Choose a plastic or fiberglass enclosure when its chemical resistance, electrical insulation and weight suit the installation. Choose metal when structural stiffness, heat spreading or electromagnetic shielding makes it the better fit. Neither material wins every application.
Voltage and three-phase power do not decide the material alone. Start with the environment, equipment requirements and applicable approvals. Then check heat, mechanical loading and every penetration. A corrosion-resistant cabinet still fails if its gasket, cable entries or internal assembly cannot handle the service.
How the materials compare
“Plastic” covers different thermoplastics, while fiberglass usually means a glass-reinforced thermoset composite. Resin, reinforcement, wall thickness and construction all affect performance. Metal also needs qualification: painted carbon steel, aluminum and stainless steel behave differently.
| Selection factor | Thermoplastic enclosure | Fiberglass enclosure | Metal enclosure |
|---|---|---|---|
| Corrosion and chemicals | Does not rust; chemical resistance depends on the polymer, temperature and exposure | Does not rust; resin system and surface condition determine chemical resistance | Depends on alloy, coating, chemicals and retained moisture |
| Impact and stiffness | Can be impact-resistant, but stiffness, cold-temperature behavior and aging vary | Can provide good stiffness; severe impact can crack or damage the composite | Often suits heavy hardware and mechanical abuse; thin walls can still dent or distort |
| Heat rejection | Low wall conductivity; verify the complete thermal design | Low wall conductivity; verify the complete thermal design | Usually spreads heat more effectively, but ambient temperature and airflow still limit rejection |
| Protective bonding | Shell is nonconductive; exposed metal parts may still require bonding | Shell is nonconductive; exposed metal parts may still require bonding | Conductive shell and other applicable metal parts require a reliable protective bonding path |
| Electromagnetic shielding | Plain polymer provides little shielding; engineered shielding requires separate design | Plain composite provides little shielding; conductive treatments require verification | Conductive walls can provide shielding, but seams, openings and cable entries control effectiveness |
| Mounting and modification | Follow limits for threads, inserts, support and tightening torque | Support cutouts and control machining dust using the manufacturer's instructions | Check wall thickness, reinforcement, corrosion protection and bonding after modification |
Compare actual enclosure data rather than ranking materials by name. For differences within the metal group, use the aluminum versus stainless steel selection comparison.
Where plastic and fiberglass win
Nonmetallic construction can be a strong choice around chemicals that attack the available metals or their coatings. It also avoids corrosion caused by damage to a protective paint layer. That does not make every polymer suitable for every chemical.
Specify the chemical name, concentration, operating temperature and exposure method. Continuous immersion, occasional splashing and cleaning spray are different duties. Check the enclosure body, gasket, transparent cover, hinges and entry fittings separately.
For example, a cabinet body may tolerate a process chemical while its gasket swells or loses compression. A generic statement such as “chemical-resistant plastic” cannot establish compatibility for that assembly. The GRP versus stainless steel junction box comparison covers that narrower material decision.
Lower weight can simplify installation where wall capacity or handling matters. However, equipment weight still loads the mounting panel, inserts and enclosure supports. Confirm those limits, especially with large transformers, drives or door-mounted equipment.
An insulating shell also avoids an exposed conductive cabinet surface. It does not remove shock hazards inside, eliminate protective bonding requirements or make energized work acceptable. Metal mounting panels, gland plates, raceways and other exposed conductive parts still need assessment.
Outdoor service adds another condition: verify suitability for ultraviolet exposure and the full temperature range. Do not infer outdoor durability from a resin name or from the absence of rust.
Where metal wins
Metal often makes sense for large doors, heavy components and installations exposed to repeated mechanical loading. It also provides a conductive boundary useful for electromagnetic compatibility. These advantages depend on the actual construction, not simply the word “steel.”
Check enclosure wall thickness, door reinforcement, mounting-panel capacity and mounting arrangement. A heavy assembly can distort an undersupported cabinet, affecting both door alignment and gasket compression. An impact that leaves the shell intact can still damage a latch or seal.
Heat rejection needs a calculation
Metal generally conducts and spreads heat more effectively than unfilled thermoplastic or fiberglass. However, wall conductivity is only part of the thermal path. Internal air movement, external convection, exposed surface area, installation position and solar heating also matter.
For equipment dissipating 100 W at 95°F [35°C] ambient, a maximum allowable enclosure air temperature of 104°F [40°C] permits only a 9°F [5°C] rise. That narrow margin needs a thermal calculation for the proposed cabinet, whichever material you choose.
Do not assume a metal cabinet makes cooling unnecessary. Likewise, do not assume every nonmetallic cabinet needs forced cooling. An internal fan redistributes heat but does not discharge it outside; the article on internal fans in a sealed enclosure explains that distinction.
Shielding depends on the complete boundary
A metal shell can help control electrical noise from drives and switching equipment. It is not automatically an effective shield after adding a large display opening, poorly bonded gland plate or unsuitable cable entry.
Specify cable-shield termination, seam treatment and penetration details with the equipment's installation requirements. Protective bonding and high-frequency shield continuity serve different purposes; one connection detail may not satisfy both. Use the discussion of RF leakage through enclosure holes and seams when evaluating openings.
Why voltage and three-phase power do not decide
A 480 V three-phase supply does not, by itself, prohibit a nonmetallic enclosure. Equally, selecting steel does not establish that a panel is suitable for that supply.
