How to calculate the power of Chint NBX series air circuit breakers?

The power calculation of Chint NBX series air circuit breakers needs to be comprehensively determined

according to the **circuit type (single-phase/three-phase)**, **load nature (inductive/resistive)**, and

**rated current of the air circuit breaker**. The following are the specific calculation methods and

selection suggestions: I. Core calculation formulas 1. Three-phase circuit (380V) Suitable for three-phase motors, air conditioners and other equipment: \[ P = \sqrt{3} \times U \times I \times \cos\phi \times \eta \] - **Calculating current when power is known**: \[ I = \frac{P}{\sqrt{3} \times U \times \cos\phi \times \eta} \] - \(P\): Power (W), \(U\): Voltage (380V), \(\cos\phi\): Power factor (take 0.8 - 0.9 for inductive loads and

1 for resistive loads), \(\eta\): Efficiency (take 0.85 - 0.9 for motors). 2. Single-phase circuit (220V) Suitable for lighting, single-phase equipment: \[ P = U \times I \times \cos\phi \] - **Calculating current when power is known**: \[ I = \frac{P}{U \times \cos\phi} \] II. Selection steps of Chint NBX series air circuit breakers Take a **three-phase 380V motor** as an example (such as a 30kW motor): 1. **Calculate the rated current** Suppose \(\cos\phi = 0.85\) and \(\eta = 0.9\): \[ I = \frac{30000}{1.732 \times 380 \times 0.85 \times 0.9} \approx 60 \, \text{A} \] 2. **Consider the starting current** The starting current of the motor is **4 - 7 times** the rated current, that is, \(60 \times 5 = 300 \, \text{A}\). The air circuit breaker needs to select the **D-type trip curve** (suitable for inductive loads), and its

instantaneous trip current is **10 - 14 times** the rated current, which can withstand short-term impacts. 3. **Match the rated current of the air circuit breaker** - **Conventional loads**: The rated current of the air circuit breaker ≥ 1.2 times the rated current

(\(60 \times 1.2 = 72 \, \text{A}\)), and **80A** can be selected. - **Frequently starting loads**: The rated current of the air circuit breaker ≥ 1.5 times the rated current

(\(60 \times 1.5 = 90 \, \text{A}\)), and **100A** can be selected. III. Parameter reference of Chint NBX series

- **C-type trip**: Suitable for **resistive loads** (such as heaters), and the instantaneous trip current is

**5 - 10 times** the rated current. - **D-type trip**: Suitable for **inductive loads** (such as motors), and the instantaneous trip current is

**10 - 14 times** the rated current. IV. Example: Selecting an air circuit breaker for a 5-horsepower 380V air conditioner (refer to Summary 1) 1. Power: \(5 \times 750 = 3750 \, \text{W}\) 2. Rated current: \[ I = \frac{3750}{1.732 \times 380} \approx 5.7 \, \text{A} \quad (\cos\phi = 1, the air conditioner compressor

is an inductive load, and the starting current needs to be enlarged) \] 3. Select a **D-type 20A air circuit breaker** (the rated current > 5.7A and can withstand the starting impact). V. Precautions 1. **Voltage matching**: The rated voltage of the NBX series is **AC 230V/400V**, which needs to be

consistent with the circuit voltage. 2. **Ambient temperature**: In a high-temperature environment, the current-carrying capacity of the air

circuit breaker will decrease (for example, the current-carrying capacity is derated by 10% at 40°C). 3. **Multi-stage protection**: The main air circuit breaker and the branch air circuit breaker need to

cooperate, and the rated current of the branch air circuit breaker < the rated current of the main air circuit breaker. Summary For calculating the power of Chint NBX series air circuit breakers, the **load current** needs to be calculated

first, and then the rated current should be selected according to the **load type (C/D type)** and **starting

current**. **Core formulas**: - Three-phase: \(I = \frac{P}{\sqrt{3} \times U \times \cos\phi \times \eta}\) - Single-phase: \(I = \frac{P}{U \times \cos\phi}\) **Selection principle**: The rated current ≥ 1.2 times the load current (select D type for inductive loads and

C type for resistive loads).