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

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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 by installing 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, you can choose the correct breaker and wire size you need based on amp requirements.

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.  

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. All commercial, residential, and industrial electrical circuits must have circuit breakers for safety. 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 electrical currents.

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 on the handle.

Your aim should be to operate appliances at a safe load, a maximum of 80% of its capacity.

Therefore, multiply the circuit breaker’s amperage by 80% to obtain this limit. 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 passes to the breaker box. Typically, most household appliances use a small current of approximately 20 amp. 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, resulting in the disconnection of electric current flow. 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 40 amp, a breaker of a similar rating will flip at 40 amp. Thus, if the appliances need 40 amp, a circuit breaker will not be possible. Therefore, you need a higher-rated breaker.

Nonetheless, it would be best if you did 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 40 amp, it would be wrong to use a 100 amp breaker. 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 40 amp limit. Also, you can damage appliances if the breaker is significantly oversized.

How to Calculate Breaker Wire Size & Wattage?

You also need to get the wire size right regardless of the circuit breaker size you are using. Wire size is significant as it determines the electricity 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 is typically a measurement of the wire’s diameter, the standard measure of the correct wire size for a circuit breaker. 

You also need to understand wire size. For instance, note that AWG measures from high to low. The higher the gauge number, the smaller the wire size. A thicker wire will undoubtedly withstand a high ampacity 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 you can use with the breaker.

Wire Size Chart

 Here is a simple wire size chart for American Wire Gauge (AWG) you can use:

Copper WireAluminum WireBreaker Size
#14 AWG15 Amp
#12 AWG20 Amp
#10 AWG#8 AWG30 Amp
#8 AWG#6 AWG40 Amp
#8 AWG#6 AWG50 Amp
#6 AWG#4 AWG60 Amp
#4 AWG#2 AWG100 Amp
#2 AWG#1/0 AWG125 Amp
#1 AWG#2/0 AWG150 Amp
#2/0 AWG#4/0 AWG200 Amp
Source: National Electrical Code

Interpreting the above table is straightforward, and thus you can quickly determine the proper size wire. 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, causing 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 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. Primarily, you will provide it 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 to determine the breaker size. Secondly, you need to choose the wire size by the current rating of the circuit breaker that you prefer. 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 amp circuit breaker 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 #14-gauge wire.

Therefore, for a current system of 10A, you need a breaker size of 15A and a #14-gauge wire. 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 #12 AWG, 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 you will find in the cable. Also, some cables indicate the presence of a ground wire. 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. It would be best if you made a perpendicular cut 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 calculate 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 indicating AWG.

What are the Key Considerations of Wire Size?

You need to get several critical considerations of wire size right.

First, you need to select a larger wire size than your ampacity requirements. 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, it would be best if you were keen on the wire material that you use. Most wires are copper. However, you will also find stranded aluminum and copper-clad wires in larger gauge feeder wires.

However, it would be best 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. A wire is likely to overheat and subsequently melt, affecting the breaker’s condition 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. It would be best to choose a circuit breaker 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. When in doubt, contact a local licensed electrician for help.


Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.