How to Calculate What Breaker & Wire Size You Need (Explained with Examples)

Have you ever had your electronics destroyed by an electrical mishap? If you haven’t experienced it firsthand, you’ve probably heard of electrical fires reported in the news. You can stop nearly all of these accidents if you install the appropriate circuit breaker and wire size. So how do you determine the proper breaker and wire size that you need for your appliances?

To determine the breaker and wire size you need, divide the total watts by the voltage to find the amperage on the circuit. You can use a breaker and wire size ampacity chart to calculate the breaker and wire size you need. Each breaker size requires a different wire size based on the breaker’s ampacity. Hence, guided by the amp requirements, you can choose the correct breaker and wire size you need.

If you don’t know how to do the calculations, worry not. Read on for an elaborate illustration of determining the perfect breaker and wire size.  

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How to Find the Proper Size of Circuit Breaker?

Having a circuit breaker is a fundamental requirement by the National Electric Code. The Institute of Electrical and Electronics Engineers (IEEE) also advocates using the proper breaker size.  For safety, all commercial, residential and industrial electrical circuits must have circuit breakers. The rationale behind this requirement is to prevent electrocution and electrical fires outbreaks.

Nonetheless, not you can use any circuit breaker in any circuit. For guaranteed safety, you must use the correct size depending on the specifications of the appliances and current.

For the proper size of the circuit breaker:

  1. Add the watt usage of all the appliances in the circuit. These include appliances such as bulbs and fridges that are always connected to the circuit. You can easily find the individual device power consumption by checking the attached labels. This will give you Total Watts.
  1. To convert the indicated power to current, divide the Total Watts by the devices’ voltage of operation. Most devices have a voltage rating of either 120V or 240V. Choose the indicated rating during the calculation.

Total Watts / Volts = Amps

  1. After you have obtained the total current of the appliances, multiply your figure by 125%. Your ideal circuit breaker rating must be above the figure that you will obtain from the computation.

Amps x 1.25

Most of the electronics used in residential circuits require 20 amperes. Therefore, a circuit breaker of this ampacity is ideal for residential circuits. Nonetheless, you can also use larger ones depending on the appliances. Also, note that the circuit breaker’s amperage is indicated on its handle.

Your aim should be to operate appliances at a safe load. This is a circuit that has a maximum of 80% of its capacity.

Therefore, to obtain this limit, multiply the circuit breaker’s amperage by 80%. You can still have a circuit with devices superseding the safe load. Nonetheless, you must be keen that this happens for a short period. Otherwise, you will jeopardize the system’s safety and risk blowing the appliances.

Calculating the Ideal Circuit Breaker Size

Let’s assume if you have 2 x 60-Watt Lights, 5 x 75-Watt lights, and 3 x 100-Watt lights you want to install

Case scenario: Consider a residential lighting circuit totaling 795 Watts using a single-phase 120V power supply. What is the ideal circuit breaker of such a circuit?

795 Watts / 120V = 6.625 Amps

Solution:  The Circuit Breaker size should be at least 125% greater than the circuit’s current.  Hence circuit breaker size of 15 amps is more than sufficient.

6.625 Amps x 1.25 = 8.28125 Amps

How does the Circuit Breaker System Operate?

When electricity enters your house, it is first passed to the breaker box. Typically, most household appliances use a small current of approximately 20A. Such a circuit requires a circuit breaker to control the flow of electricity. The larger the current demands of the appliances, the larger the breaker and wire size requirement.

The circuit breaker and the wires supplying the current are one unit. The circuit will trip if you pass a current greater than the resistance of these components. With excess current, you can expect the wires to weaken and melt.

Once they are exposed, there is a risk of a fire outbreak. Here is where the circuit breaker becomes necessary.

The circuit breaker and the wires work in tandem in regulating the amount of current that passes in the circuit. When the electric current is below the circuit breaker rating, the circuit breaker allows passage of the electricity.

However, when the current rises above the circuit breaker rating, the circuit breaker trips. This results in disconnection of the circuit and no current flows. Thus, the appliances on the system are protected from damage by the circuit overload.

The fundamental standard is choosing a circuit breaker with a 25% higher current size than the circuit’s current. For example, if a circuit carries 40A, a breaker of a similar rating will flip at 40A. Thus, if the appliances need 40A, it will not be possible with such as circuit breaker. Therefore, you need a higher-rated breaker.

