NETA MTS Severity Classifications
| Priority | ΔT Range between similar components | ΔT Range vs. ambient air |
Action |
|---|---|---|---|
| 4 (Low) | 1-3°C (2-5°F) | 1-10°C (2-18°F) | Possible deficiency; investigate |
| 3 (Medium) | 4-15°C (7-27°F) | 11-20°C (20-36°F) | Probable deficiency; repair when convenient |
| 2 (High) | — | 21-40°C (38-72°F) | Monitor until corrective measures completed |
| 1 (Critical) | >15°C (>27°F) | >40°C (>72°F) | Major deficiency; repair immediately |
Important: Use the left column to compare components of the same type under similar load, and the right column to compare against ambient air. NFPA 70B (2023) requires measuring and documenting the temperature difference but does not set severity levels: the NETA MTS table is used.
Load Correction
Important: Temperature rise is proportional to I²R. A connection at 50% load shows ~25% of the rise at full load. This is an approximation: the exponent usually falls between 1.6 and 2, so do not extrapolate from low loads.
| Measured Load | Multiply ΔT by |
|---|---|
| 40% | 6.25× |
| 50% | 4× |
| 75% | 1.78× |
| 100% | 1× (direct reading) |
Emissivity Reference
| Material | Emissivity |
|---|---|
| Oxidized steel | 0.85-0.95 |
| Painted surfaces | 0.90-0.95 |
| Aluminum (oxidized) | 0.20-0.30 |
| Copper (oxidized) | 0.65-0.75 |
| Electrical tape | 0.95 |