If no ambient temperature is marked, what range does the equipment cover?
Abstract
-20 °C to +40 °C. Under IEC 60079-0 up to the 2017 edition, equipment for this normal range needs no ambient marking; any other range must be marked (Ta or Tamb) or covered by an X condition. Edition 8 (2026) requires the range to be marked always.
Equipment with no ambient temperature marking is designed for -20 °C to +40 °C. IEC 60079-0 treats this as the normal ambient range: equipment built for it did not need any extra marking, while equipment for any other range had to show it with the symbol Ta or Tamb, or with an "X" pointing to the certificate. Since edition 8 of IEC 60079-0 (June 2026), the range must be marked on equipment certified to that edition, even when it is the normal one.
Short answer
- No Ta or Tamb on the label, and no X condition about temperature: the rated ambient range is -20 °C to +40 °C.
- Up to the 2017 edition, any range other than -20 °C to +40 °C counts as special. It must appear in the marking, for example
-40 °C ≤ Ta ≤ +60 °C, or, where this is impracticable, the certificate number ends in X and the range is in the specific conditions of use. - The temperature class or maximum surface temperature is only valid within the rated ambient range.
- IEC 60079-0:2026 keeps -20 °C to +40 °C as the normal range but requires the range to be marked on every Ex marking.
- Always confirm on the certificate and instructions; they take precedence over assumptions from the label.
What IEC 60079-0 says
IEC 60079-0 has long required that equipment is normally designed for ambient temperatures between -20 °C and +40 °C, with no additional marking in that case. Equipment designed for a different range is called special: the manufacturer states the range, and the marking must include Ta or Tamb with the range, or the symbol X where marking the range is impracticable. The standard summarises this in a table that pairs "normal" (maximum +40 °C, minimum -20 °C, no additional marking) with "special" (range stated by the manufacturer, marked with Ta, Tamb or X).
Edition 8 (IEC 60079-0:2026, published 16 June 2026, replacing the 2017 edition) keeps the normal range but removes the exemption: the ambient temperature range is now a required marking, even for -20 °C to +40 °C. The list of changes in its foreword also removes the option of replacing the ambient marking by an "X" condition; only where equipment has several ambient ranges and marking them all is impractical are the details given as specific conditions of use. Equipment certified to earlier editions keeps its marking, so labels without Ta will remain common in the field for many years.
Do not confuse this with the -20 °C to +60 °C in the scope of IEC 60079-0. That range, together with 80 to 110 kPa and normal oxygen content, describes the atmospheric conditions under which an explosive atmosphere is assumed; it is not the equipment's rated ambient.
Why the range matters
The maximum surface temperature that sets the temperature class is determined with the equipment at its maximum rated ambient temperature. A T4 box rated for +40 °C has been shown to stay at or below 135 °C with 40 °C around it. At 50 °C its surfaces will run hotter and the T4 rating is no longer demonstrated. Seals, cable glands, plastics and batteries also have temperature limits.
The lower limit matters too. Below -20 °C some plastics and elastomers become brittle, impact resistance changes and seals may lose their tightness. Equipment for cold climates is tested and marked for lower temperatures.
| What the label shows | Rated ambient range |
|---|---|
| No Ta or Tamb, no X | -20 °C to +40 °C |
Ta -40 °C to +55 °C (or -40 °C ≤ Ta ≤ +55 °C) | -40 °C to +55 °C |
| Certificate number ending in X, no Ta (editions up to 2017) | Check the specific conditions of use; the range may be stated there |
| Equipment to IEC 60079-0:2026 | Range always marked |
Example: An Ex d junction box marked II 2 G Ex db IIB T4 Gb, no Ta, no X, is planned for a plant in the Middle East where the shade temperature reaches 48 °C and the box will sit in direct sun. Its rated range ends at +40 °C, so it is not suitable. A box rated, for example, up to +60 °C, possibly with a sun shade, is needed. The opposite case, a unit for an outdoor installation in northern Scandinavia at -35 °C, also falls outside the default range.
Where national rules differ
North American codes use similar but not identical defaults. In the 2017 edition of the US National Electrical Code, Article 500 (Class and Division) takes -25 °C to +40 °C as the normal ambient and Article 505 (Zones) -20 °C to +40 °C. For North American markings, follow the applicable code and listing.
Common mistakes
- Assuming "no Ta" means "any temperature". It means -20 °C to +40 °C.
- Ignoring solar radiation and nearby hot equipment. The ambient around the equipment can exceed the local air temperature.
- Reading the -20 °C to +60 °C of the standard's scope as the equipment rating.
- Forgetting the minimum. Outdoor sites in cold regions often go below -20 °C.
Related on excodex
- Ambient temperature range Ta: what -20 °C to +40 °C really means
- How to read an Ex marking: decoding II 2 G Ex db IIC T4 Gb step by step
- Can Ex equipment be used above +40 °C ambient, for example in the Middle East?
- Does solar radiation count when checking the ambient temperature Ta?
- Tools: Ex marking check; standards space IEC 60079-0
Standards and further reading
- IEC 60079-0:2026 (edition 8), equipment general requirements, including the ambient temperature marking: IEC webstore; list of changes in the preview
- IEC 60079-0:2004, clause on ambient temperature and the table of normal and special ranges (historical text): law.resource.org
- IEC 60079-0:2017 (edition 7), preview of the previous edition: standards.iteh.ai
- NFPA 70, National Electrical Code, Articles 500 and 505.
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.