Temperature derate for direct sunlight?

[shockking]

Does anyone apply temperature derating for transformers, conductors, etc. that are installed in direct sunlight?

I've seem forum discussions that suggest adding anywhere from 5°C to 40°C to the ambient temperature; that makes sense to me, but I haven't found anything to advise on exactly what value to pick in that range. Is there another way to go about this, or any suggestions on how to pick a modifier?

I have some oil filled transformer in operation, in direct sunlight. For existing installations, I could collect some IR surface temperature measurements (with or without load) and see what the surface temperature is reaching. For these xfmr's I can look directly at the oil temperature, but it's a lot easier to collect surface temperature than oil temperature data. Is the surface temperature the right measurement location to compare with the ambient temperature rating in the specs for a transformer? (i.e. Is there a way to measure "effective ambient temp in direct sunlight" or should I just take an air temperature and add a modifier?)

 

[ramsy]

Is there a way to measure "effective ambient temp in direct sunlight

For raceways or cables exposed to direct sunlight on or above rooftops, NEC 310.15(B)(2) uses an ambient temperature adder of 33°C to correct for conductor ampacity, with exceptions & informational notes.

A more specific PV Watt calculator can estimate average solar energy in your region.
You would need a formula for ambient adjustment, from equipment surface area exposed to regional sunlight.

The risk with PV Watt calculators is missing regional climate changes, with record breaking extremes recorded each year.

 

[Julius Right]

See IEC 60287-2-1
2.2.1.2 Cables directly exposed to solar radiation
In calculation process the inner part of the cable remains as cable is in air protected from solar radiation.
Only the external thermal resistance has to be modified accordingly.

 

[brycenesbitt]

Thermal mass comes into it also. A huge oil filled transformer will stay mostly at the average of night and day.

I have applied temperature ratings to underground raceways: they sit quite close to the average yearly temperature in an are, so clearly are good for more amps ;-)