosures are more than a box" (non-metallics pass wireless signals)
- Interference Technology, EMI shielding (plastics negligible unless coated)
- Interstate Plastics, Polycarbonate fabrication guide (coolants and stress cracking)
- Stahlin, cutting and drilling (tools, speeds and feeds; punches and polycarbonate)
- Polycase, fiberglass vs polycarbonate (eye and respiratory protection when cutting FRP)
- BSI, IEC 62262 (IK code)