Temperature Class Reference
Cable temperature ratings describe the maximum conductor temperature the insulation can withstand continuously without degradation. Higher temperature ratings allow higher ampacity — more current per conductor cross-section — and suitability for elevated ambient environments.
| Temperature Class | Typical Insulation | Applications |
|---|---|---|
| 60°C (140°F) | TW, XHHW (wet) | General residential and light commercial wiring in low-ambient environments |
| 75°C (167°F) | THW, THWN, XHHW-2 (wet) | General commercial and industrial wiring — most common termination temperature rating |
| 90°C (194°F) | THHN, THHW, THWN-2, XHHW-2 (dry) | High-density conduit fills, elevated ambient, commercial and industrial — most common modern specification |
| 150°C (302°F) | Silicone rubber (SIS) | Switchgear, appliance wiring, industrial control panels with high internal temperatures |
| 200°C (392°F) | PTFE/FEP fluoropolymer | High-temperature industrial equipment, aerospace, instrumentation with sustained high ambient |
| 250°C (482°F) | PTFE/Mica composites, fiberglass | Furnace wiring, industrial ovens, ceramic kilns, extreme industrial environments |
| 650–1000°C | Pure mica, nickel-mica | Nuclear facilities, steel mills, foundries, experimental and aerospace applications |
Voltage Class Reference
Cable voltage ratings define the maximum system voltage the insulation is designed to withstand. Select cable with a voltage rating equal to or greater than the highest system voltage the cable will encounter.
| Voltage Class | Typical Cable Types | Application Note |
|---|---|---|
| 300V | Light appliance wiring, fixture cable, NM-B Romex, low-voltage controls | Do not use in systems above 300V phase-to-ground |
| 600V | THHN/THWN-2, MTW, building wire, most industrial applications | Standard for most commercial and industrial 120–480V systems |
| 1kV (1000V) | High-voltage silicone wire (JHXG), some VFD cable | Required for 600–1000V systems including some VFD installations |
| 5kV | MV-90, XHHW medium-voltage | 2.4kV and 4.16kV distribution systems |
| 15kV | MV cable, class 15kV | 13.8kV distribution feeders, substations |
| 35kV | MV cable, HV distribution | 33kV and 34.5kV high-voltage transmission and substation feeders |
NEC vs IEC Insulation Codes
North American (NEC) and international (IEC) cable standards use different naming conventions for insulation types, but map to similar performance requirements. Here are the most common cross-references:
| NEC Designation | IEC Equivalent | Temperature |
|---|---|---|
| THHN / THWN-2 | H07V-R / H05VV-F (approx) | 90°C dry / 75°C wet |
| XHHW-2 | XLPE (IEC 60502) | 90°C wet or dry |
| Silicone (SIS) | IEC 60245 SiH-F | 150°C |
| PTFE / FEP | IEC 60754-2 PTFE | 200–250°C |
| MTW (Machine Tool) | IEC 60204-1 | 60–90°C |
| TC-ER (Tray Cable) | Roughly IEC 60502 cable tray | 90°C |
FAQs
What does AWG mean for cable sizing?
AWG stands for American Wire Gauge — the US standard for conductor sizing. Counter-intuitively, a smaller AWG number means a larger conductor. 4 AWG is larger than 12 AWG. For metric equivalents, see mm² cross-sectional area: 12 AWG ≈ 3.3 mm², 10 AWG ≈ 5.3 mm².
What voltage rating do I need for 480V systems?
Standard 480V systems typically use 600V rated cable (UL 83 THHN/THWN-2). The 600V cable rating provides a safety margin above the nominal system voltage. For medium voltage above 600V, you need separate MV-rated cable.
What is the difference between 90°C and 75°C rated cable?
A 90°C cable can carry more current than a 75°C cable of the same AWG because the conductor can run hotter before exceeding insulation limits. However, when terminating into equipment rated only for 75°C conductors, the NEC requires you to use 75°C ampacity values even with 90°C cable.
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