How to size wire and breakers?

For sizing wire and breaker, you need to divide the total watts by the voltage to get the amperage on the circuit. Depending on the ampacity of the breaker, each size of breaker requires a different wire size. As a result, you can select the appropriate breaker and wire size based on the amp requirements.

How to Determine the Correct Circuit Breaker Size?

The National Electric Code states that having a circuit breaker is a fundamental requirement. Using the appropriate breaker size is another recommendation made by the Institute of Electrical and Electronics Engineers (IEEE). Circuit breakers are a requirement for all electrical circuits in commercial, residential, and industrial settings. To avoid electrocution and the spread of electrical fires, this requirement is justified. However, you are not permitted to use any circuit breaker in any circuit. You must use the proper size in accordance with the electrical currents and appliance specifications to ensure safety.  The circuit breaker should be the following size:

  • Add up the combined wattage of all the circuit's appliances. These include constantly connected electronics like lights and refrigerators. By looking at the labels attached to each device, you can quickly determine how much power it consumes. You will then be given Total Watts.
  • Divide Total Watts by Operating Voltage of the Devices to convert the indicated power to current.
    The majority of appliances are either 120V or 240V voltage rated. During the calculation, pick the suggested rating and it is Amps = Total Watts / Volts 
  • After determining the total current drawn by the appliances, multiply your result by 125 percent.
    Your ideal circuit breaker rating needs to be higher than the result of the calculations and it is, Amps x 1.25

20 amps are needed for the majority of the electronics in residential circuits. So a circuit breaker with this ampacity is perfect for household circuits. However, based on the appliances, you can also use larger ones. Take notice of the amperage on the handle of the circuit breaker as well. Your objective should be to run appliances at a safe load, which is no more than 80% of their capacity. To determine this limit, multiply the amperage of the circuit breaker by 80%. The safe load can still be exceeded by devices on a circuit. But you must be careful that this only lasts a short while. Otherwise, you run the risk of blowing the appliances and endangering the safety of the system.

How to Determine the Optimal Circuit Breaker Size

Let's say you want to install four 40-watt lights, six 60-watt lights, and three 80-watt lights. Consider a single-phase 120V residential lighting circuit with a total power consumption of 760 Watts. What kind of circuit breaker is the best for such a circuit?

760 Watts / 120V = 6.33 Amps

The circuit breaker size must be at least 125% larger than the current flowing through the circuit.
Therefore, a 15 amp circuit breaker is more than adequate. 

1.25 times 6.33 amps equals 7.91 amps

What is the mechanism of the circuit breaker system?

Electricity travels to the breaker box after entering your home. Most home appliances typically draw a small current of 20 amps or less. A circuit breaker is necessary for such a circuit to regulate electricity flow. The requirement for breaker and wire size increases as the current demands of the appliances increase. The wires supplying the current and the circuit breaker are one unit. If you pass a current through these components that is greater than their resistance, the circuit will trip. The wires will likely deteriorate and melt if there is an excessive current flow.

There is a chance of a fire starting once they are exposed. The circuit breaker is required at this point. Together, the wires and the circuit breaker control how much current flows through the circuit. The circuit breaker permits the passage of electricity when the electric current falls below its rating. But when the current exceeds the circuit breaker's rating, the circuit breaker trips and cuts off the flow of electricity. As a result, the system's appliances are shielded from damage by a circuit overload.

The basic rule is to select a circuit breaker that has a current size that is 25% greater than the circuit's current. For instance, a breaker with a similar rating will flip at 40 amps if the circuit carries 40 amps. Therefore, it will not be possible to use a circuit breaker if the appliances require 40 amps. As a result, you need a breaker with a higher rating. However, it would be best if you avoided using a very high-rated breaker because doing so would endanger the operation of the applied components.

For instance, it would be improper to use a 100 amp breaker on a circuit carrying 40 amps. A circuit would not be shielded from an overload by such a circuit breaker. When the current exceeds the 40 amp limit, the wires will melt, which could cause a fire accident. Additionally, if the breaker is significantly oversized, you risk damaging appliances.

How are breaker wire size and wattage calculated?

Regardless of the size of the circuit breaker you are using, you must also use the proper wire size. The size of the wire is important because it affects how much electricity flows through the system. It also establishes the wires' resistance to the amount of electricity that flows through them. You must first determine the wire's American Wire Gauge in order to calculate its size and wattage (AWG).

ASTM International, a global standards organization, provides the AWG as a standard wire size measurement. The standard measurement of the ideal wire size for a circuit breaker is the diameter of the wire, also known as the gauge. Additionally, you must comprehend wire size. Consider how AWG measures range from high to low. The size of the wire decreases with increasing gauge number. A thicker wire will unquestionably be able to handle a higher ampacity than a thinner one.

Additionally, you must correctly interpret the wire size ampacity chart provided by National Electric Code when performing calculations. Keep in mind that if you select the incorrect wire gauge, the breaker's functionality will be impacted. Additionally, choosing the incorrect wire size can result in melting during a current surge, sparking an electrical fire. Therefore, consider the size of the electric current you intend to use when selecting the appropriate wire gauge. 


How to Measure the Size of the Wire and Breaker?

