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When selecting the right AC Moulded Case Circuit Breaker, most engineers initially focus only on the rated current (In). However, correct sizing is not just about choosing a value from the nameplate. Environmental factors, specifically ambient temperature and installation altitude, have a major influence on performance.
If MCCB derating is ignored, the consequences can be costly:
Overheating of internal contacts.
Nuisance tripping (breaker trips below its rated load).
Shortened MCCB lifespan.
Insulation breakdown risk.
In this guide, Nuomak explains the science behind derating, focusing on temperature and altitude correction factors, so you can size your breaker correctly for any environment.
Derating means adjusting the rated current of a device to a lower, allowable current level based on specific installation conditions.
The nominal rating of a standard AC Moulded Case Circuit Breaker is typically tested under strict reference conditions:
Reference Temperature: Usually 30°C or 40°C (depending on the standard, IEC 60947-2).
Reference Altitude: Sea level (below 2000 meters).
Once the operating environment exceeds these limits, the actual safe operating current decreases. Derating is the mathematical way to ensure your electrical safety margins are maintained.
Why Temperature Affects Performance
Most standard AC MCCBs use thermal-magnetic trip units. The “thermal” part relies on a bimetallic strip that bends when heated by current.
Higher Ambient Temperature: The bimetallic strip is “pre-heated” by the environment. This causes the breaker to trip faster or at a lower current than its rating.
Risk: If you don’t derate, a 100A breaker might trip at 90A in a hot panel (50°C), causing downtime.
Key Takeaways for Temperature
Higher Temp = Lower Usable Current.
Thermal trip units are highly sensitive to panel internal temperature (not just room temp).
Solution: For environments with extreme temperature fluctuations, consider using Nuomak’s AC Adjustable Current MCCB. These electronic breakers use microprocessors and are less sensitive to ambient heat.
Why Altitude Matters
At higher altitudes (mountains or plateaus), air density decreases. This affects an AC moulded case circuit breaker in two critical ways:
1.Reduced Cooling: Thinner air carries heat away less efficiently, leading to potential overheating.
2.Reduced Dielectric Strength: Thinner air is a poorer insulator. This increases the risk of arcing and reduces the voltage withstand capability.
Altitude Rules
Below 2000m: No derating required.
Above 2000m: You must apply correction factors for both Current and Voltage.
To determine the final usable current of an AC MCCB, you must combine both correction factors.
The Derating Formula
Actual Safe Current = Rated Current × Temperature Factor × Altitude Factor
Rated Current (In):The value on the MCCB nameplate.
Temperature Factor (Kt): Found in the manufacturer’s temperature chart.
Altitude Factor (Ka): Found in the manufacturer’s altitude chart.
Calculation Example
Let’s say you have a 100A Nuomak MCCB installed in a mining site at 2500m altitude, with a panel temperature of 40°C. (Assuming the reference temp is 30°C).
| Parameter | Value | Correction Factor (Example) | Calculation / Explanation |
| Rated Current | 100 A | 1.0 | Base Rating |
| Ambient Temperature | 40°C | 0.91 (Kt) | From Temperature Derating Chart |
| Altitude | 2500 m | 0.95 (Ka) | From Altitude Derating Chart |
| Final Derated Current | 86.45 A | – | 100 × 0.91 × 0.95 |
Result: In these conditions, your “100A” breaker can only safely carry 86.45A. If your load is 90A, you must upgrade to the next frame size (e.g., 125A).
When facing harsh environments, proper selection is essential.
1.Verify the Reference: Check if the manufacturer’s reference temp is 30°C or 40°C.
2.Avoid Oversizing Blindly: Don’t just pick a huge breaker. Calculate the precise derating to ensure the cable is still protected.
3.Go Electronic: For high-altitude or hot regions, Electronic MCCBs (Adjustable type) offer better stability and often require less temperature derating than thermal-magnetic ones.
Nuomak supplies AC Moulded Case Circuit Breakers designed for harsh environments and compliant with IEC 60947-2 standards.
Does humidity affect AC MCCB derating?
Humidity generally does not affect the current rating calculation directly, but high humidity coupled with temperature changes can cause condensation and corrosion. Proper enclosure protection (IP rating) is recommended.
What is the standard reference temperature for Nuomak MCCBs?
Nuomak AC MCCBs are typically rated at a reference ambient temperature of 30°C or 40°C (check specific datasheet). This serves as the baseline (Factor = 1.0) for all calculations.
Do electronic MCCBs also need altitude derating?
Yes. While electronic trip units are less sensitive to temperature, altitude still affects the physical cooling and dielectric strength of the device. Therefore, altitude correction must still be applied for AC Adjustable Current MCCBs.
Can temperature and altitude derating be combined?
Yes, they must be combined. As shown in the formula above, you multiply both factors to find the safe operating limit.
Correct sizing of an AC Moulded Case Circuit Breaker is not simply choosing a nameplate rating. By considering ambient temperature and installation altitude, you ensure reliable operation and extended service life.
Applying the correct derating factors allows your Nuomak breaker to perform safely in the real world, from hot industrial panels to high-altitude mining sites.
Ready to select the right MCCB?
Contact Nuomak for technical support, detailed derating charts, and expert selection guidance.
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