How to Match T-Code (Temperature Class) with On-Site Gases in the NEC Class/Division System
Date： 2025-05-15
In the North American NEC system, the T-Code is a critical safety parameter for explosion-proof equipment. It defines the maximum surface temperature the equipment can reach, which must be lower than the auto-ignition temperature (AIT) of the hazardous gases present.
1. T-Code Table (NEC)
| T-Code | Max Surface Temperature | Suitable AIT Range | Common Gas Examples |
|---|---|---|---|
| T1 | ≤ 450°C | > 450°C | Hydrogen, Acetylene |
| T2 | ≤ 300°C | > 300°C | Ethanol, Propane |
| T3 | ≤ 200°C | > 200°C | Gasoline, Diesel |
| T4 | ≤ 135°C | > 135°C | Diethyl ether (most common) |
| T5 | ≤ 100°C | > 100°C | Carbon disulfide |
| T6 | ≤ 85°C | > 85°C | Highly sensitive gases |
2. How to Match T-Code with On-Site Gases
- Identify Class and Group first — Confirm it is Class I (gases) and the specific Group (A, B, C, or D).
- Find the Auto-Ignition Temperature (AIT) of the gas — Check the MSDS/SDS or NFPA 497 tables.
- Matching Rules: Equipment's maximum surface temperature must be lower than the gas AIT. Recommend a safety margin of at least 20°C. When multiple gases are present, use the lowest AIT.
3. Practical Matching Examples
| Gas | AIT (approx.) | Recommended T-Code | Notes |
|---|---|---|---|
| Hydrogen | 560°C | T1 ~ T4 (commonly T4) | Very common |
| Natural Gas/Propane | 450–540°C | T1 ~ T4 | Common |
| Gasoline Vapor | 280°C | T3 or lower | Gas stations |
| Diethyl Ether | 160°C | T4 or lower | Careful selection |
| Carbon Disulfide | 90°C | T6 | Very strict |
4. Key Notes
- T-Code is mandatory and must appear on the equipment nameplate.
- T4 is the most commonly used rating in oil & gas and chemical industries.
- For high ambient temperatures (>40°C), use de-rated or specially certified equipment.
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