Every current rating on a low voltage assembly is conditional. The condition is written into the standard, it is reasonable for the climate the standard was drafted in, and it is the single assumption most often carried unexamined into a Saudi switchroom.
IEC 61439-1 sets normal service conditions for the assemblies it covers. For indoor assemblies the ambient air temperature runs from −5 °C to +40 °C, with the average over 24 hours not exceeding +35 °C. For outdoor assemblies the range is −25 °C to +40 °C. Altitude is limited to 2000 m. Anything outside those conditions is a special service condition, to be agreed between the manufacturer and the user.
Those are the conditions the verification is performed under. They are not a prediction about your site.
Rise, not temperature
The verification in clause 10.10 is of temperature rise — how far above the surrounding air a part gets. The limits are expressed in kelvin, and they exist because the absolute temperature reached by a terminal, a busbar joint or a device determines the life of the insulation around it.
So the arithmetic is unforgiving and entirely linear. A part verified to rise 70 K sits at 105 °C in 35 °C air. Put the same assembly in 50 °C air at the same current and the same part sits at 120 °C. Nothing about the assembly has changed. The margin to the insulation limit has simply been spent by the room.
Where the capacity actually goes
Three mechanisms stack, and they are not alternatives.
- The assembly. The verified rating holds at the declared ambient. Above it, the current that produces the verified rise is lower. The relationship is not linear in the convenient direction — losses go with the square of current, so recovering headroom costs more copper than intuition suggests.
- The devices. Moulded-case and air circuit-breakers carry their own ambient derating, set by the thermal element and by the manufacturer’s curve. A frame that is comfortable at 40 °C may be at its limit at 55 °C inside the compartment.
- The arrangement. A higher form of separation restricts convection, and so raises the in-enclosure air temperature at the same load. Form and ambient are not independent variables, which is why they have to be settled together and before the layout freezes.
Forced ventilation is not the answer to any of this in a specification sense. A well-written specification requires that the assembly meet its ratings by natural ventilation — the IOGP JIP33 specification for low voltage switchgear states exactly that, and requires continuous ratings after derating for the specified service conditions. Fans are a maintenance item and a single point of failure; a rating that depends on them is a rating that expires when a filter blocks.
Altitude, briefly
Above the standard’s 2000 m reference, two things move at once. Air is a poorer coolant, so temperature rise increases at the same current. Air is also a poorer insulator, so dielectric withstand falls and clearances that were adequate at sea level may not be. Highland sites in the Kingdom can sit above that line. If yours does, it is a special service condition and belongs in the enquiry, not in a discovery during commissioning.
How to specify it so the bids are comparable
| State | Why the bid is unusable without it |
|---|---|
| The design ambient at the switchroom, from site data | Without it every bidder assumes 35 °C and the cheapest bid is the one that assumed hardest |
| Whether the room is air conditioned, and what happens when the cooling fails | A rating that depends on HVAC is a rating that depends on another contractor |
| Site altitude where above 2000 m | Changes both cooling and dielectric withstand |
| The form of separation, in the same breath | Form and in-enclosure temperature are the same conversation |
| The assumed loading of the outgoing circuits | Temperature-rise verification is performed against a loading pattern, not against every circuit at full load |
| That ratings shall be continuous, by natural ventilation, after derating | Removes fans from the rating and makes two bids comparable |
Where we stand
We quote against the design ambient you give us, and where none is given we ask rather than assume. Where the stated ambient takes an arrangement outside its verified envelope, the proposal says so and shows the options — a larger enclosure, a lower form, more copper, or a lower declared rating — with the price consequence of each. The declared ambient and the loading basis behind every rating are stated on the proposal, not buried in the verification file.