Nonetheless, you must not use an extremely high-rated breaker as this will jeopardize the functioning of the applied components. For example, if you have a circuit carrying 40A, it would be wrong to use a breaker of 100A. Such a circuit breaker would not protect the circuit from an overload. It can result in a fire accident as the wires will melt when the current passes the 40A limit. Also, you can damage appliances if the breaker is significantly oversized.

How to Calculate Breaker Wire Size & Wattage?

Irrespective of the circuit breaker size you are using, you also need to get the wire size right. Wire size is significant as it determines the electricity that is flowing through the system. It also determines the resistance of the wires to the amount of electricity that passes through.

To calculate the wire size and wattage, you first need to identify the wire’s American Wire Gauge (AWG). AWG is a standard measurement of wire size provided by global standards body ASTM International. Also called the gauge, it is typically a measurement of the wire’s diameter. This is also the standard form of measuring the correct wire size for a circuit breaker. 

You also need to understand how wire size operates. For instance, note that AWG measures from high to low. This means that the higher the gauge number, the smaller the wire size. A thicker wire will undoubtedly withstand a high ampacity as compared to a thinner one.

Also, during calculations, you need to interpret the wire size ampacity chart accurately. This table provides the best AWG versus the maximum ampacity that you can use with the breaker.

 Here is a simple wire-size ampacity chart that you can use.

Maximum AmpacityWire Gauge (AWG)
952
704
556
408
3010
2012
1514
1016
718

Interpreting the above table is straightforward, and thus you can quickly determine the proper wire size. Note that you must choose the correct wire gauge; otherwise, the operation of the breaker will be affected. Also, selecting the wrong wire size can lead to melting when there’s a current surge. This can easily cause an electrical fire. 

Therefore, take time to choose the proper wire gauge while guided by the electric current size you intend to use.

How to determine wire and breaker size?

It is crucial to ensure that you choose the appropriate wire and breaker size to avoid common accidents. Also, remember to check the appliance amp requirements first. As earlier mentioned, you can easily access this information from the appliances’ power rating label. Mostly, it will be provided in Watts, and you, therefore, need to convert it to amperes. Divide the power (Watts) by the voltage requirements to get the ampacity.

Once you have the total ampacity of the appliances, calculate the safe load. This will help in determining the breaker size. Secondly, you need to choose the wire size. This should be guided by the current rating of the circuit breaker that you choose. You can easily select the appropriate AWG from the above ampacity chart.

Each set of appliances will require a separate AWG and circuit breaker. For instance, consider an ampacity of 10 Amps. This is sufficient to operate common household appliances such as a fridge, dishwasher, and washing machine. You can determine the wire and breaker size by following two simple steps.

  1. First, you need to calculate the safe load. Remember that circuit breaker ampacity should be at least 125% of the load. Thus, in this case, the best breaker size = 10A X 125%= 12.5A. 

Note: the available breaker size ranges include 15A, 20A, 30A, 40A, 50A, etc. Therefore, in our case, a breaker size of 15A is perfect.

  1. Next, you need to determine the correct wire size to match the breaker that you have selected. This data is available from the above wire size ampacity chart. A maximum ampacity of 15 A requires a wire of gauge 14.

Therefore, for a current system of 10A, you need a breaker size of 15A and a wire of gauge 14. Similarly, you can determine the breaker and wire size for other current systems using the above two steps.

How to Interpret the Wire Labels?

When buying wire, you must be keen on purchasing the correct gauge. This requires efficient interpretation of the wire labels. You can find all the information on the wire’s surface. However, this is not the case in some cables, and you thus need to look elsewhere keenly. So how do you read the wire labels to ensure that you are using the right size?

In most wires, you will find the AWG indicated on the surface of the cable. For instance, for a wire of an AWG of 12, the inscription will read “12.” You will also see another number next to the AWG tag. For example, you can find a number such as “12/4.”

The second number represents the number of wires that you will find in the cable. Also, some wires indicate the presence of a ground wire in the cable. You will find this inscribed as “G.” Note that the presence of the ground wire does not interfere with the cumulative total of the service wires.

Note that not all wires have written text of the AWG. You can still interpret their gauge, but you will need to use an alternative process. First, you require a wire cutter to cut the wire. You should make a cut that is perpendicular to the cable’s run. Once you have access to the interior of the cable, measure the diameter. AWG is usually in inches, and therefore you must measure in inches.  

