Circuit Breaker Derating: How Ambient Temperature Affects Sizing

You should think about how heat or cold changes electrical systems. Hot or cold weather can affect how a circuit breaker works. If you do not pay attention, the breaker might trip for no reason. It could also cause equipment to stop working. Circuit Breaker Derating helps you pick the right breaker size. This keeps things safe and reliable. Using the correct size protects your equipment. It also helps your facility keep working.

Key Takeaways

Ambient temperature changes how circuit breakers work. Hot air makes breakers trip faster. Cold air lets them carry more current.
Derating is needed if it gets hotter than 40°C. Pick a breaker with a higher rating or lower the load to stay safe.
Use manufacturer charts to find derating factors. These charts show how much current your breaker can handle in different temperatures.
Always think about where the breaker is put. High temperatures can cause overheating. Low temperatures can make parts stiff.
Follow rules like NEC and what the manufacturer says for sizing. This keeps your electrical system safe and following the rules.

Ambient Temperature and Circuit Breaker Sizing

Temperature Effects on Breaker Performance

It is important to know how temperature changes affect circuit breakers. When the air gets hotter, the breaker reacts faster. It trips at lower currents. This means power can shut off even if you do not use more electricity. Cooler air lets the breaker handle more current before it trips.

Hotter air makes the breaker trip more easily.
Colder air lets the breaker carry more current.
If the temperature goes up by 10°C, the breaker can handle about 8% less current. This change affects how much electricity your breaker can safely carry.

Heat can cause other problems, too. Your breaker might trip sooner than you think. Overheating is a big reason for unwanted tripping. The bimetallic strip inside bends more when it gets hot. This bending makes the breaker trip more often.

Here is a quick look at how temperature affects breaker reliability:

Temperature Condition	Impact on Circuit Breakers
High Temperatures	Breaker carries less current, so you need derating.
Low Temperatures	Parts inside can get stiff or break easily.

Why Derating Is Needed

Circuit Breaker Derating keeps your electrical system safe. If the air around your breaker is hotter than 40°C, you need to derate it. You should pick a breaker with a higher rating or lower the load. If you do not, the breaker may not protect your system. Too much heat can hurt your equipment or even start a fire.

You need to derate breakers when it is hotter than 40°C.
High temperatures can cause overheating and make breakers fail.
Circuit Breaker Derating helps your system stay safe and work well.

For motor protection, you must derate breakers if it is hotter than 65°C. Always check the temperature where you put your breaker. This step helps you choose the right size and keeps your system safe.

Circuit Breaker Derating Principles

Standard Calibration Temperatures

You need to know the calibration temperature for your circuit breaker. Companies like Square D set a standard temperature. Most circuit breakers use 104°F as their standard. In the past, 77°F was used more often. The calibration temperature changes how the breaker trips. If you put a breaker in a place hotter or colder than this temperature, you must change the size.

Calibration Temperature	Description
104°F	Most circuit breakers use this as the standard.
77°F	This was the old standard before 104°F became common.

You should always look at the manufacturer’s instructions. Square D and NEMA AB 3 say to ask the manufacturer if the temperature goes above or below their limits. This helps you pick the right size and keeps your system safe.

The 10°C Rule and Bimetallic Strips

Circuit Breaker Derating depends on how the breaker senses heat. Most breakers have a bimetallic strip inside. This strip bends when it gets hot from electricity. The bending makes the breaker trip. The strip has two metals that grow at different speeds. This lets the breaker handle small overloads without tripping right away.

Bimetallic strips bend because the two metals expand differently when heated by electricity.
The bending is set to trip the breaker at certain current levels, usually needing a temperature rise of 50–100°C.
This design lets the breaker ignore small overloads, so it does not shut off too soon.

You can use the 10°C rule to guess derating. For every 10°C above the calibration temperature, the breaker can carry about 8% less current. If the temperature goes down, the breaker can carry more current before it trips. The bimetallic strip reacts to these changes, so you must change your calculations.

Tip: Always check the air temperature where you put your breaker. This helps you use the right derating factor and keeps your system working well.

