Two small numbers buried in most thermal rating and LiDAR survey calculations – emissivity and absorptivity – can shift a conductor’s calculated temperature by tens of degrees, depending on which measurement method is in use. Getting them right is a minor input with an outsized effect on network capacity, clearance, and survey accuracy.
What Are Emissivity and Absorptivity?
Emissivity describes how much radiative heat an object emits, ranging from 0.0 (no heat emitted) to 1.0 for an ‘ideal black body’ – a term coined by physicist Gustav Kirchhoff in 1860 to describe a body that absorbs all incident electromagnetic radiation. Absorptivity describes how much solar radiation a surface absorbs, on the same 0.0-to-1.0 scale.
These two properties affect conductor temperature calculations in three distinct areas of power line engineering.
1. Determining Conductor Temperature from Weather and Electrical Load
This calculation establishes the as-surveyed conductor temperature used to validate a LiDAR survey. Absorptivity and emissivity pull in different directions depending on conditions:
- High solar radiation: absorptivity dominates. Too high an estimate and the calculated temperature reads higher than actual; too low, and it reads lower. The conservative approach is to err toward a higher absorptivity.
- High conductor temperature relative to ambient: emissivity dominates. Too high an estimate reads the temperature as lower than actual; too low reads it higher. The conservative approach is to err toward a lower emissivity.
2. Thermal Rating Calculations from Allowable Temperature, Weather Data and Electrical Load
A higher estimate of both emissivity and absorptivity increases solar gain (making the conductor hotter) and radiative cooling (making it cooler) at the same time. Where electrical load and solar gain effects are similar in scale, these two factors tend to balance out. Where solar gain is less of a concern, the conservative approach is a lower estimate of both properties – less radiative cooling, and therefore lower allowable capacity for a given electrical load.
But be careful: an unnecessarily low emissivity estimate can constrain a network’s rated capacity for no good reason. Where there’s a legitimate engineering basis for a higher emissivity value, using it can unlock meaningful additional capacity — typically around 5%.
3. IEEE-738 vs Thermography-Based Temperature Determination
When using an IR camera to determine conductor temperature, a higher emissivity estimate corresponds to a higher indicated temperature. If the goal is establishing an as-surveyed conductor temperature, the higher emissivity estimate is generally both the more accurate and the more conservative choice.
Under IEEE-738, though, the effect of emissivity on calculated conductor temperature is comparatively small: modelling shows an emissivity of 0.9 (fairly weathered/black) reflects a temperature of around 100°F, while an emissivity of 0.4 (relatively new) reflects around 107°F – a variance of roughly 5–7°F. That’s a useful buffer for increased rating, provided there are sufficient technical grounds to justify the assumption.
Again, be careful: modelled against thermography instead of IEEE-738, that same emissivity change has a far larger effect – an emissivity of 0.9 gives an estimated conductor temperature of around 75°F, against roughly 125°F at 0.4. That’s an uncertainty of 50°F from a single assumption.
Getting Emissivity and Absorptivity Right
Emissivity and absorptivity are small inputs with an outsized effect on conductor temperature accuracy – and the scale of that effect depends heavily on which measurement method is in use. Applying them correctly takes engineering judgement, not a default value pulled from a table. This matters most when working with IR thermography, where the uncertainty introduced by emissivity is far greater than under IEEE-738.
NM Group applies conservative, engineering-led emissivity and absorptivity assumptions on every Aerial LiDAR survey and thermal rating study we deliver, rather than defaulting to a single assumed value across a network. Combined with our Caydence AAR solution, that means ratings and clearance assessments that reflect the network’s real, current condition – not an assumption made at the design stage.
Contact NM Group today to discuss a LiDAR survey or thermal rating program for your network.