The short answer
For most housings and enclosures, start with flame-retardant PC/ABS or FR polycarbonate. Use FR ABS for low-cost indoor housings, modified PPE when halogen-free and dimensional stability matter, glass-filled FR PBT or nylon for connectors and terminal blocks, and PPS or PEI when the part runs hot. Then check four numbers on the grade: flame rating, RTI, CTI and glow wire.
An electrical housing has to do more than pass a burn test. It has to stay strong after years at temperature, resist tracking where live parts sit close together, and survive drops and sometimes sunlight. This page compares the resins that do that. For how to get a flame rating specified and matched, see your part needs a flame rating.
The four numbers to check
- Flame rating (UL 94): HB, V-2, V-1, V-0, then 5VB and 5VA for enclosures. A rating holds only at the thickness and colors stated for the grade, so match it to your thinnest wall. See UL 94 flame ratings explained.
- Relative thermal index (RTI, UL 746B): the long-term temperature rating, with separate electrical, mechanical-with-impact and mechanical-without-impact values. Your part's hottest spot should sit below the RTI that applies.
- Comparative tracking index (CTI): resistance to surface tracking. UL groups it as PLC 0 (600 V and up), PLC 1 (400–599 V), PLC 2 (250–399 V), PLC 3 (175–249 V), PLC 4 (100–174 V) and PLC 5 (below 100 V). A higher CTI can allow shorter creepage distances.
- Glow wire (IEC 60695-2-12 and -13): GWFI and GWIT, called for by appliance standards such as IEC 60335-1. A V-0 grade does not automatically pass glow wire.
Resins for housings and enclosures
| Resin | Typical flame ratings available | Electrical RTI (approx.) | CTI | Impact | UV / outdoor | Notes |
|---|---|---|---|---|---|---|
| FR ABS | V-0, some 5VA | ~60–90°C (140–194°F) | Moderate to high | Good | Poor | Lowest-cost indoor housings |
| FR PC/ABS | V-0 (~1.5 mm typical); 5V in some grades | ~80–105°C (176–221°F) | Moderate to high; varies by grade | High | Poor to fair | The default for electronics housings |
| FR polycarbonate | V-0 thin walls; 5VA at ~2.5–3 mm | ~115–130°C (239–266°F) | Moderate (~175–399 V) | Very high | Fair; UV grades good | Clear options; outdoor grades with f1 |
| Modified PPE, FR | V-0 / V-1, often halogen-free; 5VA in some grades | ~90–110°C (194–230°F) | Moderate; varies by grade | Good | Poor to fair | Low moisture uptake, good dimensional stability |
| FR PBT, glass-filled | V-0 down to thin walls | ~130–140°C (266–284°F) | Varies; high-CTI grades exist | Fair | Fair | Connectors, terminal blocks, coil forms |
| FR PA66, glass-filled | V-0 thin walls | ~120–150°C (248–302°F) | High (~400–600 V) on many grades | Good | Fair; black or stabilized grades | Breakers, terminal blocks, glow-wire parts |
| PPS, glass-filled | V-0 without additives; 5VA some grades | ~200–240°C (392–464°F) | Low to moderate | Low to fair | Good | High heat, very stable; brittle |
| PEI | V-0 without additives; 5VA some grades | ~170–180°C (338–356°F) | Low | Fair | Good | High heat, strong, costly |
| Polyester BMC / SMC (thermoset) | V-0 and 5VA grades | ~130–155°C (266–311°F) | High | Low | Good | Breaker housings, meter and outdoor boxes |
Approximate ranges for typical FR or electrical grades; values vary widely by grade, thickness and color. Flame rating, RTI and CTI must be read from the UL listing for the specific grade at your wall thickness. The finished product's approval is the manufacturer's responsibility.
How the main options compare
Housings and enclosures
- FR PC/ABS: tough, flows into thin walls, paints well. See PC/ABS.
- FR PC: highest impact and heat of the three, clear grades, 5VA at moderate thickness. See polycarbonate.
- Modified PPE: halogen-free FR, very low moisture uptake, steady dimensions. See PPO / PPE.
- FR ABS: lowest cost, lower heat and RTI.
Connectors and live-part carriers
- FR PBT, glass-filled: thin-wall V-0, low moisture, good flow. See PET and PBT.
- FR PA66, glass-filled: tough, high CTI on many grades, common for glow-wire parts.
- PPS: V-0 without additives, high RTI, very stable. See PPS.
- PEI: high heat and strength for demanding parts.
Details that trip people up
- Halogen-free: a restriction on the flame-retardant chemistry, separate from the rating. Phosphorus- and nitrogen-based grades exist for PC, PC/ABS, PPE, PBT and nylon.
- Outdoor enclosures: standard PC yellows and loses toughness in sunlight. Use a UV-stabilized grade, and look for an f1 rating under UL 746C, which means the material passed both UV and water exposure tests. See plastics for outdoor use.
- Chemical exposure: PC and PC/ABS can stress-crack with some cleaners and greases used around electrical gear. Test with the real chemicals.
- Thermosets: polyester BMC and SMC are compression or injection molded thermosets with high heat and tracking resistance. They are molded on different equipment and are not drop-in alternatives to the thermoplastics above.
What to send us for a quote
- Flame rating, wall thickness and color required, and the standard it comes from
- RTI, CTI or glow-wire values called out on the drawing
- Halogen-free requirement, if any, and indoor or outdoor use
- The grade you run now, annual volume and ship-to location
We source FR and electrical grades of PC, PC/ABS, ABS, PPE, PBT, nylon, PPS and PEI. See electrical and electronics.
Common questions
What plastic is used for electrical enclosures?
Flame-retardant polycarbonate and PC/ABS are the most common for housings and enclosures. FR ABS covers lower-cost indoor housings, modified PPE covers halogen-free needs, and glass-filled FR PBT and nylon cover connectors, terminal blocks and breaker parts.
What flame rating does an electrical enclosure need?
The end-product standard decides. Many internal parts need UL 94 V-0 or V-1, fire enclosures often need V-1 or better, and larger equipment can call for 5VB or 5VA. The rating applies only at the thickness and colors listed for the grade.
What is the relative thermal index (RTI)?
RTI, set under UL 746B, is the temperature at which a grade keeps enough of a property after long-term aging. There are separate values for electrical, mechanical with impact and mechanical without impact, and they depend on thickness.
What is CTI in plastics?
The comparative tracking index is the voltage at which a material resists forming a conductive track across its surface under wet, contaminated conditions. UL groups it into performance level categories, from PLC 0 (600 V and up) to PLC 5 (below 100 V).
What glow-wire rating do appliances need?
Under IEC 60335-1, insulating parts near connections carrying more than 0.2 A in unattended appliances generally need a GWFI of at least 850°C (1562°F) and a GWIT of at least 775°C (1427°F), or the finished part must pass a 750°C (1382°F) glow-wire test. Confirm the edition that applies to your product.
Which plastic is best for outdoor electrical enclosures?
UV-stabilized polycarbonate is the usual thermoplastic choice, ideally a grade with an f1 outdoor rating under UL 746C. Polyester BMC and SMC thermosets are common for larger outdoor boxes and meter enclosures.
Are halogen-free flame-retardant plastics available?
Yes. Halogen-free FR grades exist for PC, PC/ABS, modified PPE, PBT and nylon, and PPS and PEI reach V-0 without added flame retardant. Properties and processing differ from halogenated grades, so trial them as new materials.
Tell us the rating and wall thickness
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province