In this article, we explain the standard circuit breaker sizes. Additionally, we detail how to calculate the appropriate circuit breaker and suggest an app to do it quickly.
A circuit breaker is a device designed to protect electrical equipment. Therefore, it is essential to correctly calculate the required circuit breaker. The breaker has an amperage rating, and if the circuit it protects exceeds this amperage, the breaker will trip, opening the circuit and preventing damage to electrical equipment.
The calculation of a circuit breaker depends on the type of device it protects, the size of the conductor, and other factors. According to the NEC, there are specific requirements to follow, such as not exceeding 250% of the rated amperage.
Typically, the breaker capacity is selected as the next standard size above the ampacity of the conductor (based on commercially available options).
We recommend using an Excel spreadsheet for performing various calculations, available on Hotmart.
We recommend an app for calculating circuit breakers for transformers and the overload relay settings for motors.
Additionally, it helps determine the conductor size and conduit type based on the amperage in accordance with the National Electrical Code (NEC).
Electrical Calculations based on the NEC is a useful tool for obtaining compliance with electrical installation standards.
Summary of standard circuit breaker sizes.
Guidelines According to the NEC for Circuit Breakers
The commercial value of the breaker must be the next standard size above the ampacity of the conductor connected to the breaker.
However, if the breaker rating is 800 A or higher, the conductor’s ampacity must match or exceed the breaker rating.
Protecting specific devices like motors, transformers, or generators requires careful consideration of their specific functions.
App for Calculating Breakers for Transformers and Motors
This app provides calculations for motor and transformer protection, as well as determining conductor sizes for copper and aluminum and the appropriate conduit.
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Importance of Protection, standard circuit breaker sizes
The Importance of Selecting the Correct Breaker Capacity
If the breaker is undersized, it will trip even without faults in the circuit.
On the other hand, if it is oversized, it may allow a short circuit to persist longer, potentially causing significant damage before tripping.
Considerations for Selecting a Circuit Breaker
To select the correct breaker, you need to consider the type of devices being powered, their capacity, and the conductor size. And the standard circuit breaker sizes
The capacity of the devices is crucial. This refers to the total amperage drawn by all devices connected to the breaker. This helps determine the breaker rating.
Additionally, the conductor size must be considered. The breaker should not exceed the ampacity of the conductor. In the event of a short circuit, the conductor should not fail before the breaker trips.
Furthermore, the type of device being powered is important. If the device has built-in protection, the main breaker should be sized according to the conductor. Otherwise, the breaker must protect the device itself, considering its starting and running currents to avoid nuisance tripping.
Selecting the Main Circuit Breaker
The main breaker protects multiple devices on the same circuit. It assumes that each device has its own short-circuit protection.
Thus, the main breaker is typically selected based on the conductor size connected to it.
According to the NEC, tables are provided to select breakers based on conductor ampacity.
The rule for selecting a breaker based on the conductor is to choose the next standard size above the conductor ampacity. For breakers rated at 800 A or higher, the conductor’s ampacity must match or exceed the breaker rating.
In other words, the commercial breaker value must be the next size above the conductor’s ampacity.
For circuits with multiple devices or loads such as receptacles and lighting, the same rule applies. First, calculate the conductor size, then select the appropriate breaker.
However, specific equipment like motors (which use overload relays) or transformers (which use fuses) may require different protection methods. For such cases, a more detailed analysis is recommended.
Examples of circuit breaker calculations and standard circuit breaker sizes.
Copper wire size 18 AWG: Maximum breaker size 7A.
Copper wire size 16 AWG: Maximum breaker size 10A.
Copper wire size 14 AWG: Maximum breaker size 15A.
Copper wire size 12 AWG: Maximum breaker size 20A.
Copper wire size 10 AWG: Maximum breaker size 30A.
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