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M & R ELECTRICAL WHOLESALING

Knowledgebase Article

How to correctly size kA ratings of circuit breakers

 

Introduction

One of the most important safety aspects to consider when designing an electrical circuit is to correctly size the circuit breaker fault current rating (commonly called the kA rating). Unfortunately many electricians are unaware of this concept, and it remains one of the most common design faults found in electrical circuits. In the last 10 years it has become very common in domestic wiring, because of changes in the way power is distributed to new houses.

 

So what is the kA rating?

The value of the kA rating determines how much current the circuit breaker can withstand under fault conditions. The circuit breaker only has to withstand this for a brief period of time, usually the time it takes for the circuit breaker to trip. For example, a value of 6kA means that the circuit breaker can withstand 6,000 amps of current during the brief time it takes to trip.

 

Why is the kA rating so important?

Under fault conditions (such as a short circuit) much more current flows through the circuit than what it was designed for. A circuit that was designed for a maximum of 20A may suddenly be drawing hundreds, if not thousands of amps. The circuit breaker will trip if this occurs.

 

However, what if during a short circuit there is more current flowing through the circuit than the kA rating of the circuit breaker? In this case the circuit breaker will fail, often in either one of two ways. One possibility is that the contacts in the circuit breaker will weld, thus preventing the circuit breaker from tripping. The best case scenario for this is that the cable in the circuit is damaged. The worst case is that a fire is started. Another possibility is that the circuit breaker explodes, as a result of the copper in the circuit breaker overheating and turning into dangerous plasma. This could be very dangerous to people nearby, for example the electrician turning the circuit breaker on after a fault.

 

How to calculate the correct kA

The maximum amount of current that can flow through a circuit is determined by the size of the transformer feeding the circuit and the length of the cable run from the transformer. This is often called the downstream short circuit current. This will determine the maximum kA rating required for the main circuit breaker.

 

For example, a typical 500kVA transformer has a short circuit current of 35kA at its terminals. The cable run from the transformer to the main breaker is 10m and is run with 90mm2 cable. The resistance in the cable limits how much current comes from the transformer, and so after calculations it was determined that the short circuit current at the end of the cable would be 26kA. In this case, a 20kA circuit breaker cannot be used in the installation.

 

It is outside the scope of this document to show how to calculate the short circuit current, but tables for this can be found on the internet. Another source is page 9-20 of the NHP Circuit Breaker Products catalogue.

 

Cascading

Fortunately, not every circuit breaker in the installation needs to be rated above 26kA. Cascading is what happens when you place a smaller kA rated circuit breaker on the load side of a larger kA rated circuit breaker (for example, a 6kA circuit breaker downstream from a 20kA circuit breaker). In these cases, the larger circuit breaker limits a certain amount of the fault current, thus enabling you to safely use smaller rated circuit breakers downstream.

 

You can determine what size circuit breakers can be cascaded from the manufacturers. These are usually listed as cascade tables. You need to consult these tables, because you can't just use any smaller breaker size downstream. For example, the cascade tables may show that you can use a 6kA breaker downstream from a 20kA breaker. However, you probably can't use a 3kA breaker - the 20kA breaker just doesn't provide enough protection.

 

 

Most common mistakes

The most common mistake on large installations is that the kA rating of the circuit breaker was not taken into account when designing the circuit. Instead, the cheapest circuit breaker is chosen that meets the required standard current draw. The author has seen this himself a number of times (in fact, the example used above in the document did actually occur - a 20kA circuit breaker was used on the site when it needed a minimum rating of 26kA).

 

It is becoming more and more common for larger transformers to be used to power domestic installations. This has created a situation where the short circuit currents are much higher than they used to be. A typical transformer to power a street of houses may have been 100kVA. Now it is not uncommon to see 300kVA or even 500kVA transformers. This can result in short circuit currents of 20kA and above. To make matters worse, a number of well known manufacturers sell cheap low quality circuit breakers that have a rating of only 3kA. Electricians are continuing to use these to wire these without understanding the full ramifications of their decision.

 

Conclusion

The kA rating of a circuit breaker is a very important safety aspect to consider when designing a circuit. Without it, there is a good chance that a serious accident will occur. It only takes a few minutes to do the calculations when you have the correct tables.

 

The author has personally seen an incident where an incorrect kA rating caused a man to receive burns to 60% of his body. It could happen to you. Be safe and always consider the kA ratings in your design.