When measuring diameter, ensure that you use the distance from one end of the cross-section to the other. Also, ensure that you maintain a straight line as you measure for an accurate reading. Lastly, ensure that you do not include the thickness of the cable in the measurements. It is not part of the gauge dimensions.

While this process will give you an estimate of the wire gauge size, it is tedious. Also, it is subject to errors, especially during measurement. Therefore, it is advisable to avoid the hassle by purchasing a wire that has an indicated AWG.

What are the Key Considerations of Wire Size?

There are several critical considerations of wire size that you need to get right.

First, you need to select a large wire size. This is irrespective of your ampacity requirements, justifying the use of small wire size. The rationale is that the larger the wire size, the safer your appliances will be.

Also, if you intend to transmit current over a distance of over 100 feet, use the largest possible wire size, especially if the wire is in an enclosure such as a conduit where heat dissipation is imperfect.  

Also, you need to be keen on the wire material that you use. Most wires are made of copper. However, you will also find wires made of silver, brass, or aluminum. Wires made of brass and silver will operate in the same way as those made of copper.

However, you need to be cautious when using aluminum wires as they are pretty different from copper. In terms of conductivity, aluminum delivers 61% of that of copper. Nonetheless, aluminum is 30% lighter. Therefore, be cautious when choosing the gauge of aluminum. It may not guarantee the same efficiency as copper.  

So considering the wire size, is a smaller or larger size better?

You must be very cautious with the wire size. If you use a smaller gauge than required, the effects can be fatal. Such a wire is likely to overheat and subsequently melt. This will affect the condition of the breaker or even the appliances in the connection. You’re also likely to experience an electrical fire accident.

If you consider using a larger wire, you’re playing it safer than using a smaller gauge. You do not risk experiencing the challenges of a smaller wire. It will, nonetheless, inconvenience you as it is thicker and more rigid. It is thus essential to use the proper wire size. All in all, the bigger the gauge, the better, especially in terms of safety.

What are the Key Considerations of Breaker Size?

There are seven vital precautions that you should know when choosing the breaker size. They include:

  1. Do not use an oversized breaker. It will be hazardous as it will not protect the appliances. For example, when using a water heater, its coil will overheat if an oversized circuit breaker protects it. The same will happen with other devices in the circuit. This is likely especially if the difference between the circuit breaker current and the appliances’ current rating is significantly large.
  1. Do not use an undersized breaker. This will keep breaking the circuit even when it is unnecessary. Always use the right size of the breaker for the effective operation of the circuit.
  1. You must also not use a single-phase rated breaker for a three-phase supply. Such a breaker is insufficient in protecting the circuit and will lead to electrical faults.
  1. Your breaker ampacity must not be larger than the ampacity of the wire to protect the wire from melting. A smaller ampacity wire will melt before the current reaches the wire. This can cause fire accidents or dysfunction of the breaker.
  1. If the circuit breaker is rated for use in a 120V power supply, you cannot use it for a 240V supply. However, you can use a breaker rated 240V in a 120V power supply. The rule of thumb is that the breaker cannot be used in a higher power supply than its rating.
  1. You can use a 3-Poles circuit breaker only for a three-phase system. During use, you may opt to use either two poles or three poles. Note that you cannot use this circuit breaker for a single-phase system unless under particular conditions. For example, if the user manual allows such use, you’re good to go.
  1. Lastly, you must always operate the breaker within the maximum safe current limit. The recommended limit is 80%.

Final Thoughts

Calculating the breaker and wire size is imperative in any circuit. You should calculate the breaker and wire size depending on the ampacity rating of your electrical appliances. You must choose a circuit breaker that is rated 25% higher than the circuit’s current. Also, you must know how to read the wire size. Always ensure that you use the correct wire size for safer connections.

Also noteworthy, do not use either an oversized or undersized circuit breaker. When it’s oversized, your circuit has no protection from current overload. Conversely, when it is undersized, it will keep breaking down unnecessarily.

Remember always to calculate the breaker and wire size depending on the appliances’ load. This way, you will always have safe connections.

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Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started HomeInspectionInsider.com to help people better understand the home inspection process and answer questions about homeownership and home maintenance.

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