The complete assembly must satisfy the equipment instructions, applicable electrical requirements and project approval conditions. Check insulation and spacing, wire-bending space, temperature limits, access protection and fault-related requirements. The enclosure material alone does not establish the panel's short-circuit current rating.
With metal construction, specify a reliable protective bonding arrangement for the enclosure and applicable internal metalwork. Do not treat a convenient screw, an ordinary hinge or incidental metal contact as a verified fault-current path. Door-mounted electrical equipment may require a dedicated door bonding connection, depending on the construction and applicable requirements.
With nonmetallic construction, do not assume the mounting panel, metal conduit or exposed fittings are exempt from bonding. The grounding and bonding guide explains the protective path without making enclosure material a substitute for electrical design.
Perform installation and bonding checks with the equipment de-energized, isolated and verified safe under the applicable electrical safety procedure. Material selection never justifies bypassing those controls.
Choose by the environment
Match the material to the actual exposure, then confirm the enclosure rating and assembly requirements. NEMA type ratings and IP codes describe defined protection characteristics; they are not universal chemical-compatibility or impact guarantees.
The catalog stainless wall-mount enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Confirm whether the project requires a listing before selecting a cabinet.
| Installation condition | Material direction | Check before deciding |
|---|---|---|
| Dry indoor controls with modest heat | Thermoplastic, fiberglass or metal may work | Equipment weight, access protection, approval requirements and thermal rise |
| Corrosive chemical area | Compare compatible nonmetallic construction with suitable stainless steel | Exact chemical exposure, gasket compatibility and internal contamination |
| Frequent cleaning or sanitation | Stainless steel may suit the cleaning process; verify alternatives individually | Cleaning chemicals, surface requirements, temperature, spray pressure and seals |
| Outdoor rain and sunlight | Use a construction documented for outdoor service | UV exposure, solar heat, condensation, mounting and entry orientation |
| Drives beside sensitive instrumentation | Metal may simplify shielding; nonmetallic construction needs deliberate EMC design | Equipment instructions, cable routing, shield termination and heat load |
| Heavy equipment or likely impact | A suitably reinforced metal cabinet may be preferable | Actual mechanical ratings, support spacing, door loading and seal integrity |
Neither stainless steel nor an insulating shell prevents moisture problems inside. Temperature cycling and moisture carried through entries can still cause corrosion on terminals and components. Include corrosion prevention inside an outdoor stainless enclosure in the outdoor design review.
High-pressure or hot washdown requires application review. Do not treat a general hose-down rating as confirmation for every cleaning process.
What to specify on the order
Write the required performance before selecting a material or catalog size. This makes substitutions easier to evaluate and exposes missing installation details.
| Specification item | What to record |
|---|---|
| Installation | Indoor or outdoor location, mounting surface, orientation and service access |
| Electrical duty | Supply voltage, phase, equipment schedule and applicable assembly approval requirements |
| Environmental protection | Required enclosure type or IP code, plus any separate listing requirement |
| Material | Exact polymer or composite specification, or metal grade and finish; avoid “plastic” or “steel” alone |
| Exposure | Chemical names, concentrations, temperatures, cleaning method, sunlight and mechanical hazards |
| Usable dimensions | Required mounting-panel height and width, panel-to-door clearance and conductor bending space |
| Mechanical loading | Equipment mass, mounting-panel loading, door-mounted devices and support arrangement |
| Thermal duty | Component losses in watts, maximum ambient temperature, allowable internal temperature and solar exposure |
| Bonding and EMC | Protective bonding points, applicable door bond, cable-shield terminations and conductive entry requirements |
| Penetrations | Dimensioned cutouts, fitting specifications, cable outside diameters and sealing arrangements |
Separate outside dimensions from usable space. Door hardware, mounting-panel offsets, cable bends and cooling equipment can consume clearance that a simple outside-size comparison misses.
When the installation requirements are established, the enclosure selector can narrow the catalog by environment, listing requirement and usable dimensions. A dimensional match does not confirm thermal suitability.
For a stainless selection, the stainless enclosure size finder filters wall-mount and HMI lines by grade and dimensions. Use it after deciding that stainless meets the environment and approval requirements, not as a substitute for that decision.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 16 × 12 × 8 in | 316L stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 316L stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 30 × 24 × 10 in | 316L stainless | 30 x 24 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 10 × 6 in | 316L stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can a fiberglass enclosure contain three-phase equipment?
Yes, when the enclosure and complete assembly meet the equipment instructions, applicable electrical requirements and project approval conditions. Three-phase power alone does not prohibit fiberglass. Check voltage suitability, spacing, temperature, mechanical loading and fault-related requirements rather than applying a material-only rule.
Does a plastic enclosure still need grounding or bonding?
Yes. An insulating shell does not remove bonding requirements for applicable exposed conductive parts. Metal mounting panels, raceways, gland plates and fittings need assessment. Specify their protective bonding arrangement rather than assuming the nonmetallic body makes every internal metal part electrically safe.
Will a metal enclosure eliminate the need for cooling?
Not automatically. Metal usually conducts and spreads heat more effectively, but total heat rejection also depends on surface area, airflow, mounting and ambient temperature. Calculate the temperature rise using actual component losses. Internal fans redistribute heat without removing it from a sealed cabinet.
Is fiberglass always better than stainless steel in corrosive areas?
Neither material is universally better. Fiberglass performance depends on its resin system and exposure conditions. Stainless performance depends on grade, chemicals, temperature and surface condition. Compare the complete enclosure, including gaskets and fittings, against the actual chemical concentrations and cleaning process.
