Myth: A Circuit Breaker Rated 100A Will Always Carry 100A Safely
Per IEC 60947-2, circuit breaker rated current assumes 40°C ambient in open air. In enclosed switchboards at elevated temperatures, actual continuous capacity can drop to 70-80A. Temperature derating, enclosure heating, and mounting orientation all reduce the nameplate rating.
The Myth
“It says 100A on the breaker. It carries 100A. End of story.”
If only it were that simple. The rated current printed on a circuit breaker is a nominal value tested under specific, standardised conditions. Change those conditions — as every real installation does — and the actual continuous current capacity changes with them.
The Reference Conditions Nobody Reads
Per IEC 60947-2, Clause 8.3.2.5, the rated current (In) of a circuit breaker is verified at a reference ambient temperature of 40°C, with the device mounted in open air (not in an enclosure), in the vertical orientation specified by the manufacturer.
Real installations differ in every one of these conditions:
| Condition | Reference (Test) | Typical Reality | Effect on Capacity |
|---|---|---|---|
| Ambient temperature | 40°C | 45–55°C inside switchboard | −5% to −20% |
| Enclosure | Open air | IP54/IP65 enclosed switchboard | −5% to −15% |
| Adjacent devices | Single device tested | Multiple breakers, contactors, busbars | −5% to −10% (mutual heating) |
| Mounting orientation | Vertical (as specified) | Sometimes horizontal | −0% to −10% |
These effects are cumulative. A 100A MCCB in a fully loaded, enclosed switchboard at 50°C internal ambient may carry only 70–80A continuously without exceeding its thermal limits.
The Derating Data Buried in the Catalogue
Every reputable manufacturer publishes temperature derating curves for their circuit breakers. They are typically on page 47 of a 200-page catalogue, in 8-point font. Nobody reads them until something trips unexpectedly on a hot afternoon.
Representative derating for a typical 100A thermal-magnetic MCCB:
| Ambient Temperature | Continuous Current Capacity | Percentage of In |
|---|---|---|
| 30°C | 107A | 107% |
| 40°C (reference) | 100A | 100% |
| 45°C | 95A | 95% |
| 50°C | 88A | 88% |
| 55°C | 82A | 82% |
| 60°C | 75A | 75% |
Note: the ambient temperature here is the air temperature surrounding the breaker inside the enclosure, not the room temperature outside the switchboard. A switchboard in a 35°C plant room with poor ventilation and full load on adjacent devices can easily reach 55°C internally. At 55°C, that 100A breaker carries 82A before it starts to thermally age or nuisance-trip.
The Reality: Design for the Actual Conditions
The correct engineering approach:
- Determine the actual internal ambient temperature of the switchboard. For existing boards, measure it. For new designs, calculate it per IEC 61439-1, Clause 10.10 (temperature rise test) or use the manufacturer’s thermal modelling tools.
- Apply the manufacturer’s derating curve to determine actual continuous capacity at that temperature.
- Account for adjacent device heating. The manufacturer’s thermal test for switchboard assemblies (IEC 61439-1, Clause 10.10) considers adjacent devices. If designing a new panel, use the assembly manufacturer’s verified power loss data.
- Select the breaker size with margin. If the derated capacity of a 100A breaker at your conditions is 82A, and the load is 80A, you are at 97.6% of actual capacity — too close. Select the 125A frame, which at the same derating gives 103A actual, providing a workable margin.
The nameplate rating is a starting point for selection, not the final answer. The final answer comes from the derating curve and the actual installation conditions.
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