Does solar radiation count when checking the ambient temperature Ta?
Abstract
No. The rated Ta is the temperature of the surrounding air, and the temperature class is proven in still air without sun. Solar heating comes on top, so for equipment in direct sun the designer must consider it and document the measures, such as shading or a higher Ta rating.
No, the rated ambient temperature does not include the sun. Ta is the temperature of the air around the equipment, and the equipment's temperature class is proven in still air without solar heating. Direct sunlight adds heat on top of that, so for outdoor equipment the designer has to consider solar radiation separately and record how it is dealt with: by shading, by a mounting position out of the sun, or by choosing equipment with enough margin in its rating.
Short answer
- Compare the shade air temperature at the mounting position with the Ta range on the marking or certificate.
- Then ask whether direct sun will heat the enclosure. If it will, the rating alone does not prove the temperature class or the life of seals, plastics and cables.
- IEC 60079-14 makes this an installation task: its 2007 text requires that, where solar radiation (or process heat) influences the equipment temperature, the effect be considered and the measures documented; the 2024 edition adds an annex section on solar radiation.
- The usual measures are sunshades or canopies, mounting on the shaded side, light-coloured enclosures, or equipment rated for a higher Ta with a clear margin.
- Sunlight also degrades plastics and cable sheaths through UV, which IEC 60079-14 treats under cable selection and adverse conditions.
Why the rating leaves the sun out
In IEC 60079-0 the maximum surface temperature is measured at rated conditions in still ambient air, with the equipment in its normal position, and the result is corrected to the maximum rated ambient temperature (this is how the 2004 edition words it). There is no solar load in that test. The same edition notes, under external sources of heating, that the influence of radiated heat may need to be considered in the final installation and refers to IEC 60079-14.
The installation standard then closes the gap. In the 2007 text of IEC 60079-14 (published in Australia and New Zealand as AS/NZS 60079.14:2009), the clause on selection by ignition temperature and ambient temperature requires that, if the ambient lies outside the equipment's range or if other factors such as process temperature or exposure to solar radiation influence its temperature, the effect be considered and the measures taken documented. The 2024 edition keeps the topic and adds an informative annex on adverse environmental conditions with a section on solar radiation covering equipment protection, cables and cable entry devices.
The ATEX guidelines make the same point from the product side: the manufacturer and the user should communicate about the surrounding area conditions, which include the ambient temperature range and the effect of additional sources of heat.
What "ambient" means on a real site
The air temperature in a weather report is measured 1.25 to 2 m above the ground, in a screen that protects the thermometer from solar radiation. That is the right starting point, but the local air around Ex equipment can be warmer:
- under a roof or canopy heated by the sun,
- inside a cabinet, shelter or analyser house,
- near hot equipment such as compressors, steam lines or furnaces.
Those are still "ambient" effects: they change the air around the equipment and must be included in the design ambient. Direct sunshine on the enclosure is different: it heats the enclosure itself, not the air, and comes on top of the design ambient.
How to deal with it in design
| Situation | What to do |
|---|---|
| Equipment in permanent shade (inside a building, under a ventilated canopy) | Use the local shade air temperature as the design ambient |
| Equipment in direct sun, good margin between design ambient and rated Ta | Ask the manufacturer for data on solar exposure, or fit a sunshade; record the decision |
| Equipment in direct sun, little or no margin | Fit a sunshade or move the equipment out of the sun, or select equipment with a higher Ta |
| Plastic enclosures, windows, cables | Check UV resistance as well as temperature |
Practical case. A gas detector is certified T4 for -40 °C ≤ Ta ≤ +60 °C. The site's highest shade temperature at the mounting point is +46 °C, and the detector faces west on an open structure. The 14 K margin looks comfortable, but nobody knows how much of it a dark housing in afternoon sun will use. A simple sunshade, open at the sides so it does not trap heat, restores the shade conditions the rating assumes, and the design file records why.
A sunshade or canopy installed in a hazardous area is part of the installation. A metal shade should be bonded like other metalwork; a large plastic shade can become an electrostatic charging surface, so its material and size need thought.
Common mistakes
- Taking the weather report as the design ambient without the local effects of roofs, enclosures and hot equipment.
- Assuming a high Ta rating covers the sun automatically. It covers hot air, not radiation on the enclosure.
- Forgetting cables and glands. Sun-exposed cables near hot equipment can exceed their rating at the entry point.
Related on excodex
- Ambient temperature range Ta: what -20 °C to +40 °C really means
- Hot surfaces in hazardous areas: luminaires, motors, heat tracing and process heat
- Can Ex equipment be used above +40 °C ambient, for example in the Middle East?
- What is the difference between ambient temperature and process temperature on an Ex device?
- Tools and spaces: Ex marking check, IEC 60079-0, IEC 60079-14
Standards and further reading
- IEC 60079-0:2004, maximum surface temperature measured in still air and corrected to maximum ambient; note on radiated heat, public copy: law.resource.org
- IEC 60079-0 edition 8 (2026), preview with the scope note on the normal ambient range: webstore.ansi.org
- IEC 60079-14:2007 text as AS/NZS 60079.14:2009, selection by ambient temperature including solar radiation: archive.org
- IEC 60079-14:2024, contents with clause 7.4.6 (UV or solar radiation) and Annex L.7 (solar radiation): cdn.standards.iteh.ai
- European Commission, ATEX 2014/34/EU Guidelines, 6th edition (January 2026), surrounding area conditions: single-market-economy.ec.europa.eu
- Hong Kong Observatory, how air temperature is measured at weather stations: hko.gov.hk
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.