How to Size a Circuit Breaker for a Motor Properly and Accurately?

If you want to know how to properly and safely size a circuit breaker for a motor, the first thing you should do is follow what is recommended by the NEC, specifically in the various sections of Article 430. You can find that information here if you want to learn how to properly and safely size a circuit breaker for a motor. You can also find the data needed from the motor’s nameplate, online if it is illegible to you.

My tutorial will focus on how you are expected to adhere to these sizing standards while keeping the motor’s type, horsepower, voltage, and size in mind. But before we get into that, let’s quickly review what a motor circuit breaker is.

Table of Contents

What Is A Circuit Breaker For A Motor?

An electromechanical device is known as a Motor Protection Circuit Breaker; MPCB protects a motor from variations in current, overload, and abrupt or unexpected interruptions in the primary electric circuit. Additionally, it protects against phase imbalance or loss and line faults in 3-phase motors. MPCBs save money and space and offer fuseless protection, which immediately shuts off the motor in the event of an overheating or other problem that could cause harm to the motor. If there is a short circuit, it will additionally safely disconnect the current. In addition, the MPCB is generally outfitted with a thermal delay that gives electric motors time to cool down.

How Does an MPCB (Motor Protection Circuit Breaker) Work?

A motor protection circuit breaker serves three essential purposes for the safety of electric motors.

The thermal protection on the MPCB protects the electric motor against overload. The electric motor will eventually be shut off if an expanding and contracting contact detects too much current. A delayed reaction is built-in to account for the strong inrush currents that motors experience when they first start. The thermal contact trips when the inrush current is prolonged for a longer time, and the engine doesn’t start.
Because it cuts off the current when a defect is detected, magnetic protection defends against short circuits and severe electrical problems.
Phase protection provides defense against phase loss or imbalance. When the three live wires of a 3-phase circuit have balanced or equitable voltages, 3-phase circuit motors can operate well. An imbalance reduces the service life and efficiency of the motors in the phase voltage of more than two percent. Further damage may result from an unexpected or sudden loss of phase voltages. The motor may run continuously in this situation, but the current flow in the other two phases may be more than the rated value. In addition, the motor windings might catch fire. When there is an imbalance or loss, a motor protection circuit breaker promptly disconnects the motor. It continuously monitors the variation in phase voltages.

Given the critical function of motor circuit protectors and motor protection circuit breakers, it is crucial to select the right one for your application. It is therefore advisable to speak with experts who can guide you in making the best decision, enabling you to decrease downtime and improve motor efficiency.

You’ll Need These Tools for Proper Sizing for

A copy of the National Electric Code

If you don’t already have one at your disposal, the NFPA may be able to provide you with free access to a similar resource. Otherwise, you might be required to purchase a subscription. You will need a copy of the Code since you will be referring to the tables inside it that provide the necessary information to determine the appropriate motor breaker size for your specific application. These tables can only be found in the Code.

See if the nameplate on your vehicle’s motor already contains the required information to make matters even better. In the first portion of the following section, I will discuss why. Doing a manual computation is yet another step that you can do, which is especially important if you do not possess a copy of the NEC.

Because the correct amperage will be delivered to you, the nameplate approach is the one I recommend for its ease of use. You can verify this information by looking at the tables in the NEC’s Articles 430.32 and 430.52. Despite this, and for the reasons I will explain below, I am not entirely dismissing the possibility of carrying out calculations by hand.

How to Properly Size Breakers for Different Types of Motors

In the end, the following guidelines and procedures can be used to determine the appropriate size of a breaker for a motor:

Examining the information on the motor’s plate will allow you to obtain precise knowledge regarding the horsepower rating of the device. If the maximum currents for both 240V and 480V are already accessible, presuming that you are located in the United States, this should provide you with the precise size you require as expressed in amps.
Suppose you are just aware of the horsepower and voltage. In that case, you can determine the type of motor, whether continuous or non-continuous, and if it requires 125 percent current protection by consulting the table in Article 430.32 of the National Electric Code.
You will also need protection against short circuits, and for this purpose, you will need to refer to the table in Article 430.52. This one depends on the overall load as well as the horsepower of the motor.