Derating Factors and Reference Ranges

You must use derating factors when sizing circuit breakers in hot or cold places. Manufacturers give charts and tables to help you. If the temperature is much higher than the calibration point, you should use a derating factor. For example, a factor of 0.8 means the breaker can only carry 80% of its rated current. In very tough conditions, you may need to use a factor as low as 0.7.

Here is how you use derating factors:

Find the breaker’s rated current.
Multiply by the derating factor, like 0.8 or 0.7.
Use this new number to size your breaker for the real environment.

Ambient Temperature	Typical Derating Factor	Application
40°C (104°F)	1.0	No derating needed
50°C (122°F)	0.8	Use for high temperatures
65°C (149°F)	0.7	Use for very tough conditions

Circuit Breaker Derating helps you stop unwanted tripping and damage. You should always use manufacturer charts and follow rules like NEMA AB 3. If you are not sure, ask the manufacturer for help. This keeps your electrical system safe and working.

Calibration temperatures are very important for how well circuit breakers trip, especially in thermal trip units. The bimetallic strip’s sensitivity, which is needed for tripping, changes with the temperature around it. In hotter places, the strip may trip faster, causing the breaker to shut off at lower currents. In colder places, the strip may wait until higher currents to trip, which can slow down needed protection.

Applying Derating in Practice

High Temperature Scenarios

Sometimes, you need Circuit Breaker Derating in hot places. Heat makes your breaker carry less current safely. If you forget this, the breaker might trip too early. It could also fail to protect your equipment.

In a data center, you may need 12 kilowatts of AC to DC power in a rack at 65°C. The MercuryFlex system gives about 2,900 watts at this heat. You would need more than one unit to get enough power.
The LiquaBlade system uses liquid cooling. It can give 16.5 kilowatts and does not have as many problems with heat.
At high altitudes, the air is thinner. At 5 kilometers, the system can lose up to 45% of its power. You must change your setup to keep things safe.

Manufacturers give you derating curves. These curves show how much current your breaker can handle as it gets hotter. Always check these charts before picking a breaker. The curves are made in labs, so add a safety margin for real use.

Tip: Pick breakers made for hot places. Give yourself extra space in your numbers to avoid trouble.

Aspect	Description
Temperature Distribution	You need to watch the temperature to keep breakers working.
Impact of Abnormal States	Too much heat can hurt insulation and make breakers wear out faster.
Monitoring Importance	Look for signs of wear to stop failures and plan repairs.

Low Temperature Scenarios

Cold air helps your breaker carry more current. The bimetallic strip inside starts out cold, so it needs more heat to trip. This means your breaker is less sensitive and can handle bigger loads before it shuts off.

Think about the air temperature when you design your system.
In very cold places, electronic trip breakers work better, but small systems may not have them.
Wires can also carry more current when it is cold.

If you use your breaker in cold places, you may not need to derate it. Sometimes, you can use a bigger load, but always check with the manufacturer.

Note: Cold can make parts stiff. Always test your system to make sure it works right.

Calculating Derated Current

You need to know how to find the derated current for your breaker. This helps you pick the right size and keeps your system safe.

Here is an easy way to do it:

Find the breaker’s rated current.
Use the derating factor from the manufacturer’s chart.
Multiply the rated current by the derating factor.

For example, if your breaker is rated for 63 amps and the derating factor is 0.7 for hot places:

Derated Current = Rated Current × Derating Factor
Derated Current = 63 A × 0.7 = 44.1 A

You can use tables to help you:

Condition	Derating Coefficient
Ambient temperature (above 40°C)	1.5% per 1°C
Altitude (above 2,000m)	0.5% per 100m
High-temperature environments	0.7
High-density installations	Minimum 0.8

If you have a breaker rated for 32 amps and need to use a factor of 0.6:

Derated Current = 32 A × 0.6 = 19.2 A

Tip: Always use manufacturer derating charts and follow the rules. This keeps your system safe and helps you avoid mistakes.

Remember, Circuit Breaker Derating is not just about math. You need to think about the real place, the breaker type, and the load. Always ask the manufacturer if you are not sure.