To prevent common accidents, selecting the proper wire and breaker size is essential. Don't forget to first verify the appliance amp requirements. As was already mentioned, the power rating label on the appliances makes it simple to find this information. You need to convert it from Watts, which is the primary unit, to amperes. To find the ampacity, divide the power (Watts) by the required voltage.

Calculate the safe load to choose the appropriate breaker size once you know the combined ampacity of the appliances. The second step is to choose the wire size based on the preferred circuit breaker's current rating.  AWG and circuit breakers are required for each set of appliances. Think about a 10 Amp ampacity, for instance. This is adequate to run typical home appliances like a refrigerator, dishwasher, and washing machine. Two easy steps will help you determine the wire and breaker sizes.

  • You must first determine the safe load. Keep in mind that the load should be at least 125% of the amp circuit breaker. The best breaker size in this situation is 10A X 125%, or 12.5A. Take note that different breaker size ranges are available, such as 15A, 20A, 30A, 40A, and 50A. Therefore, a 15A breaker would be ideal in our situation.
  • The right wire size must then be determined in order to match the chosen breaker. A wire gauge of #14 is required for a maximum ampacity of 15 A.

As a result, you require a 15A breaker and #14-gauge wire for a 10A current system. The two steps mentioned above can be used to determine the breaker and wire size for other current systems. 

What are the Main Factors Affecting Wire Size?

A few crucial wire size considerations must be correct.

  • First, choose a wire size that is larger than what is required for ampacity. According to this logic, your appliances will be safer the larger the wire size.
  • Use the largest wire size possible if you want to transmit current over a distance greater than 100 feet, especially if the wire is inside of a conduit or another enclosure with poor heat dissipation.
  • Additionally, it would be best if you were particular about the type of wire you use. Copper wires predominate.
  • Larger gauge feeder wires also come with stranded aluminum and copper-clad wires. Since aluminum wires differ significantly from copper wires, it would be best to exercise caution when using them.
  • Aluminum has a conductivity that is 61% that of copper. Aluminum is still 30% lighter. Therefore, exercise caution when selecting the aluminum's gauge.
  • It might not promise copper's level of efficacy.

Is a smaller or larger size better when it comes to wire size?

The wire size requires extreme caution. The consequences of using a smaller gauge than necessary can be fatal. It's possible for a wire to become overheated and then melt, which could damage the breaker or even the connected appliances. A mishap involving an electrical fire is also possible. When compared to using a smaller gauge, using a larger wire is the safer option. You don't run the risk of dealing with the difficulties of a smaller wire. It will still cause you trouble because it is thicker and more rigid. Use of the correct wire size is therefore crucial. Overall, a larger gauge is preferable, particularly in terms of safety.


What are the Important Breaker Size Considerations?

When selecting the breaker size, there are seven crucial safety precautions that you need to be aware of.
They consist of:

  1. Use only standard-sized breakers. Because the appliances won't be protected, it will be dangerous. For instance, if a water heater is protected by an oversized circuit breaker while in use, the coil will overheat. Other circuit devices will experience the same thing. This is especially likely if there is a sizable disparity between the circuit breaker current and the current rating of the appliances.
  2. Use only properly sized breakers. This will continue to disrupt the circuit even when it is not required. For the circuit to operate properly, always use the correct size of breaker. 
  3. Additionally, you cannot use a three-phase supply with a single-phase rated breaker. Such a breaker falls short of adequately securing the circuit and will result in electrical problems.
  4. To prevent the wire from melting, your breaker ampacity must not be greater than the ampacity of the wire. A wire with a lower ampacity will melt before the current gets to it. This may result in fire incidents or breaker malfunctions.
  5. You cannot use a circuit breaker for a 240V supply if it is rated for use in a 120V power supply. A breaker with a 240V rating can, however, be used in a 120V power supply. The general rule is that a breaker cannot be used in a power supply that is greater than its rating.
  6. A 3-Poles circuit breaker can only be used with a three-phase system. You can choose to use two poles or three poles when using. Keep in mind that unless specific circumstances apply, you cannot use this circuit breaker for a single-phase system. For instance, you're good to go if the user manual permits such use.
  7. Last but not least, the breaker must always be used within the maximum safe current limit. The suggested upper limit is 80%.

In any circuit, determining the breaker and wire size is essential. Depending on the ampacity rating of your electrical appliances, you should determine the breaker and wire size. The ideal circuit breaker to use is one that is rated 25% higher than the circuit's current. You also need to understand how to read the wire size.

Always use the proper wire size for connections to be secure. Notably, neither an oversized nor an undersized circuit breaker should be used. Your circuit is not protected from current overload when it is oversized. On the other hand, if it is too small, it will keep failing needlessly. Always remember to determine the breaker and wire size based on the load of the appliances. You'll always have secure connections if you do this.

Prasun Barua

Prasun Barua is an Engineer (Electrical & Electronic) and Member of the European Energy Centre (EEC). His first published book Green Planet is all about green technologies and science. His other published books are Solar PV System Design and Technology, Electricity from Renewable Energy, Tech Know Solar PV System, C Coding Practice, AI and Robotics Overview, Robotics and Artificial Intelligence, Know How Solar PV System, Know The Product, Solar PV Technology Overview, Home Appliances Overview, Tech Know Solar PV System, C Programming Practice, etc. These books are available at Google Books, Google Play, Amazon and other platforms.


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