Whenever I am determining the appropriate size of breakers for motors, I do not go above these three criteria. These three processes never fail to provide me with the specific sizing information that I require, even though there will be differences in the types of motors and circuit breakers available.

In addition, if you do not live in the United States and are looking for motor circuit breakers ised in sizing calculator, I strongly suggest you utilize the one that Electrical Technology offers. It outperforms any custom circuit breaker used in motor sizing chart I’ve ever seen in my research. You can put it to use in the following ways:

Provide the information that will be used for the calculation. In this particular instance, the breaker size is unmistakably, so go with that.
After that, select the nation and the voltage that corresponds to it that you wish to examine. There is no need to adjust the breaker’s amps or wattage. The bottom portion will provide you with the minimum and recommended breaker sizes. Isn’t it as easy as pie?
Refer to the tables I mentioned if you are unsure of something or want to double-check the result.

What happens, though, if you don’t have a copy of the Code on hand or if you can only recall specific passages of it? Or, let’s say you suddenly discover that you do not have access to the Internet.

Situations like this emphasize how important it is to make calculations by hand when trying to calculate the breaker size for a motor.

When you need precise information, you won’t have to get a copy of the Code or use a breaker size for a motor calculator like you would if you didn’t have this skill. This is one of the many apparent benefits of having this skill. It could also come in handy if you’re employing a different kind of circuit breaker, such as an inverted time breaker, which has an overall capacity equivalent to 250 percent of the load’s total capacity.

How to Perform a Hand Calculation to Determine the Appropriate Size of a Circuit Breaker for a Motor

When performing calculations by hand, there are a lot of different things that need to be taken into mind. These are the following:

The motors and horsepower.
Whether the motor is a three-phase or a single-phase supply
Power factor
Efficiency
Type of circuit breaker used

Calculate the pullout current first

For this, a particular formula is required. Let’s assume that the motor has 25 horsepower. Its voltage is 220V, and its efficiency is 90% with a power factor of 0.8. We’ll arrive at the following equation using these values as a starting point:

I = (25 x 746) / (√3 x 220 x 0.8 x 0.9)

67.98 amps is the total pullout current.

Find out the precise ampere rating for the circuit breaker you’re using.

If you’re utilizing an inverse time circuit breaker, you must adhere to the formula shown below:

I = 67.98 amps multiplied by 250

As a result, the overall amperage is 169.95. Assuming you haven’t already committed to memory the table in section 240.6 that details the ampere ratings pertinent to inverse time circuit breakers, you will undoubtedly need to consult the Code.

We will therefore need to utilize a 175A breaker because the Code specifies that it should always be somewhat greater than the former. After all, our computation put us near a 170A rating.

This again emphasizes how crucial it is to consider the circuit breaker you’re employing because it might significantly alter the size you’ll need for your particular needs.

Additional Considerations to Bear in Mind

Keep in mind that the motor of your particular piece of equipment may significantly impact the ultimate size of the circuit breaker and the kind that should be used. For instance, the National Electrical Code (NEC) stipulates in article 440.22 that circuit breakers for air conditioners cannot have a rated load current greater than 175% of the motor’s rated load current.

It is also determined by the total starting current of the motor whether or not an increase in the percentage of the rated load current should be made. It is possible to boost the overall load current by 225 percent for alternating machines with high starting currents.

Conclusion

Have you found my article on how to determine the appropriate circuit breaker size for a motor to be helpful? If you have any ideas on the matter, please leave them in the comments box below. I’d value hearing from you. If you did enjoy it, you could make my day and the day of others if you shared this post with others who you believe will find the information to be helpful.

Regarding the sizing of motor breakers, you should remember that the NEC recommendations cannot be beaten. It is necessary to ensure that a particular electrical system continues to function correctly while guaranteeing the complete configuration is risk-free. The information embedded on the nameplates also applies to a greater or lesser extent.

For more details on circuit breaker used in motors or other electrical parts, contact us at ICRFQ. We manufacture the best electrical components in China.