Standards and Best Practices

Key Standards (NEC, IEC, NEMA AB 3)

You must follow rules when picking circuit breakers for different temperatures. These rules help keep your electrical system safe and working. Here are some main ideas:

Circuit breaker derating depends on the air temperature around it. Hotter air means the breaker can handle less current and may trip faster during problems.
Most derating charts use 30°C (86°F) as a starting point.
NEC Article 110.14 says to use the lowest temperature rating of any device when you figure out how much current wires can carry.
The NEC gives rules to help you use circuit breakers safely in many places.

Here is how these rules work:

Rule Type	Description
Lowest Equipment Temperature	Use the lowest temperature rating of any equipment in the circuit.
Ampacity-Based Rule	For circuits 100A or less: Use the 60°C column for wire size. For circuits over 100A: Use the 75°C column.
Temperature Correction Factors	Use these if the air is hotter than 30°C (86°F). Check Table 310.15(B)(1)(1) for changes.

Manufacturer Guidelines

Manufacturers give special advice for sizing circuit breakers in hot or cold places. Always check these tips before you pick a breaker.

High temperatures lower how much current a breaker can handle. Pick breakers made for hot places.
Manufacturer charts show how much current a breaker can carry at different temperatures.
Use these charts to stop unwanted tripping or damage.

Steps for Proper Sizing

You can follow easy steps to pick the right circuit breaker when temperatures change. This keeps your system safe and working.

Know what kind of load you have.
Check the voltage and frequency your system needs.
Choose the right current rating for your breaker.
Think about temperature, humidity, and other things in the area.
Test the breaker to make sure it works well.

Temperature changes how much current a breaker can handle. Look at this table to see how ratings change with temperature:

In (A)	-10°C	0°C	10°C	20°C	30°C	40°C	50°C
6	7.92	7.44	6.96	6.48	6	5.52	5.04
16	21.1	19.8	18.6	17.3	16	14.7	13.4
20	26.4	24.8	23.2	21.6	20	18.4	16.8
32	42.2	39.7	37.1	34.6	32	29.4	26.9
40	52.8	49.6	46.4	43.2	40	36.8	33.6
63	-	-	-	-	63	58	52.9

You need to size circuit breakers the right way to stop fires, protect equipment, and follow the rules.

Importance of Correct Sizing	Description
Preventing Fires	Too much current can make things too hot and cause fires.
Protecting Equipment	The right size breaker keeps equipment safe from too much current.
Compliance	Following the rules makes sure your system is safe and legal.

Tip: Always use manufacturer charts and follow standards like NEC, IEC, and NEMA AB 3. This helps you use Circuit Breaker Derating the right way and keeps your system safe.

You have learned that the air temperature affects how much current a circuit breaker can handle. Derating helps keep your system safe and working well. If you follow the rules and listen to the manufacturer, your equipment stays safe and you can stop unwanted shutdowns.

Derating puts less stress on parts and helps stop things from breaking early.
It helps your electrical system last longer and makes it easier to plan repairs.
Use these steps for every project. This way, you will make electrical systems that are safer and last longer, no matter where you use them.

FAQ

What does circuit breaker derating mean?

Circuit breaker derating means you lower how much current the breaker can handle. You do this when it gets very hot or cold. This helps keep your electrical system safe. It also stops the breaker from tripping when you do not want it to.

When should you apply derating to a circuit breaker?

You need to use derating if the air is hotter or colder than the breaker’s set temperature. Most companies say to derate when it is over 40°C (104°F).

How do you find the correct derating factor?

Look at the manufacturer’s chart or table for the right number. Multiply the breaker’s current by this number to find the safe amount.

Does altitude affect circuit breaker derating?

Yes, altitude changes derating. When you go higher, the air gets thinner. This makes cooling harder. You need to use a special number for altitude. You can find it in the manufacturer’s guide.

Can you use the same breaker in hot and cold places?

You should not use the same breaker everywhere. Pick a breaker made for the temperature in your area. Always read what the manufacturer says before you install it.