The rating is the specification
Buyers often start with a drawing and a material, then discover at certification that the enclosure does not meet the ingress protection the market expects. The order should be reversed: the rating drives the material, the wall, the gasket and the fasteners.
Two rating systems dominate: IEC 60529 (IP codes, used worldwide) and NEMA 250 (North America, with types such as 4, 4X, 12 and 13). They are not equivalent. A NEMA 4X claim involves corrosion resistance that IP66 does not address, and an IP66 test does not include the hose-directed spray from a specific angle that NEMA 4 defines.
Reading the IP code correctly
The first digit covers solids, the second covers water:
| Code | Solids | Water |
|---|---|---|
| IP54 | Dust protected | Splashing water |
| IP65 | Dust tight | Water jets, 12.5 mm nozzle |
| IP66 | Dust tight | Powerful water jets, 6.3 mm nozzle |
| IP67 | Dust tight | Temporary immersion, 1 m for 30 min |
| IP68 | Dust tight | Continuous immersion, depth by agreement |
| IP69K | Dust tight | High-pressure, high-temperature jet wash |
The suffix K in IP69K matters for food processing and vehicle washdown: 80 °C water at 80–100 bar from four angles. Few gasket designs pass it, and the ones that do use a silicone or EPDM seal in a captured groove with a rigid lid, not a flat foam gasket.
Material selection
| Material | UV outdoor | Impact | Chemical | Flame | Notes |
|---|---|---|---|---|---|
| ABS | Poor without ASA cap | Good | Poor against solvents | UL 94 HB to V-0 | Cheapest, indoor use |
| PC | Good with UV stabiliser | Excellent down to −40 °C | Attacked by alkalis | V-0 to 5VA | Standard for outdoor and industrial |
| PC/ABS | Moderate | Very good | Moderate | V-0 to 5VA | Best cost/performance balance |
| PA66-GF30 | Good | Good | Good | V-0 | Higher temperature, tighter tolerance |
| PPO/PPE blend | Good | Good | Good | V-0 | Dimensionally stable, good for seals |
| PP | Poor | Moderate | Excellent | HB | Chemical plants, non-flame applications |
For an enclosure that houses mains wiring, UL 94 5VA is commonly required rather than V-0, because 5VA is tested on the actual enclosure shape for burn-through. A resin supplier's V-0 rating on a test bar does not automatically give a 5VA enclosure — the wall thickness and the part geometry are part of the test.
Outdoor service in North America usually brings UL 746C with the f1 suffix, which is a UV and water immersion rating on the material, not on the finished part.
Wall thickness and stiffness
| Enclosure size | Typical wall | Note |
|---|---|---|
| Under 200 mm | 2.0–2.5 mm | Ribs rather than thicker walls |
| 200–400 mm | 2.5–3.0 mm | Bosses at corners, rib grid on the base |
| Above 400 mm | 3.0–4.0 mm | Consider a metal frame for lid rigidity |
Thickening the wall is the expensive way to add stiffness. A rib grid on the base and a return flange on the lid give more rigidity per gram, and they also reduce sink marks on the visible face. Rib thickness should stay at 50–60% of the wall to avoid sink; the aspect ratio (height to thickness) should stay under about 3:1 to avoid ejection and fill problems.
Gasket design is where IP ratings are won or lost
A gasket works by compression, and compression is a design number, not a feel.
| Gasket type | Compression target | Rebound | Best fit |
|---|---|---|---|
| EPDM solid profile | 25–30% | Good | Outdoor, UV, ozone |
| Silicone solid profile | 25–35% | Excellent | Wide temperature, IP69K |
| PU foam | 30–40% | Moderate | Indoor, low clamp force |
| TPE moulded-in place | 20–30% | Good | High volume, no assembly step |
| Liquid gasket (FIPG) | 20–25% | None | One-time seal, service-hostile |
Four rules decide whether the seal holds:
- Groove, not a flat land. A captured groove stops the gasket walking sideways under compression and keeps the compression ratio constant around the perimeter.
- Continuous compression. Screw spacing must be tight enough that the lid does not bow between fasteners. As a starting point, screw pitch should not exceed 12–15 times the wall thickness, and corners always get a screw.
- Seal inside the screw line. Put the gasket inboard of the fasteners so water that reaches a screw thread still has to cross the seal.
- No sharp corners in the gasket path. Radius the groove at every corner; a 90° turn in a compressed gasket is a leak path.
Moulded-in-place TPE gaskets remove the assembly step and the risk of a missing gasket, but they add tooling cost and make field replacement impossible.
Fasteners and bosses
Self-tapping screws into plastic bosses are the default. Boss design drives both sealing and service life:
- Boss outside diameter 2.0–2.5× the screw diameter
- Boss wall 0.6× the nominal wall, with a radius at the base
- Pilot hole diameter per the screw supplier's specification for the resin — too small and the boss splits, too large and it strips
- Boss depth at least 2× the screw diameter of engagement
- For repeated service, brass inserts or machine screws with a threaded insert
Screw torque is a specification, not an operator skill. Overtorque cracks bosses; undertorque leaves the gasket under-compressed. Publish a torque value with a tolerance band and use a calibrated driver.
Electrical and thermal requirements
| Requirement | Reference | Practical implication |
|---|---|---|
| Creepage and clearance | IEC 60664-1 | Sets minimum distances between live parts and the wall |
| Flammability | UL 94 5VA / V-0 | Material and wall thickness |
| Enclosure for industrial control | UL 508A | Assembly-level requirements, not just the box |
| Outdoor polymer | UL 746C f1 | UV and water exposure |
| Temperature rise | IEC 61439 or product standard | Ventilation or derating |
| Earthing | Local wiring code | Metal insert or earthing boss if required |
Sealed enclosures trap heat. A gasketed IP66 box with a 30 W load inside can run 15–25 °C above ambient, and that is what limits the components, not the enclosure. Plan vents with a breathable membrane if condensation is a risk: a fully sealed box in a cycling temperature environment will accumulate water from the air trapped at assembly.
Knockouts, cable entries and accessories
- Knockouts weaken the wall where they are moulded. Keep them at least one wall thickness from a corner and specify the wall thickness that remains.
- Cable glands need a flat land on the outside and clearance on the inside for the locknut. Moulded M16/M20/M25 thread forms save a punching step but cannot be repaired if stripped.
- DIN rail mounting needs a moulded rail clip with a metal spring; all-plastic clips creep under load.
- Mounting feet should be outside the gasket line so a fixing screw cannot create a leak path.
RFQ checklist
- Required IP and NEMA rating, with the test standard and the test house
- Flammability class on the finished part, and the wall thickness assumed
- Indoor or outdoor, with the UV exposure expected
- Operating temperature range, including solar gain
- Internal heat load in watts
- Gasket material, compression target, and whether it is supplied loose or moulded in
- Screw type, torque specification and service access frequency
- Cable entry sizes and knockout positions
- Chemical exposure: cleaning agents, oils, disinfectants
- Tooling ownership and the expected annual volume
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
Standards referenced
- IEC 60529 — degrees of protection provided by enclosures (IP code)
- NEMA 250 — enclosures for electrical equipment
- UL 50 / UL 50E — enclosures for electrical equipment
- UL 94 — flammability of plastic materials, including the 5VA surface burn test
- UL 746C — polymeric materials in electrical equipment, including the f1 outdoor rating
- IEC 60664-1 — insulation coordination, creepage and clearance
